Space & Astronomy/Space and Astronomy (October 1993).iso/mac/TEXT/STSCI/NOV91.TXT

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stsci/newsletters/december91

Highlights of This Issue

    New Science Results on N87, Crab Pulsar
    COSTAR Progressing Well
    Answers to your Questions about HST Data
    Cycle 2 Peer Review Underway

 HST Science Highlights

 WF/PC Observations of the Stellar Cusp in M87

    The photograph on the left shows one of a set of images of the
central regions of the giant elliptical galaxy M87, obtained in June 1991
with HST's Wide Field and Planetary Camera (WF/PC). Analysis of these
images has revealed a stellar cusp in the core of M87, consistent with the
presence of a massive black hole in its nucleus.

    A combined approach of image deconvolution and modelling has made
it possible to investigate the starlight distribution in M87 down to a
limiting radius of about 0".04 from the nucleus (or about    3 pc from the
nucleus if the Virgo cluster is at       16 Mpc). The results show that the
central structure of M87 can be described by three components: a power-law
starlight profile with an r-1/4 slope which continues unabated into the
center, an unresolved central point source, and optical counterparts of the
jet knots identified by VLBI observations.

    In both the V- and I-band Planetary Camera images, the stellar cusp
is consistent with the black-hole model proposed for M87 by Young et al. in
1978; in this model, there is a central massive object of about 3 x 109 Mo%
.

    The central luminosity spike remains unresolved with the Planetary
Camera, and is at least as blue as the rest of the M87 jet. In a paper
reporting these results (now in press in The Astronomical Journal), the
WF/PC Investigation Definition Team (IDT) argues that the central spike is
entirely due to nonthermal (synchrotron) radiation.

    The WF/PC images, as well as recent Faint Object Camera (FOC)
images, also show M87's well- known jet with unprecedented optical
resolution. The FOC and WF/PC data will be discussed at the upcoming
American Astronomical Society meeting in Atlanta.

-Tod Lauer



 HSP Observations of the Crab Pulsar

    In late October, the High Speed Photometer (HSP) team obtained
photometry of the optical pulsar in the Crab Nebula. These data demonstrate
the capabilities of HSP to observe short-timescale phenomena free of the
effects of atmospheric scintillation. The observations were taken through
the F400LP filter with image-dissector tube 3, with a sample time of 10.74
microseconds. A total of 703,244,160 samples was collected during four
intervals of about half an hour each. Each interval covered about 50,000
rotations of the pulsar. The sample arrival times were adjusted for the
motion of the Earth about the barycenter of the solar system using the JPL
DE-200 Planetary Ephemeris, and for the motion of HST about the Earth using
the SOGS predictive ephemeris. An additional adjustment, tentatively
identified as the difference between the true HSP clock frequency and the
manufacturer's specification value, has also been made.

    In the accompanying figure, data covering one half-hour have been
phased onto the known rotational period (33.396206 milliseconds) of the
pulsar. The main pulse and the interpulse are shown with very high S/N.
Details of the light curve, such as the luminosity between pulses and the
pulse structure, are being analyzed now.

    The HSP team will make more high-speed observations of this object
in both broad and narrow wavelength bands in the visual and ultraviolet
regions of the spectrum.

-Bob Bless & Jeff Percival



 HST Scientific Program Makes Substantial Progress

    The current scientific program of the HST is comprised of 347
individual observing programs, of which 100 have now been completed. The
present pool consists of Guaranteed Time Observer (GTO), Cycle 1 General
Observer (GO), and Director's Discretionary (DD) programs. These programs
were approved for execution during Cycle 0 (the interval up to

July 1, 1991) or Cycle 1 (July 1, 1991 through June 30, 1992), although the
actual dates of execution have not necessarily been limited to those
intervals. Among the 100 completed programs are 75 GTO, 20 GO, and 5 DD
programs. Of the remaining 247 scientific programs, 61 (31 GTO and 30 GO)
have begun execution (i.e., some but not all of the proposed data have been
obtained) and an additional 40 (16 GTO, 20 GO, and 4 DD) are planned to
begin execution during the next two months.

    The remaining 146 scientific programs consist of 56 programs that
utilize the GHRS and 90 other programs. Implementation and scheduling of
the latter group is under way, but work on the 56 GHRS programs has been
suspended pending resolution of the operational status of the GHRS, or
revision of the programs by the observers to use other scientific
instruments. Nearly all programs will be completed by the end of Cycle 1,
but due to scheduling constraints it will be necessary to complete some
after the beginning of Cycle 2. For example, this will occur for those
programs that contain observations that must be executed at a particular
date, and may occur for some programs utilizing the GHRS due to the
suspension of GHRS observations.

    In addition to the scientific program, utilization of the HST
Observatory is assigned to the Orbital Verification (OV), Science
Verification (SV), and Engineering (ENG) and Calibration (CAL) programs.
The Science Assessment Tests (SAT) and the Early Release Observations (ERO)
were parts of the last category. The OV program is now nearly complete,
with only two Fine Guidance Sensor tests remaining to be executed. The SV
program has made substantial progress and will end on November 30, 1991,
although a small number of tests will be executed after that date due to
scheduling constraints. The Engineering and Calibration program is an
ongoing effort that will utilize 10-15% of the observing time.

-Larry Petro & Duccio Macchetto



 THE HST OBSERVATORY

 From the Director's Office

    For the STScI, which operates the science mission of the HST around
the clock, the summer months are distinguished more by the steamy Baltimore
weather than the peaceful study and spiritual healing longed for by the
academician. With an orbiting spacecraft, the detailed planning and data
handling cannot let up. New spacecraft problems must be addressed and
observers notified.

    This year, the HST proposal cycle also peaked in mid-summer, with
last-minute updates and the flurry of incoming proposals. Those staff
astronomers who traveled to the IAU General Assembly, or spent several
weeks observing or at workshops, returned to new challenges and an
increasing stream of General Observers, science data, calibrations, and the
reviews needed before the next Telescope Allocation Committee meeting this
December; in other words, "routine operations."

    Despite its aberrated optics, the HST Observatory is providing the
astronomical community with unique and exciting high-resolution UV,
optical, and near-IR images and UV spectroscopy. Of the more than 300 GTO
and GO science programs accepted for Cycles 0 and 1, we will have completed
about one-third by the time this Newsletter appears.

    Unfortunately, because of the failure of the Goddard High
Resolution Spectrograph (GHRS) side 1 power-distribution system in July
(see page 12), programs using the GHRS have had to be put on hold until the
use of Side 2 can be restored. The calibration program for the Fine
Guidance Sensors is beginning after an extensive collimation effort, and
some early science observations will soon be executed. The science team for
the High Speed Photometer has now essentially completed its calibration
program, thus permitting the initiation of UV photometric and polarimetric
observations.

    We are pleased by the continuing strong demand for HST observations
by the international astronomical community. As described in more detail on
p. 19, we received over 480 proposals for Cycle 2 programs, requesting
nearly 12,000 hours of spacecraft time. As gratifying as the demand was the
breadth of international interest, indicated by the fact that the Principal
Investigators represent 19 different countries. The number and quality of
these programs will ensure a high scientific productivity for HST until the
first servicing mission is able to restore its originally intended
performance.

    As discussed in the previous Newsletter and elaborated below, the
planning and development required for the first shuttle servicing mission
have intensified. With the second gyro failure in June, and the GHRS
failure in July (both reported in detail elsewhere in this Newsletter), the
menu of desired servicing activities has become extensive. The baseline
mission now includes the installation of new solar arrays as well as at
least two gyro assemblies (containing two gyros each).

    The installation of the replacement Wide Field and Planetary Camera
(WF/PC II) and COSTAR will retire the current WF/PC and HSP, respectively.
In addition, there is a good chance that the GHRS can be repaired on-orbit
during the same mission. In order to accomplish all these tasks with some
contingency for unforeseen needs, the HST Project at Goddard Space Flight
Center is working with the Johnson Space Center to develop a mission with
two astronaut teams to carry out four extravehicular-activity periods.
While efforts of greater magnitude will be required to construct the space
station in the second half of this decade, the HST servicing mission is
shaping up to be one of the most ambitious missions yet attempted.

-Peter Stockman



  HST's Observing Efficiency

    The August 1991 issue of the ST-ECF Newsletter contained an
excellent article on HST observing efficiency by Piero Benvenuti and Benoit
Pirenne. This article considered the actual amount of time HST devotes to
collecting photons from targets, and plotted the amount of collection or
"exposure" time per instrument for the period November 1990 to June 1991.
The time that HST does not spend exposing on targets may be regarded as the
overhead time required for the conduct of operations with the HST
Observatory.

    Both the ST-ECF analysis and our independent review of six months
of operations (January to July 1991) indicate that the "exposure time"
efficiency is about 9%. Since this differs significantly from the
"time-on-target" efficiency of 30-35% that is our operational goal (see
below), we have broken down the accounting of overhead activities to
understand better how the remaining time is being used and where
significant improvements may be made.

    The utilization of the HST Observatory may be characterized by six
types of serial activities: guide-star acquisition, target acquisition,
exposure, control of the scientific instruments and spacecraft, Earth
occultation of the target, and other overhead items such as passage through
the South Atlantic Anomaly (SAA), slewing of the telescope, and unscheduled
deadtime.

    The typical proportions of time spent on these activities are shown
in an accompanying figure, and are based on the recent months' GO, GTO, and
Science Verification programs. As mentioned above, the figure shows that
scientific exposures account for an average of 9% of the Observatory time.
This is the time spent actually integrating on targets. The remaining time
spent pointing at a target is consumed by acquiring the guide stars used to
control the telescope pointing (8%), acquiring the target itself in the
scientific instrument aperture (3%), and configuring the scientific
instrument for use and reading out the science data, usually to the onboard
tape recorder (16%).

    During a typical observation, time is also lost to occultation of
the target by the Earth (typically 45%). The remainder of the Observatory's
time (about 19%) is comprised of time spent during passage of the
spacecraft through the SAA (which affects both the guidance system and most
of the instruments), to unschedulable time (usually short time intervals
during which no short exposures are available to be scheduled), and time
spent slewing to that target (at HST's slew rate of 360 degrees per hour).

    In the future, it should be possible to make incremental
operational improvements in each of the five overhead categories. A task
force led by the Science and Engineering Systems Division at STScI has been
established to review current HST operations and recommend specific
improvements. For instance, software improvements are about to be installed
in the long-range planning system, which should significantly reduce the
amount of time lost to target occultations and passage through the SAA. In
addition, although we are currently using two-pair guide-star acquisitions
for most scientific observations, experience has shown that single-pair
acquisitions should be sufficient.

    Although several such areas for reducing the overhead times have
been identified, it is difficult to forecast the ultimate HST science
efficiency in terms of exposure time. Many of the overheads are "built in"
by the design of the current scientific instruments, or are required by the
fact that we must operate at HST's low orbital altitude.

    However, we can gauge the capabilities of the ground system by
referring to the goals that were established for on-target efficiency based
upon pre-launch studies. The on-target time is defined as the elapsed time
between the completion of the slew to a target and the completion of
observations of that target, excluding target occultations, SAA passage,
and end-of-orbit deadtime. In terms illustrated in the first figure, the
on-target time is computed as the sum of exposure, S.I. control, guide-star
acquisition, and target acquisition times. The goals established were for
20% on-target efficiency during Cycle 0, and an average of 27.5% efficiency
during Cycle 1.

    The efficiency achieved for external science targets during each
week of the past year is shown in the second accompanying figure. The
values achieved are typically within the range 25 - 35%, with the lowest
values being due to the spacecraft or a scientific instrument being placed
in safe mode. Thus the ground system is already surpassing the near-term
goals for planning science observations. For Cycle 2, the goal is 32.5%,
and for Cycle 3 it is 35%. The improvements mentioned above concerning the
avoidance of SAA passages will probably be sufficient to achieve the Cycle
2 and 3 goals. Since the planning system will soon be near its ultimate
performance level, further efficiency improvements must come from possible
changes in the operation of the scientific instruments and the actual
design of the scientific observations.

    The scientific efficiency of the HST Observatory is affected
strongly by the actual scientific programs specified by the observers. For
instance, a program containing many short WF/PC images will have a small
ratio of exposure time to instrument overhead due to the time required for
preflashes, shutter control, and data readout.

    Therefore, it is on-target or "spacecraft" time, rather than
exposure time, that has been allocated to observers, in order to allow them
to optimize the exposure time obtained within their spacecraft-time
allocations. To calculate the spacecraft time required for a program,
observers have been provided with the Resource Estimator as a part of the
Remote Proposal Submission System. The version of this estimator provided
for Cycle 1 was written well before the launch of HST and therefore before
any experience had been obtained in scheduling the science program.
After-the-fact accounting has shown that the consumed Spacecraft Time is
approximately 1.3 times the value computed with the present Resource
Estimator. Revisions to the Resource Estimator are in process with the goal
of matching after-the-fact accounting within 5% on average.

    In summary, the goal for Cycle 2 is to improve the on-target
efficiency by more than 5 percentage points. This will increase both the
amount of time available for observing and the number of executed
observations. In addition, optimizing the operation of the spacecraft and
the scientific instruments may realize a similar improvement. Finally, the
results of the study now underway by the STScI Efficiency Task Force will
be useful in improving the accuracy of the RPSS Resource Estimator and in
helping observers optimize their science observations.

-Larry Petro, Peter Stockman, & Brad Whitmore



 COSTAR Progress

    As described in previous Newsletters, a strategy for recovering
HST's scientific capabilities has been developed. This strategy calls for
replacing the High Speed Photo-meter with COSTAR (the Corrective Optics
Space Telescope Axial Replacement), an instrument that will deploy
corrective elements into the optical paths in front of the Faint Object
Camera, Faint Object Spectrograph, and Goddard High Resolution
Spectrograph. Installation of COSTAR, along with replacement of the Wide
Field and Planetary Camera by a new camera (WF/PC II) with built-in
corrective optics, is expected to remove the effects of the HST primary
mirror's spherical aberration and restore the originally designed
performance of the scientific instruments.

    Development of COSTAR has proceeded well since program inception in
January 1991. The conceptual definition phase was completed on schedule in
February 1991, and the Preliminary Design Review (PDR) was completed as
scheduled in early May. The next major milestone is the Critical Design
Review, scheduled for early December 1991. The Quarterly COSTAR Review held
in late August at Ball Aerospace Corp. showed the program to be on schedule
for this December review. (Further details of this review are given below.)

    Future critical milestones to support the December 1993 launch of
the first HST servicing mission include completion of fabrication and
assembly in September 1992, followed by test and verification through May
1993. Delivery of the instrument to the Goddard Space Flight Center (GSFC)
will occur the following month. The interval between delivery and launch is
reserved for testing at GSFC, and launch preparations at the Kennedy Space
Center.

    The PDR was a critical juncture for COSTAR. The review was held at
GSFC with over one hundred people in attendance. The outcome was that the
technical review committee found that, although COSTAR would be
challenging, they saw no technical "showstoppers." While some details have
changed since the PDR, the fundamental design (see figure) has remained
unchanged since inception.

    The STScI COSTAR project was concerned with the relatively small
clearance between the FOS beams and the multi-layer insulation (MLI)
blankets that are installed at the interfaces between the four axial
Scientific Instruments. An optical redesign that increases the nominal
clearance to greater than 28 mm at all points has alleviated concerns about
possible vignetting by bulging or displaced MLI.

    The COSTAR optical design has now matured, and the mirrors are on
order from two vendors, Tinsley and UTOS, each of which will produce a full
set of flight optics. The first pieces are due for delivery in December
1991 and all the mirrors should be on hand in March 1992. The
specifications for figure and surface roughness are exacting, to assure
that the throughput and image-quality goals will be met, but are within the
demonstrated capabilities of both manufacturers. A problem with the sizing
of the FOS M2 mirrors was encountered in September, but was rapidly
circumvented with an inventive adaptation of the mounting scheme that has
been under development.

    Topics discussed in detail at the August Quarterly Review included
system design and performance, the end-to-end wavefront budget, the optics,
the design of the refractive beam simulator (RAS) that will be used to test
COSTAR, the Hubble Optical Mechanical Simulator that will hold COSTAR and
the test components, and the plan for aligning and testing COSTAR. The
project has reached the point that the detailed designs and analyses of the
deployable optical bench, the mechanisms, and the optics can be compared to
the error budget. This comparison shows that COSTAR can be built to
stringent system-level specifications. Finally, Ball Aerospace Corp. showed
enough detailed planning at the Quarterly Review to convince the HST
Project and STScI that COSTAR can be aligned and tested within the time
allotted in the schedule.

    To ensure that COSTAR will work, there will be extensive testing
using independent measurements and analyses by four groups. The European
Space Agency (ESA) will provide a Structural and Thermal Model (STM) of the
Faint Object Camera that exactly duplicates the optics of the in-flight
FOC, although replacing the photon-counting detectors with CCDs. This will
be tested in Europe with an aberrated-beam simulator to verify that COSTAR
corrects it exactly. The images will be analyzed by Ball, STScI, and ESA.
The goal will be to check that COSTAR, when aligned properly, can deliver
the performance outlined in the Level I specification for HST, and to
investigate how the images can be used to align the instrument when
deployed on-orbit.

    Another level of verification will come from the Independent
Verification Team, set up by the HST Project at Goddard to check
independently the optical prescriptions and alignments and verify the test
results. They will also perform their own tests of the RAS and COSTAR+RAS
wavefront errors using a wavefront analyzer. These independent
verifications of the critical aspects of the design will ensure that the
type of error that led to the spherical aberration in the HST primary will
not recur with COSTAR.

    For readers who are interested in further details, the high-level
specifications for COSTAR follow. Two specifications control image
sharpness. The first is the Strehl ratio, which is the ratio of the peak
intensity in the observed image to the peak intensity in the image from a
perfect telescope. The second is the encircled energy. The Strehl ratio
specification is that the corrected image quality for the FOC f/96 channel
shall give a minimum ratio of 0.55 at 6328 
, with a design goal of 0.60.
Had the HST primary been within specifications, the Strehl ratio would have
been approximately 0.8. The encircled-energy specifications are that 60% of
the light must be within a 0".1 radius at the center of the FOC and FOS
fields, and within a 0".125 radius at the center of the GHRS field.
Scattering from the surfaces of the COSTAR mirrors in the UV is controlled
by the specification that the rms surface roughness of the mirrors for all
spatial scales less than 1 mm must be less than 10 
 rms. Finally, the
optical throughput of each pair of correcting mirrors must be no less than
56% at 1216 
 and 72% at 6328 
.

-Jim Crocker, Holland Ford, George Hartig, & Robert Jedrzejewski



 HST Spacecraft Operational Status

    Of the changes in HST's operational status that have occurred since
the last issue of the Newsletter, the one with the greatest impact is the
failure of a low-voltage power supply in the GHRS. This failure prevents
use of side 1 altogether, and allows only intermittent readouts from side
2. Astronomers and engineers at STScI are working with NASA and Ball
engineers on the failure analysis, on-orbit engineering tests, and
development of work-arounds and side-switch procedures. For further details
of this failure, see Ron Gilliland's article below (p. 12).

    A number of operational improvements have been made since June.
Several improvements have been made in the use of the Fixed Head Star
Trackers, resulting in a reduction of the failure rate of position updates
after slews to essentially zero. This in turn has removed the biggest
source of failed guide-star acquisitions. In fact, all of the guide-star
acquisitions in the last two months have been successful.

    The planning system at STScI has been modified to schedule
interleaved WF/PC Earth flat fields properly. Starting in July, we have
been taking many Earth flats in support of the WF/PC calibration program,
as described in more detail below (p. 9). We are in the last stages of
testing modifications to the planning system that will allow us to obtain
parallel observations (i.e., simultaneous observations with two of the
scientific instruments). The first on-orbit test of parallel observations
is expected in early January 1992.

    While it had essentially no effect on on-going scientific
observations, there was another gyro failure (this time of gyro #4) this
past summer. This gyro behaved intermittently for several weeks, and then
apparently failed completely. At the time of the initial failure, HST was
operating in its normal four-gyro configuration. The failure resulted in a
saturated output from the gyro, e.g., it indicated the maximum possible
rotation rate. The flight software recognized the erroneous input
immediately and took the gyro out of the control loop, falling back to a
three-gyro configuration. The saturated condition lasted for about 80
seconds, after which the gyro output returned to normal although the flight
software remained in the three- gyro configuration.

    After investigation, we returned gyro #4 to the control loop. A
very similar event occurred again three days later, except that the gyro
stayed saturated for 10 minutes. Finally, a week later, the gyro failed
with a saturated output and remained saturated.

    Analysis indicates that the likely cause of the failure was a
failed wire bond in one of the hybrid circuits in the gyro electronics
package. Although in a different circuit, it is a similar failure to the
gyro #6 failure that occurred last December.

    Gyro #4 was left running, although its output obviously was not
used to control the telescope. In late October, it unsaturated again, and
produced reasonable data for a period of about a week. The HST Operations
Control Center at GSFC is continuing to monitor its performance.

    Following the failure of gyro #4, we turned on the last spare gyro,
gyro #1, and after calibration it was placed into the control loop,
returning HST to a four-gyro configuration. The four-gyro configuration
allows the flight software to perform checks on gyro data and detect
failures, as it had done with gyro #4. Science operations are also possible
with only three gyros, as indeed was done for a period this summer while
the problems were being diagnosed. Vehicle pointing performance is
essentially the same in the three- gyro configuration as in the four-gyro
configuration.

    These events led to a review of the flight software "sanity" checks
that are available when operating in three-gyro mode and possible failure
modes. As a result, modifications are being made to the flight software
that will provide more robust checks of gyro performance while in
three-gyro mode. A zero-gyro safemode that is designed to maintain the
health of the spacecraft has also been added, but (obviously) it cannot
carry out a science program.

    In addition to the gyro #4 failure, there have been two other gyro
anomalies since the last Newsletter. Gyro #5 has shown an increase in
operating current, but there has been no change in its performance. Gyro #1
has had one short episode of increased noise. These anomalies are distinct
from those exhibited by gyros #4 and #6 as they were failing. Both
situations are being analyzed, but at this time they are not believed to be
precursors of further failures.

-Rodger Doxsey



 HST Gyros

    Since its launch on April 24, 1990, HST has experienced failures of
two of its six gyros, the mechanisms used to point and stabilize the
telescope (see previous article). Because there are only four gyros left,
and the telescope needs at least three to operate, it is natural to ask
what the gyros are and how they work.

    The principle of the gyros is similar to that of a child's toy
gyroscope: the inertia of a spinning mass provides a restoring force that
counters any displacement of its spin axis. In the HST case, the gyros have
only one degree of freedom-the spin axis is supported by only one gimbal.
In the accompanying diagram, rotation about the input axis causes the
gimbal to precess about the output axis. (You can see why this happens if
you remember that the angular-momentum vector is parallel to the spin
axis.)

    What our engineers call a "gyro" is actually composed of some very
sophisticated electronics, which together with the above hardware form a
feedback system that works in concert with the Fine Guidance Sensors and
Fixed Head Star Trackers to point the telescope. These electronic
"reference" gyros are called Rate Gyro Assemblies. They are made up of a
Rate Sensing Unit and an Electronic Control Unit, both of which can be
replaced in orbit. There are three of these RGAs, each containing two gyros
(x- and y-axes) whose input axes are skewed with respect to the telescope's
axes. This offset means that three-dimensional control of the spacecraft
can be maintained with any configuration of three or four gyros. That is,
if the six gyros were aligned exactly with the telescope axes, and two on
the same axis malfunctioned, then only the two remaining axes could be
controlled.

    The gyros are located on the equipment shelf near the back of the
telescope, in order to isolate them from vibrations caused by the reaction
wheels and the high-gain antenna. The reaction wheels are four flywheels,
each with mass of about 45 kg, which control and modify spacecraft attitude
based on RGA information processed and interpreted by the Control Law
Software. The reaction wheels are also mounted with angled spin axes to
provide redundancy.

    There is a separate set of backup gyros called the Retrieval Mode
Gyro Assembly, which are used if fewer than three main gyros are available.
There is no direct ground communication with the RMGA-it is enabled by the
Power Distribution Unit. The telescope is then safely kept in "hardware sun
point" mode with the aperture door closed, but of course it cannot then be
used for scientific observations.

-Pete Reppert



 Optical Telescope Assembly Collimation

    The present secondary-mirror position in the telescope was first
set on day 323 in 1990. The secondary was positioned in decenter and tilt
to remove coma in the WF/PC and FOC (approximately), and to remove
astigmatism as measured by the Optics Control System (wavefront sensors).
Since then, with very few exceptions, the secondary has been maintained at
this position with occasional adjustments to preserve the focus setting, as
the graphite-epoxy metering truss shrinks due to water desorption.

    One attempt, on day 66 in 1991, was made to arrive at a better
setting, and it led to better FGS performance. Unfortunately the camera
images were too comatic, so the secondary was reset to the day 323/90
position. The Science Working Group then asked Dan Schroeder to convene an
optical alignment panel to review all the data, and propose additional
observations as necessary to improve the collimation.

    These observations consisted of a series of images at different
coma settings to establish a more accurate zero-coma setting taken both in
the FOC and the WF/PC. The secondary was then placed temporarily at this
position, and a series of four pairs of images were taken with the FOC to
fix the astigmatism. Finally a focus sweep was run in both cameras to see
if the focus setting is optimal.

    The conclusion of all of these experiments was that the day 323
position, modified only by the present desorption corrections, is optimal
in astigmatism and focus but could be improved in coma. However, the effect
of this improvement on the camera images is slight and was not felt to be
of any scientific value.

    Similarly the throughput of the spectrographs was not expected to
be affected. A "five points of light" test was then run at the zero-coma
position to assess the three FGSs; neither the astrometry nor the guiding
was improved significantly. The residual amount of coma involved (about
1/15 wave) is not felt to be a problem for the next-generation instruments,
although the telescope may be recollimated just before the servicing
mission.

    In conclusion, the telescope collimation is now better understood,
and although marginal improvements are possible it was felt that they did
not compensate for the increased spacecraft time that would be needed to
repeat some calibrations. The secondary mirror will therefore be left in
the day 323/90 position, and PSF observations taken since then will be
applicable to almost all data taken since then (although there may be some
differences in focus setting). The OTA has almost stopped shrinking, and
the secondary will now be maintained within 5 microns of its present
position.

-Chris Burrows


 Scientific Instruments

 Wide Field and Planetary Camera

1. WF/PC safing and thermal-anomaly history. The calibration of the Wide
Field and Planetary Camera (WF/PC) is adversely affected by transient
changes in its thermal state. Such changes have resulted from HST safing
events, internal power cutoffs, and decontamination procedures. Nearly all
of the calibration changes are the consequence of the removal of a part of
the UV-flood charging while the CCD detectors are at an elevated
temperature.

    In-flight experience has shown that raising the CCD cold junctions
above approximately -40 C results in the formation of a visible light
contamination (known as "the measles"), which seriously degrades image
quality. Since the equilibrium temperature of the cold junctions is about
-35 C when the thermo-electric coolers are powered off, the WF/PC must be
"decontaminated" following every such power-off episode. This is
accomplished by warming the CCDs so that the cold junctions reach between
-2 and +10 C for more than 1 hour. This causes some loss of UV-flood and,
therefore, a calibration change in both photometric zero point and
flat-field structure, since the UV flood increases the quantum efficiencies
of the CCD detectors in a location-dependent manner.

    Since the first on-orbit UV-flood of the WF/PC on December 27-28,
1990, detector warm-up episodes have occurred as described in the table.



2. Status of WF/PC flat-field calibration program. The current calibration
being performed in the pipeline processing of WF/PC data is limited by the
availability of calibration data and does not represent the full
capabilities of the instrument. While this situation was expected during
Science Verification (SV), it has continued into Cycle 1 because at present
WF/PC is performing the GO/GTO Cycle 1 program while still taking the SV
observations needed to populate the calibration data base.

    As a result observers should plan on recalibrating their WF/PC data
after the ongoing flat-field observations are completed, the WF/PC IDT has
created flat-field calibration files, and these files have been placed in
the STScI data bases. These tasks should be completed by late December
1991.

    The WF/PC flat-field behavior changes greatly between the on-orbit
UV-flooded state and the unflooded state in which it was launched. The
first (and so far only) on-orbit UV flood was performed in late December
1990, as described above. In early 1991 a limited number of Earth-flat
observations were made in a mode that prohibited any other observations
during the unocculted part of the orbit. It was determined that a large
number of exposures of the Earth during occultation would be required to
obtain a sufficient number of properly exposed images at several roll
angles (needed to remove the streaks in the images produced by spacecraft
motion during the exposure). Additional Earth flats were placed on hold
until "interleaved" observations could be supported. Interleaving permits
Earth flats to be taken during occultation and other science exposures to
be taken during the unocculted part of the orbit, thus greatly increasing
the observing efficiency.

    At the start of April 1991, the following preliminary Planetary
Camera flats were delivered by the WF/PC IDT to the STScI and placed in the
calibration data base: F230W, F284W, F336W, F439W, F555W, F702W, F785LP,
and F889N.

    Since that time the WF/PC has been warmed twice to remove
contaminants (following spacecraft and instrument safings). This changed
all of the flat fields slightly (~1-2%) each time, but caused larger
changes on CCDs P7 and P8 (as expected from ground-based testing). Except
for the PC filters listed above, all data processed to date in the pipeline
have been flattened with pre-launch flat fields or dummy flats-both of
which yield rather limited results.

    Starting in July 1991, interleaved observations have been supported
and the WF/PC SV program has begun to collect large numbers of exposures of
the Earth for the generation of flat-field reference files. As explained at
the November 1990 Users' Workshop and in the June 1991 STScI Newsletter,
the initial flat- field calibration of the WF/PC will concentrate on a
limited set of camera/filter combinations (selected mainly on the basis of
frequency of use). However, Earth flat-field exposures will be obtained
during Cycle 1 for all camera/filter combinations scheduled for use in
Cycle1.

    The following is a summary of the expected flat-field calibrations
that should result from these efforts. It is expected that these
observations will run until approximately November 1991. The main
difficulty at present is that the shortest possible exposure (0.11 s) of
the sunlit Earth often saturates frames taken through broadband filters.



WFC Flat-Field Calibrations:

F194W, F230W, F284W, F336W, F375N, F439W, F487N, F502N, F547M, F555W,
F622W, F631N, F656N, F658N, F664N, F673N, F702W, F785LP, F889N



PC Flat-Field Calibrations:

F230W, F284W, F336W, F375N, F439W, F469N, F487N, F502N, F517N, F547M,
F555W, F622W, F656N, F658N, F664N, F673N, F702W, F718M, F785LP, F889N



    To determine which type of flat field has been used on an existing
processed dataset, one should examine the keyword FLATFILE in the science
image header. This gives the flat-field reference file name in the format:
wref$<name>w.r6h. If the <name> field starts with one of the character
strings "9", "a1", "a2l", or "a2m" then the reference file is a dummy flat
field of all ones (i.e., it has no effect on the data). If <name> starts
with "a2q" or "a3" the reference file is a ground-based pre-launch
calibration file. If <name> starts with "b"

the reference file is an on-orbit flat

field from the preliminary IDT calibration delivery.

    A more detailed description of the status of the flat-field
calibration program is available on STEIS in the directory
instrument_news/wfpc_flat_fields_status.



3. Contamination at Visible Wavelengths. As previously reported, the WF/PC
contains contaminants that make observations shortward of 2000 

impractical. These contaminants build up with time and decrease the
throughput in the F230W filter at a rate of about 50% per month. Present
evidence indicates that the sensitivity at F336W is decreasing by about 5%
per month, and there are preliminary indications of a slight decrease at
F439W. At longer wavelengths the throughput is stable to a few percent over
several months. A spectrophotometric standard star is being observed in
F230W, F284W, F336W, F439W, F555W, and F785LP on WF2 and PC6 once per month
to monitor the WF/PC's quantum efficiency. A more complete photometric
calibration by the IDT is scheduled for December 1991.

    There are also indications that longer-wavelength observations may
be affected by the contamination. At F555W we observe small dark regions
that are growing in size with time. It may not be possible to correct these
regions with flat-field observations. The amount of scattered light seen on
the CCDs beyond the pyramid edges is also observed to increase with time
and to decrease abruptly after a decontamination. The contamination is
believed to be on the cold field-flattening windows in front of the CCD
detectors.



4. Interrupted Exposures. WF/PC exposures longer than 300 s may be
interrupted if the FGSs lose lock on guide stars. In such cases the WF/PC
shutter will be closed until lock is established again. The missing
exposure time is not currently reflected in the keyword EXPTIME in the
science image (.d0h) file headers. This has been fixed in the next release
of the ground-system software, scheduled for implementation in late October
1991.

    If loss of lock was reported during an observation, the Standard
Header Packet (.shh) file provided with the data should be examined.The
keywords WFOCTM01, 02, 03, ..., 15 contain the shutter movement history. If
WFOCTM02 is nonzero, then the exposure was probably interrupted (the
trailer file should also be read). The actual exposure time may be
determined by subtracting the lost time as recorded by the WFOCTMxx
keywords (which is in units of 0.125 seconds mod 216) from the commanded
exposure time.

-John MacKenty



 Faint Object Camera

    The importance of UV imaging as a unique capability of HST is now
clear, and so it is reassuring that the UV response of the FOC has been
measured (and monitored) since launch and shown to be close to the expected
values as given in the FOC Instrument Handbook.

    Near the end of November 1991, the calibration pipeline will be
changed to improve the calibration of FOC science data. For imaging modes,
the flat-field correction had been applied before the geometric correction.
After the change the order will be reversed; the geometric correction will
be applied before the flat-field correction.

    Previously the data products of the pipeline for imaging mode had
been: (a) a raw image, (b) a photometrically corrected image, and (c) a
photometrically and geometrically corrected image. The data products will
be changed so that now there will be: (a) a raw image, (b) a geometrically
corrected image, and (c) a geometrically and photometrically corrected
image. The STScI Instrument Science Report FOC-051 describes the processing
in more detail and the motivations for the changes. This document can be
obtained from Nancy Fulton at STScI (410-338-4955, userid FULTON).

-Perry Greenfield & William Sparks



 Faint Object Spectrograph

    The Faint Object Spectrograph (FOS) is working well, with a
combination of Science Verification (SV), General Observer (GO), and
Guaranteed Time Observer (GTO) observations being scheduled and executed
regularly. The programs that are part of SV are designed to verify and
calibrate the basic modes of the FOS, allowing observers to acquire and
analyze their scientific data properly. These programs have included the
measurement of instrumental characteristics such as dark counts, scattered
light, flat-field behavior, aperture positions, wavelength scales, and
photometric and polarimetric performance and stability.     In
addition, the operational characteristics of the instrument are being
determined, including background rates at various galactic and ecliptic
latitudes, and wavelength offsets between the internal calibration lamps
and external sources. Target-acquisition methods and techniques are being
established and are discussed further below.

    SV is progressing well, with approximately 90% of the necessary
data in hand and undergoing analysis by FOS IDT members. Photometric
stability appears to be good, including in the far ultraviolet. Time-
resolved mode and spectropolarimetry mode have both been used successfully
for GO proposals.

    In addition to the SV program that was defined before launch, the
discovery of geomagnetically induced motion (GIM) of the spectrum on the
detectors (especially on the red side) created the need for a program to
characterize this phenomenon. (See the article on the FOS in the March 1991
STScI Newsletter for more information.) Testing has shown this effect to be
highly repeatable and readily modelled. The operational fix in the near
term is to break long integrations into exposures of two minutes each.
These separate integrations can then be shifted by the appropriate amount
during data reduction, and then summed to yield the final corrected
spectrum.

    A new version of CALFOS that supports these operations has been
submitted and will be the standard for pipeline processing by the time this
Newsletter appears. In the long term, an operational "real-time" fix will
simply dither the deflection coils by the appropriate amount to remove the
effects of GIM during an observation. This fix will be included in a future
operations software update, currently scheduled for February 1992.

    Various target-acquisition modes have now been thoroughly tested. A
bug in the way the scheduling program handled proper motions was revealed
the hard way when a target acquisition failed for a long observation.
Improved aperture positions and aperture-to-FGS alignments have reduced the
need for "big" peak-ups (i.e., a 6 by 2 pattern with the 4".3 acquisition
aperture) and targets are usually found within the initial 4".3 aperture
position. BINARY SEARCH works well, although targets can still be left off
center by as much as three pixels (0".24) with this acquisition mode,
depending on the brightness of the target star and the effects of GIM and
jitter during the acquisition. The user community should be aware of the
importance of setting proper BRIGHT and FAINT limits in order to have
successful results with BINARY SEARCH acquisitions.

    WF/PC-assisted target acquisition has been tested on the blue side
of FOS and has worked well, placing a target star within 0".25 of the
desired location in the 4".3 acquisition aperture. The instrument has gone
into safe mode twice due to relatively minor technical problems that have
now been resolved. In mid-July, a test star slightly exceeded the
"overlight" limit, which had been set conservatively low, and safed the
instrument. The value of this limit has been increased by 50% to prevent
unnecessary future occurrences. At the end of July, a BINARY SEARCH target
acquisition failed due to a combination of circumstances (GIM, spacecraft
jitter, and the star's initial position for the search). This should not
normally safe the instrument, but the particular circumstances caused the
FOS to attempt execution of a vestigial program branch in the operations
software, and the instrument safed. This problem has also been addressed
and should not recur. It was during this "safed" period in July that the
instrument was officially handed over from the IDT to STScI.

The special HST spectroscopy issue of Ap. J. Letters in August 1991
contains four articles by the FOS team, reporting observations of the
quasars 3C 273 and UM 675, and the active galaxies NGC 1068 and NGC 1566.
These papers contain considerable details on instrument performance that
should be of interest to potential FOS users. In addition to some early ERO
and SAT observations, the remaining FOS GTO observations are in full swing.
While some spectroscopy on QSOs and stellar objects has been done, much of
the early GTO data have been in the form of "early-acquisition" images with
either the PC or WFC. These images were intended mainly to set up future
FOS spectroscopy, but many of the images are interesting in their own
right. Considerable effort has gone into the proper reduction and analysis
of these data. The issue of "accurate astrometry" has become a major
concern as we try to prepare for the main body of our FOS spectroscopy
program. Getting a 0".3 aperture onto a given knot in an active galaxy
nucleus through a blind offset, for instance, is a non-trivial matter with
HST.GOs should be wary of such problems and be prepared with their own CCD
images or other astrometric data for complicated acquisitions.

-William P. Blair



 Goddard High Resolution Spectrograph

    In July 1991 the Goddard High Resolution Spectrograph (GHRS)
suffered a major component failure, which will have a significant impact on
users of the spectrograph. The failure occurred in the low-voltage power
supply on side 1 of the GHRS. It will have the following implications:

1. Side 1 of the GHRS, which carries out observations with G140L, Ech-A,
and G140M (the prime far-UV capability), has become unavailable for
scientific use, and is expected to remain so unless an on-orbit repair of
side 1 can be performed during the first HST servicing mission.

2. Side 2 data are currently routed through a science data formatter (SDF)
powered from the side 1 electronics; thus usage of side 2 has become
unreliable, in addition to side 1 having failed altogether.

3. A solution that would restore reliable usage of side 2 would require
switching communications for all of the scientific instruments, including
GHRS, to the spare side (side B) of the HST electronics. (Side B is a
redundant spare that duplicates the currently used Side A of the spacecraft
electronics.) Sides 1 and 2 of the GHRS SDF interfaces are hard-wired to
sides A and B of the spacecraft, respectively, requiring routing of
communications from side 2 through side 1 when side A of the spacecraft is
in use.

    It should be noted that this new problem is not related to the
earlier intermittent failure of the side 1 carousel control (reported in
the June 1991 STScI Newsletter), which can be worked around effectively.
The history and technical details of these failures, and current testing
aimed at understanding the precise constraints on continued use of the
GHRS, are discussed below.

    During a routine science observation on July 24, 1991, the GHRS
experienced its first loss of SDF interface control. The transfer of
commands to the GHRS and return of data from the GHRS to the HST computer
is routed through the SDF. Any transfer of information requires
"handshakes" between the instrument and spacecraft sides; if such an
acknowledgement is not received, then the SDF interface shuts down until a
reset command is encountered. The glitch on July 24 was associated with a
fluctuating current on the side 1 emission-line comparison lamp. It was
suspected at that time that a problem with the lamp could have induced a
voltage glitch in side 1, leading to failure of the SDF interface
handshake. After an automatic reset, the interface worked without problems
for the next ten days.

    On August 5, 1991, however, the GHRS again had a loss of the SDF
interface. This time, the SDF interface did not return to normal operations
after a reset, suggesting a continuing problem. Normal full telemetry is
returned from either side of the GHRS only when the low voltage is fully up
for science operations, which is not the case for side 1 when it is only
serving as a conduit for side 2 data. The August 5 observation used side 2.
After the SDF problem continued for several hours, a decision was made to
enable low voltage on side 1 to capture a telemetry profile of voltage and
current distribution. This was done and resulted in (a) a good set of
telemetry that has allowed a secure diagnosis of the failure point, and (b)
confirmation that many voltages were detected as out of range by the
instrument monitoring system, resulting in a safing of the GHRS.

    Engineering analysis of the August 5 telemetry showed that the
voltage distribution throughout the instrument could be explained in a
unique way via failure of a specific solder joint in the low-voltage
power-distribution system. Analysis (and ground-testing experience) also
showed that an unbalanced power distribution, as sometimes exists on side 1
now, can lead to hard failures of other components. Therefore full
low-voltage operation of side 1 has not been attempted since August 5, and
is not likely to be attempted again in the future.

    In the mode where side 1 operates only as a data conduit to side A
of the spacecraft, the standby power- distribution system supplies proper
voltages to all required elements. This includes instrument heaters, which
have now been off-loaded to side 2. By this off-loading it was hoped that
smaller currents across a (possibly) high-resistance solder joint failure
might allow maintenance of sufficient voltage at the SDF for its continued
operation. This has not turned out to be the case with any degree of
reliability. Continued operations have shown only a 50% rate of success in
recovering side 2 data without the SDF interface being lost.

    An engineering test has been designed that will allow
near-continuous testing of the SDF interface once every 5 minutes. The
purpose of this test (scheduled for late October) is to determine if
certain operational conditions (e.g., electronics temperature) may be
correlated reliably with the times when the SDF interface can be kept up.
If such a robust correlation can be found, it might be exploited to restore
reliable use of GHRS side 2.

    If a reliable correlation that will allow continued, predictable
use of GHRS side 2 cannot be found, then only three options remain:

1. Use the GHRS side 2 only for projects where very high priority for
scientific return has been established that favorably balances the
decreased probability of success. This would be the desired solution for
conducting further GHRS science in the near future only if more positive
options are not deemed possible. Under this scenario, time allocations
would need to be balanced against the realized probabilities of any
observation succeeding.

2. Switch all of the scientific instruments to side B of the spacecraft.
The decision to do this will have to be considered in light of the risk
this would entail for other components of the HST system.

3. Repair GHRS during the 1993 servicing mission. At this early stage of
planning it appears that an astronaut could carry out a fix by: (a) opening
the appropriate access hatch on the HST, (b) removing some 70 (!) screws
holding a GHRS access panel in place, (c) either installing a spare
low-voltage power supply, or simply wiring a bypass to the failed
electronics line, and (d) closing up the original (or a newly designed)
access panel and closing the hatch. It is believed that such a repair would
restore the full capabilities of both sides of the GHRS.

-Ron Gilliland



 A Comparison of GHRS and FOS sensitivities

    To facilitate changes that may be necessary for Cycle 1 proposals
that currently use the GHRS side 1 low-resolution grating, G140L, but will
be changed to the FOS, the following table provides a direct comparison of
GHRS and FOS photometric sensitivities for point sources well centered in
the aperture.         The sensitivities in cols. 2, 3, and 4 are
expressed in counts s-1 diode-1 (erg s-1 cm-2 
-1)-1.

    For the GHRS with the G140L grating, column 2 gives the sensitivity
with the 2".0 square Large Science Aperture (LSA). For observations with
the Small Science Aperture (SSA), the GHRS sensitivity should be multiplied
by the ratio in the final column. With G140L the diode spacing is 0.572 
,
and the resolutions of the SSA and LSA are 1.1 and 2.0 diodes,
respectively.

    The FOS sensitivities are also on a per-diode basis, for the
1".0-diameter circular aperture. The diode spacings are 1.0 
 and 1.46 

for the G130H and G190H gratings, respectively, and the resolution of the
1".0 aperture is 1.4 diodes.

    A sample calculation: at 1200 
, one would infer from the above
that GHRS G140L was a factor of 5 faster than FOS on a per-diode basis. On
a per-Angstrom basis, the ratio at 1200 
 is 8.9. But the FOS does have
greater wavelength coverage, and starts gaining ground rapidly with
increasing wavelength in terms of relative sensitivity.

    Throughputs for the various FOS apertures at 1500 
, as determined
from direct comparison on a point source, are as follows:



        Aperture          Throughput

          (arcsec)



         4.3 x 1.4            49%

              1.0            27%

              0.5            20%

              0.3            13%

       0.25 x 2.0            20%



    The GHRS to FOS ratios derived from the table should typically be
within 20% of truth, allowing for good exposure-time planning.

-Ron Gilliland & George Hartig



 High Speed Photometer

    As a consequence of the vastly improved performance of the HST
pointing-control system and a long series of High Speed Photometer (HSP)
tests, it is now possible to center a star in most of the HSP science
apertures to within a few hundredths of an arcsecond. This has enabled much
of the HSP SV program to be carried out. The instrument continues to
perform well.

    One of the more interesting tests performed recently was a 5.5-hour
continuous observation of the ninth- magnitude rapidly oscillating Ap star
HD 60435, taken through the F240W filter on image-dissector tube 3 with a
sample time of 82 msec. In addition to the intrinsic variability of the
star, the data show three anomalous features, none of which is understood
fully as yet: (a) a sinusoidal modulation with the orbital period of HST
and a semi-amplitude of 0.9%; (b) a monotonic increase in the signal of
about 1% over the interval of observation; and (c) several data dropouts
lasting a few seconds each.

    Many parameters vary with the HST's orbital period, and it is not
clear which is responsible for the observed modulation of the star's
brightness. Since the star itself should have become slightly fainter
during the observation period (its brightness is known to be modulated on
the rotational period of the star), it should not have caused the slow
increase that was seen. The data dropouts are likely a result of spacecraft
jitter, since they occur most strongly at the night-to-day transitions.
Apart from these effects, the noise in the data is due nearly entirely to
photon statistics. A Fourier transform of the data shows a 1.9 mmag peak,
well above the noise of about 0.5 mmag, with a period of 11.7 minutes. This
is known to be one of the oscillatory modes of the star.

    The first GTO program using the HSP has recently been completed,
namely the observation of  the luminous blue variable P Cygni. This was
done as part of a P Cyg campaign that occurred during September 1991.
Another GTO program, the occultation of a star by Saturn's rings, will soon
be performed, as well as an SV observation of the Crab pulsar (see article
on p. 2). The SV tests remaining to be completed are the calibration of the
POL detector and the determination of the best entrance-aperture position
for the PRISM mode on image-dissector tubes 2, 3, and 4.

-Robert C. Bless



 Fine Guidance Sensors

    The SV phase of operations for the Fine Guidance Sensors (FGS) has
begun. SV and some parts of the Orbital Verification (OV) activities have
been delayed owing to the continuing effort to find the optimum placement
for the HST secondary mirror. Even so, both the Early Release Observations
(ERO) and the Science Assessment Tests (SAT) have been completed and the
data reduced. The former are in press (AJ, 1992), and the latter have
already appeared (ApJ, 377, L17, 1991).

    These activities have been carried out mainly by the University of
Texas Space Telescope Astrometry Team, but STScI's FGS Instrument Team has
been busy too. The Instrument Scientist Cycle 1 calibration plan was
finalized, the Version 1.0 astrometric data-processing pipeline has been
completed (see below for a brief description), a new FGS Instrument
Handbook-which will include examples of completed proposal forms-is being
written, the Cycle 2 Instrument Scientist calibration program is in the
design phase (the details depend on the SV and Cycle 1 observations), and
we are starting to tackle the problem of jitter elimination using the 32
kilobit telemetry acquired on the Guide Stars. We also are supporting the
re- reduction of the Guide Star Catalog (Version 1.2), using new methods
developed at STScI. In addition, we are continuing to transform one of the
engineering-only FGS modes into a GO mode because we expect it to minimize
the deleterious effects of the spacecraft jitter. Finally, we are
anticipating the data from the Fine Lock test that was designed to
demonstrate that most of the loss of sensitivity induced by the primary
mirror figure can be regained by better control of the Fine Guidance
Electronics inside the FGS.

    The scientific FORTRAN code for the astrometry pipeline has been
completed. It contains three major components: (a) code to treat
astrometric data (which arrive on the ground via the Astrometry and
Engineering Data Processing telemetry channel rather than the Routine
Science Data Processing telemetry channel as is the case for the other five
instruments), (b) code to process TRANS-mode observational data, and (c)
code to process POS-mode observational data. The latter includes all the
basic astrometric corrections and therefore has pieces that will be used by
TRANS-mode observers too. A scientific description of the code will appear
in the forthcoming version of the FGS Instrument Handbook. At the moment
the code is being reconfigured to reside in the IRAF format. After that
there is one more stage of software configuration to go through before it
becomes available to GOs in STSDAS.

-Mario G. Lattanzi and Larry Taff



 NEWS FOR HST OBSERVERS AND PROPOSERS

 Frequently Asked Questions about HST Data and Data Analysis

    We are moving into an era where increasing numbers of General
Observers (GOs) are receiving HST data and/or coming to STScI to analyze
their data. Here are some answers to a few of the most common questions
that are asked by GOs, Archival Researchers, and other persons interested
in HST data.

    The text below is taken from a much longer article, which has been
posted to observer/frequently_asked_questions on STEIS. The full version
contains considerably more detailed information about the questions raised
below and numerous other topics, and all observers should consult it.

    1. What happens to the science data once an observation is made?
The science data are separated from the engineering data, and passed
through Routine Science Data Processing (the RSDP "pipeline"), where they
are calibrated. They are then written into the archive and a copy is sent
to the Principal Investigator (PI).

    2. What am I going to receive? The PI will receive magnetic tapes
containing the data in FITS format, a listing of the contents of the tape,
a summary of the observations (if it exists) which complements the
information in the files, and hard copies of the data (either glossy
photographs in

the case of imaging data or plots for

the spectra).

    3. When will I receive the tape? The data are received by the Data
Archive Operations Group (DAOG) approximately one day after they are taken.
About 8 hours later the tape is written and sent to the PI (or to the
person indicated in the Data Distribution Form), or kept at the DAOG office
until picked up at STScI.

    4. What is on the tape? The tape will have the raw (uncalibrated)
data, plus all the files generated by the calibration pipeline. There are
routines in STSDAS for reading the tape.

    5. Are the data calibrated? Maybe. The data that the PI will
receive have been calibrated with the best currently available calibration
reference files. In some cases, however, complete sets of reference files
for all the possible combinations of setups and filters are not yet
available. Observers should contact us (410-338- 1082, userid ANALYSIS) for
further information. See also the discussion of WF/PC flat fields earlier
in this issue (p. 9).

    6. Are the calibration reference files included on the tape I will
receive? No, the reference files are not routinely included in the data
tape you will be receiving. However, all the reference files are public and

can be requested to recalibrate your

observations.

    7. Are the images deconvolved? No, the data are not deconvolved
routinely. Software for deconvolution of HST images and spectra is
available in STSDAS.

    8. Are there PSFs that will help me deconvolve my images? STScI
maintains a library of point-spread functions (PSFs), both observed and
calculated using the Telescope Image Modelling (TIM) software developed in
house. (TIM can be downloaded from STEIS using anonymous ftp.)

    9. Is it useful to visit STScI for the data analysis? Yes! We
strongly recommend that you visit us at least the first time you receive
HST data. We have the facilities to analyze your data. Also, all your
instrument-specific queries can be addressed immediately by in-house
experts. HST data are complex and unless you have had some experience with
them, we would strongly advise you to visit us.

    10. How can I arrange a visit to STScI? Contact the User Support
Branch (USB) (800-544-8425 in the U.S. or 410-338-4413, userid USB). We
request that you give us two weeks advance notice that you are coming.

    11. Do I have to know IRAF/STSDAS before coming? No. Technical
support staff can help you learn the data analysis system.

    12. Where do I call if I have an STSDAS question? Call
410-338-5100, or send e-mail to userid HOTSEAT.

    13. How can I obtain copies of non-proprietary data from the HST
archive? See the article below (p. 18).

    14. Is there documentation available? Yes, a large number of
documents are available covering all aspects of STScI, HST, the scientific
instruments, and data reduction and analysis. Please download the STEIS
version of this article for a

complete listing, or contact USB to

request copies.

    15. What if I am a European (ESA) HST user? European HST users with
their own data or with a desire to make extensive use of the HST archive
are strongly encouraged to visit the ST-ECF at ESO in Garching, Munich.
Staff at the ST-ECF are available to help users begin to reduce and analyze
their data. Limited facilities mean that, after an exploratory phase at the
ST-ECF, most of the analysis will take place at the user's home institute
but considerable assistance can be given with instrument-specific questions
and the full facilities of the archive are available for any recalibration
work. In order to arrange a visit to Garching, contact: (e-mail on
span/decnet) ESO::STDESK; (phone) [phone] and ask for STDESK;
(fax) [phone], attention STDESK; or contact Bob Fosbury at
ST-ECF.

-Daniel Golombek



 User's Guide to the STScI

We have just finished revising a short document called "A User's Guide to
the STScI," and have mailed copies to the General and Guaranteed Time
Observers. This guide is intended for users who wish to visit the STScI,
and provides current information on travel directions, on-site "logistics,"
technical and scientific support, remote and on-site information resources,
hotels and restaurants. Two short forms are also included: one for user
feedback and one for requesting institute documentation. The guide was
prepared by Sheryl Falgout with help and input from many Institute staff.
If you are considering a visit to the STScI, we strongly recommend that you
contact the User Support Branch and request a copy of the user's guide.

-Bruce Gillespie



 Observation Problem           Reports

    STScI is most interested in receiving feedback from observers on
the degree of success of their HST observing programs. Earlier this year,
we developed the HST Observation Problem Report (HOPR, known locally as a
"hopper" and available from the User Support Branch), which is a form to be
filled out by GOs and GTOs when there are apparent problems with HST
scientific and calibration data. This form was devised both to provide us
with invaluable information on how well HST is performing in a scientific
sense and to offer Principal Investigators (PIs) a way to request
rescheduling of failed observations.

    If you determine that data from your GO, GTO, or calibration
programs are defective in any way, please let us know by submitting a
Problem Report, even if you are not requesting that the observations be
repeated. All types of problems should be reported, ranging from the
obvious (e.g., no data due to loss of guide-star lock) to the more subtle
(e.g., signal-to-noise different than expected).

    Each problem report is examined by our Telescope Time Review Board
and forwarded to expert staff for analysis. When a retake of a failed
observation is specifically requested by the PI, the Review Board will make
a recommendation to the STScI Director. The decision to repeat the
observation will be based on the investigation of the problem, within the
context of how repeating the observation would contribute to the broader
HST science program.

    It is important to note that STScI will not initiate the retaking
of failed observations without an explicit request submitted on an HST
Observation Problem Report form and signed by the PI. The only exception to
this policy is in the relatively rare case of observations lost during
telescope and instrument "safing" events; observations lost due to safing
will be rescheduled automatically where possible. Otherwise, we will only
reschedule failed or defective observations when an explicit request has
been received from a PI, following review and Director's approval.

    In all cases, the analysis and disposition of Observation Problem
Reports are communicated to the PI, generally within a month of receipt. If
you have questions on how to use the form or need additional copies, please
contact the undersigned (410-338-4723, userid GILLESPIE).

-Bruce Gillespie



 WF/PC-Assisted Early           Acquisitions

    Some common problems related to WF/PC early-acquisition images
(i.e., images obtained in order to determine the telescope pointing for
subsequent observations with other instruments) may not have been taken
into proper account by some observers. These problems are discussed in
detail in the Target Acquisition Handbooks and in recent Phase II
materials, but they are highlighted again here. This article also describes
the corrective actions that STScI is taking.



1. WF/PC geometric distortion. This problem affects the ability to perform
accurate relative astrometry within the WF/PC field of view. The distortion
is introduced by the WF/PC optics, and the departure from linearity can be
as high as 3 pixels over one CCD chip. We have obtained data to calibrate
this distortion, and expect to have a preliminary calibration of W2 and P6
by year's end, with the other chips following soon after. The calibration
will be accompanied by a STSDAS tool that will enable observers to go from
the x,y positions directly to right ascension and declination without
having to worry about the geometric correction. Note that the WF/PC team
has recently circulated a correction algorithm for the WFC based on a
ray-tracing program.



2. WF/PC mosaicking. The data gathered for the calibration of the geometric
distortion will also provide a very good estimate of the relative distance
and rotation of the various CCDs with respect to each other. Until then,
mosaicking of the CCDs will prove difficult, and, as in the past, we do not
advise observers to attempt astrometry across CCDs. Moreover, each chip is
treated independently by most of the software, and this will cause obvious
problems if one wants to construct an astrometrically "sensible" mosaic.
And in any case, the long-term stability of the CCD positions relative to
one another cannot be assessed at the moment, so the advice to restrict
oneself to a single chip for astrometry will remain valid.

-Roberto Gilmozzi



 Users' committee meets

    The first meeting of the Space Telescope Users' Committee (STUC) in
Cycle 1 was held on September 26-27, 1991. The three major issues which the
committee addressed were the recent changes to the WF/PC II capabilities,
the problems with the GHRS, and the on-going operations of HST, including
the experiences and recommendations of the first General Observers (GOs).
Joe Rothenberg, the HST Project Manager at GSFC, described the difficulties
that the WF/PC II development had encountered at JPL. Faced with risks to
both the on-orbit performance and development schedules, the WF/PC II
science team agreed to a reduction in the number of relay camera/CCDs (from
4 to 3 in the Wide Field and from 4 to 1 in the Planetary Cameras) and the
addition of on-orbit actuators on two of the three Wide Field relays
(thereby ensuring good on-orbit optical performance over most of the
original field of view). The STUC expressed concern about the WF/PC II
development and recommended the highest management attention by NASA.

    The STUC was briefed by Preston Burch, a NASA manager, on the
technical status of the GHRS and the possible workarounds to the failure of
the side 1 low-voltage power supply. The STUC recommended to NASA that
procedures for switching data channels (which could restore the operation
of GHRS side 2) be validated and that the method for restoring GHRS side 2
operations be established prior to the Time Allocation Committee
deliberations. It also endorsed further study of recovering side 1
operations during the next servicing mission. While the experiences of the
early GOs have generally been positive, several early GOs have taken the
opportunity to communicate some criticisms and recommendations to the STScI
Director and the STUC Chair. These and the corrective actions begun by the
STScI were discussed at length. Progress in the development of parallel
observations and tracking moving targets was also reviewed. The STUC
commended all elements of the HST program for achieving a vastly improved
level of operations. Future GOs are encouraged to communicate any concerns
to the STScI and/or the STUC. The members of the ST Users' Committee are
listed in the table, along with the elected Chair and Vice-Chair.

-Peter Stockman



 STAC Meeting

    Partly in response to the power supply failure in the GHRS, NASA
formally requested advice from the STScI Director concerning the
development and priorities for future servicing missions. To consider these
issues from the broadest scientific perspective, Riccardo Giacconi
reconvened the Space Telescope Advisory Committee (STAC), with Jerry
Ostriker as the Chairman. Meeting on October 9-10, the STAC received
briefings from Joe Rothenberg, the HST Project Manager, on the current
development plans; from Bruce Woodgate and Rodger Thompson, the PIs for the
Space Telescope Imaging Spectrometer (STIS) and Near Infrared Camera (NIC);
and from Ed Weiler, the acting HST Program Manager. The STAC was also
provided extensive written material by the STIS and NIC science teams.
Prior to the meeting, an independent technical review of the STIS, NIC, and
COSTAR programs was conducted by astronomers from the STScI and five other
research institutions. The results of that review were provided to the STAC
and copies may be requested from the Chair of the review team, Chris Blades
(userid BLADES).

    In summary, the STAC recommendations to the STScI (which generally
comprise the STScI response to NASA) place the highest priority on
restoring the capabilities of the GHRS and the timely completion of WF/PC
II and COSTAR for a late-1993 servicing mission. Since the scientific
impacts of all three developments are comparable and complementary, the
STAC did not attempt to prioritize among the three and urged tight
management of their development. In this regard, the STAC commended the
recent reduction of the WF/PC II as necessary to restore most of the
original HST capabilities by the earliest possible date. As for the
relative priorities of the second generation instruments (STIS and NIC),
the STAC considered both instruments to offer compelling scientific
capabilities in the ultraviolet and near-IR compared to current
ground-based facilities and HST instruments. They urged NASA to develop
both instruments in parallel for an on-orbit installation targeted for
1997. The STAC recognized that, with instrument costs around $100 million
each, such a parallel development would not be feasible without further
reductions in their capabilities and cost as well as belt-tightening in
other HST Project elements.

    The members of the STAC for 1991-92 are listed in the table. The
STAC report may be requested through the STScI Director's Office.

-Peter Stockman



 STEIS Usage Increases        Five-Fold

    The Space Telescope Electronic Information Service (STEIS) is an
anonymous ftp (file transfer protocol) account set up over a year ago to
provide current information about HST to the astronomical community.
Observers use STEIS to obtain observing schedules, instrument status
reports, information about (and source code for) calibration and
data-analysis software, and software to help in creating observing
proposals. The ftp address is stsci.edu.

    The figure shows that usage has increased five-fold since the
beginning of the year. The large jump in July resulted from the August
proposal deadline. This chart is based on the number of times the string
"LOGIN" appears in our accounting file; each login does not necessarily
represent a new user. All of the directories have shown an increase in
activity-apparently STEIS users like to browse.

    A questionnaire about STEIS has been posted on STEIS, and we have
received many positive responses, as well as constructive criticisms and
suggestions. Common requests are for immediate notification of newly posted
items, on-line help, and greater ease in finding and downloading files.
Some users have experienced trouble making connections with a particular
brand of ftp, or are not fluent with available commands (the system is
based in UNIX). For example, people often try to download directories,
mistaking them for files. This problem is easily avoided by using the "ls
-F" command to provide a listing that distinguishes directory names from
file names.

    The daily HST status reports written by Joe Ryan of NASA/GSFC have
answered the demand for up-to- date HST news, and are probably the most
popular item on STEIS. Sometime in the future these reports might become
accessible via telnet.

    There are sometimes discrepancies in our HST scheduling
information, which arise when the long-term scheduling is revised in the
short term. We have some possible solutions for this problem, but may never
be able to promise 100% concurrence, since schedules are frequently
modified after the reports come out. The weekly timeline is usually the
most reliable schedule.

    As always, we welcome comments and suggestions. Anyone wishing to
post information relating to HST should contact

the undersigned (410-338-4551, userid REPPERT).

    There will be a poster paper on (and probably a live connection to)
STEIS at the January 1992 AAS meeting in Atlanta.

-Pete Reppert



 How to Obtain Archival Data

    Scientists who wish to obtain non-proprietary archival HST data and
who do not seek funding for their Archival Research may request copies of
the data by filling out the form "Request For Copy of HST Observations,"
which is available from the User Support Branch or can be downloaded from
STEIS (file observer/dsob2.ps).

    A list of the thousands of archived HST images is available on
STEIS, under the filename AEC.CATALOG in the
observer/completed_observations directory.

    A request must specify the "root" or "dataset" name of the desired
data. These names can be obtained either from AEC.CATALOG or by querying
the HST archive catalog using STARCAT, a menu-driven archive-searching
system which works on any standard terminal. A guide to using STARCAT,
which accesses stsci via telnet, is available upon request from the User
Support Branch. STARCAT can display a field specifying the date on which
each observation will become public. (The normal proprietary period is one
year, but some special observations may have shorter periods, including the
Science Assessment and Early Release Observations and many calibration
data.)

    Questions about the data archive may be directed to Mario Livio,
head of the STScI Data Management Facility (410-338-4439, userid MLIVIO).

-Mario Livio & Pete Reppert



 PROPRIETARY STATUS OF HST CALIBRATION DATA

    The following statements are intended to clarify the policies
related to the availability and release of HST calibration data.



1. All HST data, including calibration data, are immediately available to
STScI instrument and calibration scientists for evaluation and analysis.



2. Calibration and engineering data obtained as part of the STScI
Calibration Program will become available to the astronomical community as
soon as the data are placed in the STScI archive.



3. Calibration data obtained as part of the OV/SV Program will become
publicly available 30 days following the observation.



4. Calibration data obtained as part of and charged to approved GTO and GO
programs have the same proprietary period as the associated scientific
data, typically one year, unless released earlier by the PI or released
later following approval by the Institute Director. Note that these data
may also be used, as appropriate, by STScI staff for updating the
calibration pipeline.

-Kirk Borne



 An HST User Survey:  The Proposal Submission System

    In both Phase I and Phase II of the proposal process, HST observers
are required to fill out an electronic proposal template and use software
(such as the Phase I Formatter and RPSS) that checks the proposal for
syntactical and feasibility errors. The proposal is then submitted
electronically and is eventually translated directly into telemetry
commands that are sent to the spacecraft, which is why the proposal file
must be carefully constructed.

    We  will be upgrading the proposal submission software in the
coming year, and we are trying to identify hardware and software
improvements that will best serve the HST user community. If you have ever
submitted an HST observing proposal, Phase I and/or Phase II, the User
Support Branch is interested in your comments and opinions regarding the
current proposal submission system. This includes the Phase I template and
formatter; the Phase II RPSS template, validation program, and spacecraft
resource estimator; and any other resources you have used.

    A questionnaire has been posted on STEIS in the main directory, and
may be downloaded, filled out, and returned to USB. This is your chance to
let us know how we can help with the difficult task of HST proposing and/or
to vent your frustrations with the current system in a constructive way.
Please return the questionnaire by January 17, 1992, by e-mail to userid
USB.

    Also, be sure to look for the STScI display at the AAS meeting in
Atlanta. The USB will be demonstrating software tools that are designed to
assist HST proposers, and we hope to get some immediate feedback from
potential users.

-Max Mutchler



 Publication of HST Research

    The rate of publication of HST results continues to increase. We
wish to remind all authors again that research papers based on HST data
should carry the following footnote:

    "Based on observations with the NASA/ESA Hubble Space Telescope,
obtained at the Space Telescope Science Institute, which is operated by the
Association of Universities for Research in Astronomy, Inc., under NASA
contract NAS5-26555."

    If the research was supported by a grant from STScI, the
publication should also carry the following acknowledgment at the end of
the text:

    "Support for this work was provided by NASA through grant number
____ from the Space Telescope Science Institute, which is operated by the
Association of Universities for Research in Astronomy, Inc., under NASA
contract NAS5-26555."

    For our records, please send one preprint of any research paper
based on HST data to:



    Librarian

    Space Telescope Science Institute

    3700 San Martin Drive

    Baltimore, Maryland 21218



    Finally, please reference the relevant HST observing program
identification number(s) in your papers so that we can cross-index
scientific papers with the original observing proposals.

    If you have questions regarding these instructions, please contact
Bruce Gillespie (410-338-4723, userid GILLESPIE) or Sarah Stevens-Rayburn
(410-338-4961, userid LIBRARY).

-Bruce Gillespie & Sarah Stevens-Rayburn



 PROPOSAL NEWS

 Cycle 2 Peer Review Underway

    The deadline for submission of Cycle 2 HST proposals occurred on
August 16, 1991. A total of 483 proposals was submitted. The proposals have
now been sent out to the peer reviewers, who will meet at STScI in December
to select programs to be recommended to the STScI Director for
implementation.

    Proposers will be notified of the outcome in January and successful
proposers will then be asked to submit their Phase II information. Cycle 2
observations are expected to begin in the summer of 1992.

    The tables present some statistics of the Cycle 2 proposal pool.
Note that large proposals are defined as those requesting more than 100
hours of spacecraft and/or parallel time. The proposal fractions by
scientific instrument total more than 100%, since some proposals have
requested more than one instrument.

-Kirk Borne & Howard E. Bond



 Approved Director's Discretionary Programs

    The following Director's Discretionary programs have been approved
since the last issue of the Newsletter (PI and Proposal Title):

S. Shore, High Resolution Observations of Nova LMC 1991 (unsuccessful due
to GHRS failure)

C. Leitherer, UV Spectropolarimetry of AG Car in its Current Outburst

J. Bahcall, Gravitational Lens Candidate 1208+101: Photometry

-Kirk Borne



 SOFTWARE NEWS

 STSDAS News

    The next major release of the Space Telescope Science Data Analysis
Software (STSDAS Version 1.2) will be available by the end of this year.
This release is being coordinated with the next release of IRAF, Version
2.10. Some 94 new tasks have been added to STSDAS since the release of
V1.1, including both instrument-specific tasks and general-purpose tools.
Several other tasks, including those for pipeline data reduction, have
undergone major revision and/or enhancement.

    Many of the new tasks are intended to facilitate the analysis
required to generate the calibration reference files for each instrument,
while several others provide additional plotting and analysis capabilities.
As with all STSDAS software, these new tasks support the GEIS (or
multi-group) format data structures that are used for HST data files. Some
of the new tasks also use the Data Quality Files to mask bad pixels from a
calculation or operation, where relevant.

    One new package, pipeline, may be of particular interest for users
who wish to recalibrate their data. Since the launch of HST, substantial
revisions and enhancements have been made to the header records of HST data
files. The bulk of these changes are scheduled to be implemented by the
time STSDAS V1.2 is released. The header changes are being done in
preparation for a reprocessing of all HST data taken since launch. This
reprocessing is intended to populate the raw and processed data headers,
and the data bases, with more correct and/or complete information, and to
recalibrate the data with improved calibration reference files. As the
headers have changed, so has the calibration software. However, until all
the data are reprocessed, a few incompatibilities will exist between some
data files as they currently exist in the archive and the most recent
version (CALxxx) of the STSDAS calibration tasks. The most substantial
revisions affect the GHRS and FOS.

    In order to provide continued access to currently archived data for
recalibration, we provide all older versions of the CALxxx tasks in the
pipeline package. A translation task is now available in each of the hrs
and fos packages to make the headers compatible with Version 28 of SOGS.
Users should run the task pipeline.versions to see which version of the
software was used for the initial calibration of their data, and be aware
that choosing the most appropriate version of the software and calibration
reference files may require some consultation with Institute staff.

-Dick Shaw & Bob Hanisch



 AURA NEWS

 AURA Appoints New Vice President

    The Association of Universities for Research in Astronomy (AURA),
Inc. is pleased to announce the appointment of Harry W. Feinstein as Vice
President for Administration, effective September 1, 1991. Mr. Feinstein,
formerly Head of Administration at the Space Telescope Science Institute,
has over 25 years of experience as a professional business manager
(including 6 years with STScI).

    AURA will miss Jay Gallagher, who has served as Vice President
since June 1989. Dr. Gallagher took up a position on the faculty of the
University of Wisconsin on August 1 as Professor of Astronomy. During his
appointment at AURA, he contributed significantly to the development of the
Gemini telescopes project and worked closely with the astronomy community.
Jay will continue his association with AURA as a senior scientific advisor.

-Goetz Oertel & Lorraine Reams



 Board Member to Serve on Presidential Science          Committee

    Congratulations! President Bush has appointed France Crdova, AURA
Director-at-Large from Pennsylvania State University, to the President's
Committee on the National Medal of Science.

-Goetz Oertel & Lorraine Reams



 HUBBLE FELLOWSHIP PROGRAM

 Third Selection Cycle Underway

    The Announcement of Opportunity for the third round of competition
for Hubble Postdoctoral Fellowships was issued at the beginning of
September 1991. The deadline for submitting applications was November 15.
The applications received by that time will be considered by the Review
Panel that meets in late January 1992. Offers to successful candidates will
be made by February 1, 1992. Further information on the Hubble Fellowship
Program can be obtained from Nino Panagia (410-338-4916, userid PANAGIA),
or by e-mail to userid HFELLOWS.



 First Hubble Symposium

    On October 22-23, 1991, the current Hubble Fellows met at STScI to
present and discuss the results of their Hubble Fellowship research
projects. It is expected that the Hubble Symposium will be an annual event.



 INSTITUTE NEWS

 A Digital All-Sky Survey

    As part of the effort to construct the Guide Star Catalog (GSC),
STScI digitized Schmidt plates covering the entire sky. The plate
collection consists of materials from the UK Schmidt in Siding Spring,
Australia, operated by the Royal Observatory Edinburgh until June 1988 and
thereafter by the Anglo Australian Observatory, and from the Oschin
telescope on Palomar Mountain, operated by the California Institute of
Technology. These digitized scans (of order 1012 bytes of data) are stored
on optical disks at STScI. Details of the digitization program, in
particular the survey and scan characteristics, are described in the
Astronomical Journal (1990, AJ, 99, 2019).


    The purpose of this announcement is to ask whether your institution
might be interested in purchasing a copy of the digitized sky. The northern
sky images that will be offered are from the original POSS E (red) plates.
The southern sky will be covered by the SERC J survey. In addition we
expect to add other digitized surveys to this collection as appropriate
arrangements are completed.

    With the cooperation and encouragement of Caltech, the National
Geographic Society, the UK Science and Engineering Research Council, and
NASA, the STScI is currently planning to distribute moderately compressed
images of the digital scans. Extensive tests have shown that essentially no
astrometric or photometric information is lost through the compression and
decompression processes. Relative positional accuracies significantly
better than 1 arcsec-ond, and stellar brightnesses accurate to better than
0.5 mag, are routinely obtained except near plate flaws and edges. The
compression algorithms and tests were reported at the Digital Optical Sky
Surveys Conference in Edinburgh this past June, and the report will also
appear as an STScI preprint.

    The breadth of the community interest will enable STScI to decide
whether to proceed with the survey distribution and to estimate better the
costs of production. Any distribution of digitized surveys by STScI will be
done on a cost-recovery basis.

    The per-copy cost of the all-sky digitized survey will depend on
the number of copies ordered. If demand is about 100 orders, then the
survey cost should be approximately $6,000 per complete set, and if 500
orders are received, the cost per set should be reduced to under $1,500.

    Of the many different mass-storage and distribution media in
widespread use, CD- ROMs (Compact Disk Read Only Memory) appear to be the
most stable and cost-effective technology available. We propose to
distribute the survey as a set of about 100 CDs. Software to read,
decompress, and display sky images on certain workstations, and to obtain
celestial coordinates from the images, will also be distributed free of
charge to survey customers.

    If your institution might be interested in purchasing a CD copy of
the digitized sky survey, please contact Michael Shara at STScI
(410-338-4743, userid SHARA). Be sure to indicate the type of workstation
and operating system you would prefer to use with the digitized scans. Such
preliminary expressions of interest, of course, imply no commitment to
purchase the survey.

-Michael Shara



 Year of First Light Proceedings Available

     The Proceedings for the Year of First Light workshop held at STScI
May 14-16, 1991 are now published and are being sent to Guest Observers,
Guaranteed Time Observers, members of the Instrument Development Teams,
members of the STScI proposal panels, and to university libraries. Copies
will also be available at the 179th meeting of the American Astronomical
Society, held in Atlanta, Georgia. There is a limited number of additional
copies available. If you would like one, please contact Sarah Stevens-
Rayburn, STScI Librarian (userid LIBRARY).



 October Mini-Workshop

    The final STScI mini-workshop of the year, attracting more than 70
participants to Baltimore on October 8-10, 1991, concerned "Nonisotropic
and Variable Outflows from Stars." Theoretical models rely largely on the
idea that stars and their environments can be described under the
assumption of spherical symmetry and time-independence. However, recent
obser- vational data-including HST results-suggest that the outflow
properties are widely dominated by disks, jets, and clumps, which often
display significant time variability. A variety of objects, including
pre-main-sequence stars, early-type stars, and novae were discussed during
the workshop, and the need for more elaborate models and observations was
emphasized.

    The proceedings of this mini-workshop will be published in the
Astronomical Society of the Pacific Conference Series.

-Claus Leitherer



 Workshop on Status of Women in Astronomy

    The Space Telescope Science Institute is planning a workshop on the
Status of Women in Astronomy, to be held at STScI on September 3-4, 1992.
The workshop will be geared toward graduate students, post- docs, junior
and senior astronomers, and administrators. The agenda will include
discussion of the current status of women in the field, the particular
challenges women face, and ways to improve the recruitment and retention of
women in astronomy.

    The organizing committee for the workshop includes (*=local): Neta
Bahcall, Peter Boyce, France Crdova, Laura Danly*, Doug Duncan*, Riccardo
Giac-coni*, Anne Kinney*, Ethan Schreier*, Meg Urry*, and Sidney Wolff. If
you are interested in receiving further details about the workshop,
including registration information, please contact: Barb Jedrzejewski,
Conference Coordinator (410-338-4836, userid ELLER), or one of the local
organizers (userids DUNCAN, GIACCONI, KINNEY, SCHREIER, CMU).

-Meg Urry



 SABBATICAL & LONG-TERM VISITORS AT STScI

In order to promote the exchange of ideas and collaborations in HST-related
science, STScI expects to provide limited funds to support visiting
scientists who wish to spend extended periods of time (three to twelve
months), typically on sabbatical leave from their home institutions or
during the summer, doing research at STScI.

In general, these visitors will have a status similar to STScI employees
and have access to the facilities available to staff members. Established
scientists who might be interested in such a visit during the summer of
1992 or during the academic year commencing in September 1992 should send a
letter specifying the suggested period for the visit and other relevant
details to the Visiting Scientist Program, c/o Tim Heckman (410-338-4442,
userid HECKMAN), at STScI. It will be helpful if candidates include a
recent curriculum vitae and a short description of their research plans.

-Tim Heckman



 Recent Staff Changes

    Mario Livio has joined STScI as Astronomer and Chief of the Data
Systems Operations Branch. Mario comes to us from the Technion Institute of
Technology in Israel and has much experience with research and teaching in
Physics and Astronomy. His research interests include theoretical studies
of novae, supernovae, accretion disks, and the interactions of close binary
stars. Mario will be taking over DSOB from Jerry Sellwood, who is leaving
the Institute for a position at Rutgers University.

    New Assistant Astronomer Stefi Baum was most recently a Hubble
Postdoctoral Fellow at Johns Hopkins University. She has joined the
SCARS/DSOB as the archive scientist. Her research centers on optical and
radio observations of extranuclear signs of activity in active galaxies.

    Rex Saffer has joined the Institute as an STScI postdoctoral
fellow. He recently completed his Ph.D. at the University of Arizona, and
specializes in spectroscopic studies of hot subdwarfs, single and binary
white dwarfs, and horizontal-branch stars.

    Brad Whitmore has been appointed Associate Astronomer with tenure.
He continues as Deputy Division Head of the Science Programs Division.

    After seven years of enthusiastic service, Colin Norman has stepped
down as Head of the Academic Affairs Division to devote more time to
research. Nino Panagia, at STScI since September 1984, and on the Academic
Affairs staff since July 1988, will assume the position of Head of Academic
Affairs on December 1, 1991. Nino has wide-ranging research interests but
is perhaps best known for his recent study of SN1987A with the HST, in
which he determined an extremely accurate distance to the LMC via analysis
of the ring around the supernova.



 Recent STScI Preprints

    The following papers have appeared recently in the STScI Preprint
Series. Copies may be requested from Sharon Toolan (410-338-4898, userid
TOOLAN) at STScI. Please specify the preprint number when making a request.

548. "New Wolf-Rayet Stars in Galactic Open Clusters: Sher 1 and the Giant
H II Region Core Westerlund 2," A.F.J. Moffat, M.M. Shara, and M. Potter.

549. "A Deep Survey for Galactic Wolf-Rayet Stars. I. Motivation, Search
Technique, and First Results," M.M. Shara, A.F.J. Moffat, L.F. Smith, and
M. Potter.

550. "A Radio-Quiet Galaxy at Redshift z=3.409," D.A. Turnshek, F.
Macchetto, M.V. Bencke, C. Hazard, W.B. Sparks, and R.G. McMahon.

551. "Accretion-Disk Phenomena," J.E. Pringle.

552. "Spectroscopy of Spatially Extended Material around High-Redshift
Radio-Loud Quasars," T.M. Heckman, M.D. Lehnert, G.K. Miley, and W. van
Breugel.

553. "On the Evolutionary Status of Beta Pictoris," F. Paresce.

554. "QSO Absorption Systems and the Origin of the Ionizing Background at
High Redshift," P. Madau.

555. "Confirmation of Dust in Damped-Lyman-Alpha Systems," Y  Pei, S.M.
Fall, and J. Bechtold.

556. "Attenuation of Lyman-Alpha Emission by Dust in Damped-Lyman-Alpha
Systems," S. Charlot and S.M. Fall.

557. "Subgiant CH Stars. II. Chemical Compositions and the Evolutionary
Connection with Barium Stars," R.E. Luck and H.E. Bond.

558. "Implications of Helium Diffusion for Globular-Cluster Isochrones and
Luminosity Functions," C.R. Proffitt and D.A. VandenBerg.

559. "Polarized Radio Emission from the Edge-On Spiral Galaxies NGC 891 and
NGC 4565," S. Sukumar and R.J. Allen.

560. "Gravitational Settling in Solar Models," C.R. Proffitt and G.
Michaud.

561. "The Relationship between the CO Intensity and the Radio Continuum
Emission in Spiral Galaxies," D.S. Adler, R.J. Allen, and K.Y. Lo.

562. "Fanaroff-Riley I Galaxies as the Parent Population of BL Lacertae
Objects. III. Radio Constraints," C.M. Urry, P. Padovani, and M. Stickel.

563. "A Model for Tidally Driven Eccentric Instabilities in Fluid Disks,"
S.H. Lubow.

564. "Simulations of Tidally Driven Eccentric Instabilities with
Application to Superhumps," S. H. Lubow.

565. "The Current Ability of HST to Reveal Morphological Structure in
Medium-Redshift Galaxies," I.R. King, S.A. Stanford, P. Seitzer, M.
Bershady, W  Keel, D  Koo, N. Weir, S. Djorgovski, and R. Windhorst.

566. "Optical Synchrotron Emission and Turbulence in Extragalactic Jets,"
D. Fraix-Burnet.

567. "The Formation of Globular Clusters in Merging and Interacting
Galaxies," K.M. Ashman and S.E. Zepf.

568. "Two High-Velocity Stars Shot out from the Core of the Globular
Cluster 47 Tucanae," G. Meylan, P. Dubath, and M. Mayor.

569. "Modelling the Evolution of Galaxies in Compact Groups," S.E. Zepf and
B  Whitmore.

570. "Optical Colors of Early-Type Galaxies in Compact Groups," S.E. Zepf,
B  Whitmore, and H.F. Levison.

571. "Proton-Initiated Electron-Positron Pair Production in Compact
Sources," A.A. Zdziarski.

572. "The Distribution of Nearby Rich Clusters of Galaxies," M. Postman,
J.P. Huchra, and M.J. Geller.

573. "Outbursts by Low-Mass White Dwarfs in Symbiotic Variables," E.M. Sion
and C.J. Ready.

574. "Search for Starbursts among X-ray Selected Galaxies: Optical
Spectroscopy," A. Fruscione and R.E. Griffiths.

575. "Properties of the SN 1987A Circumstellar Ring and the Distance to the
Large Magellanic Cloud," N. Panagia, R. Gilmozzi, F. Macchetto, H.M. Adorf,
and R.P. Kirshner.

576. "Eclipse Studies of the Dwarf Nova HT Cas. II. White Dwarf and
Accretion Disk," J.H. Wood, K. Horne, and S. Vennes.

577. "Polarization Variability among Wolf-Rayet Stars. VII. The Single
Stars in WR 14, WR 25 and WR 69," L. Drissen, C. Robert, and A.F.J. Moffat.

578. "Discovery of a Low-Redshift Ultraluminous TE+A' Galaxy," W.R.
Oegerle, J.M. Hill, and J.G. Hoessel.

579. "Inverse-Compton Gamma Ray Emission from Chaotic, Early-Type Stellar
Winds and its Detectability by GRO," W. Chen and R.L. White.

580. "Coronal Lines and Starburst Features in the Two New IRAS AGN: IRAS
04493-6441 and IRAS 22419-6049," S. Lipari and F. Macchetto.

581. "Binary-Star Observations with the Hubble Space Telescope Fine
Guidance Sensors," O.G. Franz, L.H. Wasserman, E. Nelan, M.G. Lattanzi, B.
Bucciarelli, and L.G. Taff.

582. "Luminosity Functions, Relativistic Beaming, and Unified Theories of
High-Luminosity Radio Sources," P. Padovani and C.M. Urry.

583. "Wind Accretion by Compact Objects: the TFlip-Flop' Instability," M.
Livio.

584. "IRAS 02366-3101: An Accretion-Disk Candidate among Luminous IRAS
Galaxies," L. Colina, S. Lipari, and F. Macchetto.

585. "Gas Flows in Spirals and Bars," J.A. Sellwood.



 Graduate Student Research Assistantships

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pursue Ph.D. thesis-level research with members of the Institute staff. The
scientific fields represented at the Institute cover much of modern
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Since STScI is not a degree- granting

organization, all students must be enrolled in the graduate program at
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    The program is intended for students who will spend at least one
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 ESA FELLOWSHIPS AT STScI

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Details of the interests of staff members at STScI can be obtained from Dr.
J. E. Pringle in the Academic Affairs Division (410-338-4477, userid
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 Newsletter Notes

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    Persons who assisted in the preparation of this issue include John
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