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Date: Fri, 6 Jul 1990 02:10:56 -0400 (EDT)
Subject: SPACE Digest V12 #18
SPACE Digest Volume 12 : Issue 18
Today's Topics:
HST / WF-PC: Update from those who know.
Re: SPACE Digest V11 #592
Re: grim tidings for the future
yspace pics
Galileo Update - 07/05/90
Administrivia:
Submissions to the SPACE Digest/sci.space should be mailed to
space+@andrew.cmu.edu. Other mail, esp. [un]subscription notices,
should be sent to space-request+@andrew.cmu.edu, or, if urgent, to
As you have no doubt heard from the press, the Hubble Space
Telescope has serious optical problems. Since the media do not always
communicate the technical details with full accuracy, I would like to
inform the astronomical community about just what is wrong and what
its consequences are likely to be.
Tests in orbit have shown that the Optical Telescope Assembly has
about half a wave (r.m.s.) of spherical aberration. If this error
were entirely due to the shape of the primary (which is only an
assumption) it would correspond to a mirror curvature that is too
shallow, with a total center-to-edge error of about two microns.
Consequently the images will not achieve their anticipated quality.
At a compromise focus, star images have cores whose full width at half
maximum is 65 to 70 milliarcsec. Although this core size is within
specification, it contains only about 15 percent of the light. The
remainder of the energy is in a faint halo that spreads over an arc
second or more.
The HST Science Working Group and the newly-formed HST Users
Committee met on June 27/28 to consider the implications of this
situation, which is obviously a major setback. The two groups
concluded that HST never-the-less has capabilities that far exceed and
complement those available on the ground and that its program should
go forward vigorously, emphasizing the unique and valuable science
that can be done with the telescope in its present condition. The
Working Group and Users Committee drafted a short report to NASA
summarizing their position, and that report is attached for your
information.
The cause and exact nature of the spherical aberration is still
being analyzed. At present we do not know whether the error is in the
primary mirror or the secondary or distributed in some way between
them. Although there are force actuators on the back of the primary,
they were designed to remove small amounts of coma and astigmatism.
The correction needed now is a relatively large motion in a direction
in which the mirror blank is quite stiff, and the actuators are not
strong enough to do that job. Consequently, the aberration does not
appear to be curable in orbit and the existing instruments will suffer
degraded performance - more or less, depending on the particular
instrument. A quick assessment of these performances is contained in
the accompanying report. On the other hand, we now believe that the
Advanced Scientific Instruments which are already planned and
scheduled for in-orbit installation can be fully compensated for the
aberration, so that two and a half or three years from now we can
expect the WF/PC-II to provide the sharp imaging that we had intended
to achieve in the present WF/PC.
It is too early to state just what changes may be required in the
HST observing program. The Project must thoroughly quantify the
present performance of the observatory , and NASA and the Space
Telescope Science Institute must reexamine their plans for observatory
usage. In the meantime, I wanted you to read the facts as I know
them. I encourage you to continue monitoring Ron Polidan's HST news
reports on email and the ST Science Institute's reports for new
information as it becomes available.
Albert Boggess
Project Scientist for HST
**************************************
POSITION PAPER OF THE
HST SCIENCE WORKING GROUP
AND THE HST USERS COMMITTEE
JUNE 29, 1990
SUMMARY
Our groups met jointly on June 27 and 28, 1990, at the GSFC and
received reports from key Project personnel about the status of the
Hubble Space Telescope, with particular emphasis on the imaging
performance. Although the program has clearly suffered a major setback
due to telescope optics that are well below specification, we are
convinced that the long-term prospects for completion of the science
program are highly encouraging. Furthermore, a valuable subset of the
scientific program can be executed with the telescope even in its
present state, with the result that HST still has the potential to
produce many key discoveries in the near future.
HST right now has capabilities that vastly exceed ground-based
observatories. Most important are spectroscopy, photometry and imaging
at ultraviolet wavelengths, none of which are possible at all from the
ground. High resolution visible-light imaging on bright objects will
also be possible, achieving to some degree the original fine details
expected in HST pictures.
For the longer term, it appears highly probable that the full
imaging capabilities of HST can be restored by straightforward
modifications to the Second-Generation Scientific Instruments. These
are already under development as Orbital Replacement Instruments
(ORIs), to be installed in the observatory over the next several
years. We recommend that the development of these ORIs be accelerated
as much as possible. With determined effort, the WFPC-II camera could
be installed in as little as 2 1/2 years, producing images that meet
the original design goals.
THE OPTICAL CAPABILITY
We understand that there is approximately one-half wave rms
spherical aberration error (2 micron center-to-edge surface error) in
the OTA wavefront, leading to images that fail to meet the Level I 70%
enclosed light specification by roughly a factor of 7. The observed
image radius is 0.7", versus a specified radius of 0.1". For certain
focal positions, the images possess sharp cores (~0.07" FWHM), so that
at some level the high spatial resolving power of HST is preserved.
However, these cores contain only ~15% of the light, the remaining
light being dispersed over a wide halo comparable in size to
ground-based images. A summary of what capabilities for science are
allowed by these image properties and what can be done to recover the
full Level I performance of the observatory is given below.
THE SCIENTIFIC CAPABILITIES
HST is an extremely versatile observatory with many modes of
observation. Loss of image quality has damaged some of these, modes,
but many remain wholly or largely intact. Temporarily being able to
use only certain of these modes will not alter the fact that we will
still be able to do forefront science 100% of the time, but the
initial scientific emphasis will have to be different. The number of
programs that can be done with the HST has always greatly exceeded the
time available for their execution, and this remains true even with
the telescope in its current state.
The near-term observing plan will now have to give greater
emphasis to ultraviolet imaging (FOC), ultraviolet spectroscopy (FOS
and GHRS), and ultraviolet photometry (HSP), plus the use of bright
core images in visible light (WF/PC and FOC). These, plus the ongoing
astrometry program (FGS), will easily occupy all the time available
and will return scientific results of great interest and utility. The
fulfillment of the original imaging programs will still be possible,
but will largely have to be scheduled later.
These near-term programs are possible because HST, even in its
present state, has unique capabilities that cannot be matched
anywhere, either in space or on the ground. The core of the sharply
focused image can yield sharp pictures for bright, high-contrast
objects such as stars and galactic nuclei. The ultraviolet
spectroscopic capability is still largely intact, although some
trade-offs are now involved. Obtaining spectra at the planned
spectral resolution and desired signal-to-noise will be possible, but
the targets will have to be brighter, or the exposure times longer.
Spectra of faint targets should also be possible, although with some
loss of spectral purity. Fortunately, many programs of the HSP will
not be impacted, and the astrometry capability of the FGS will
apparently not be compromised at all. Realizing these capabilities
will require some new effort; for example, it will be necessary to
revise target acquisition methods for the small-aperture instruments
and to develop image deconvolution algorithms far the cameras.
However, these are not major tasks.
Most encouraging of all, it should be possible ultimately to
realize the full capability of the HST through the use of the
Second-Generation Scientific Instruments (SIs). The WF/PC-II
instrument that is currently planned for flight some 3 years after
launch has an optical design that will permit complete correction of
the errors in the OTA. The other two new Scientific Instruments, both
already in development, will also be able to correct simply for the
limitations of the telescope optics. The NICMOS instrument will
extend the wavelength range of the HST and open up a new window to
infra-red observations. The STIS instrument will expand the
spectroscopic power of the HST by at least an order of magnitude
beyond the goals for the first generation of spectrographs.
Completion of all three of the Second-Generation Instruments is now
clearly more urgent than ever.
CONCLUSIONS AND RECOMMENDATIONS
While some of the important scientific goals of the Hubble Space
Telescope are currently not achievable because of the spherical
aberration in the telescope optics, other unique scientific goals do
remain viable, and HST therefore still has the potential to produce
many important discoveries during its first years of operation. The
original goals for the 15-year mission of the HST continue to be
achievable. We therefore recommend that all necessary actions be
taken to insure that HST will operate as productively as possible in
the short-term, and that activities directed at restoring full
capability over the long-term be pursued vigorously. In the short
term, this means that on-going HST operations must be fully supported
and that the telescope and instrument performance evaluations and
analyses must be continued. We also need to adjust to the larger
images by developing new target acquisition techniques and by
modifying various other capabilities as indicated by the results of
ongoing studies.
For the longer term, it is clear that the full imaging
capabilities of HST can be restored by suitable modifications to the
Second-Generation Scientific Instruments, which are planned as Orbital
Replacement Instruments (ORIs), to be installed in the observatory
over the next several years. We recommend that the development of
these ORIs be accelerated. Finally, we recommend that the first
Maintenance and Refurbishment Mission, which will install the WF/PC-II
in place of the original WF/PC, be directed to proceed as
expeditiously as possible, with the goal of being completed in less
than 2 1/2 years rather than the planned 3-year schedule.
By way of conclusion, we would like to emphasize that an enormous
number of technically difficult challenges involved in the design of
HST have been successfully met. These include the basic smoothness of
the optics, the high degree of accuracy required from the Pointing
Control System, and the proper functioning of a suite of complex
scientific instruments, to name only a few. In comparison to the huge
investment already made in HST, the effort involved in correcting the
optical aberrations is fairly small. In comparison to the planned
15-year lifetime of HST, the few-year delay until delivery of WFPC-II
is also small. With this perspective in mind, we feel it is important
to keep our eyes on the long-term future and work diligently to
realize the full potential of the observatory, which is still a fully
viable goal.
For the HST Science Working Group For the HST Users Committe
Albert Boggess Arthur Davidsen
****************************************************************************************************************************************************************Posted by: Eric Jaderlund
