The Starbase One Astronomy & Space Collection/STARBASE_ONE.ISO/animate/satws.doc

Text File | 1991-09-03 | 4KB | 65 lines

This is the documentation file for SATWS1.GIF, SATWS2.GIF, SATWS3.GIF,
SATWS4.GIF, SATWS5.GIF, SATWS6.GIF, and SATWSP.GIF.

SATWSn.GIF are "stills" from a movie of the Saturn White Spot constructed from
data obtained by the Hubble Space Telescope. These are 400 by 400 by 256 color
GIFs. SATWSP.GIF is a polar projection of Saturn showing the entire planet
North of about -6 degrees latitude.  This is a 604 by 604 by 256 color GIF.

These frames show the Saturn white spot, a great storm in the equatorial
region of Saturn, discovered by amateur astronomers in September, 1990.  Such
storms are rare: the last one in the equatorial region occurred in 1933.  By
November, the storm extended completely around the planet, in some places
appearing as great masses of clouds and in others as well organized
turbulence.

Knowing that this storm is probably a once in a lifetime event, scientists and
engineers of an ad-hoc White Spot Observing Team, the Wide Field/Planetary
Camera Team, the Space Telescope Science Institute, and the Goddard Space
Flight Center reprogrammed the observing schedule of the Hubble Space
Telescope.  They were able to get several days of Saturn observations in mid-
November, shortly before Saturn would be too close to the Sun for
observations.

The movie was constructed from red, green, and blue Planetary Camera images
obtained during eight successive HST orbits on November 17.  Each of the 24
frames was processed to remove instrumental artifacts and the effects of the
HST spherical aberration.  The frames were then combined to make a movie by
interpolating images of Saturn at uniform intervals of about ten minutes, or
six degrees of rotation of Saturn. The SATWSn.GIF frames include every tenth
frame from the movie. The color is approximately true color as the center
wavelengths of the filters are 718, 547, and 439 nanometers. The occasional
dark swaths running North-South are an artifact of joining the individual
frames. The processed frames reveal detail down to about 700 km (440 miles),
but there is some loss in resolution as a result of the interpolation. For
comparison, the diameter of Saturn is about 120,000 km (75,000 miles).

The images used to construct these frames are only about fifteen percent of
the data acquired during the November observing session.  By studying all the
data, scientists hope to better understand wind speeds in Saturn's atmosphere,
the composition and altitude of the clouds, and perhaps the cause of this
great storm.

More detail on the construction of the movie:  Each of the 8 frames in each of
the three colors is projected onto a rectangular latitude/longitude grid.
Then the frames in each color are joined to give maps of Saturn covering about
96 degrees in latitude and more than 360 degrees in longitude. These long,
thin maps (sometimes called snakes!) are projected back to a sphere.  One can
make as many projections (with different longitude centers) as desired in
order to construct a movie showing the rotation of the planet. I chose 60
frames since that's as much as will fit in the memory of my workstation.  Of
course, the rings, and the Southern part of the planet obscured by the dusky
ring, do strange things when projected onto a latitude/longitude grid, so they
must be excluded from these projections.  To make Saturn appear properly, the
rings from one of the original frames are simply pasted onto all the movie
frames! The polar projection image is made from the snakes by using a
different projection algorithm.

The images of Saturn are fainter towards the edges because the Sun is shining
obliquely on the limbs of the planet.  The joints between frames are near the
edges, and are somewhat darker than the rest of the map.  This is the reason
for the North/South dark swaths in the frames.

February 5, 1991 - Edward J. Groth for the HST WF/PC team.