SWUIS Project Background

A New Capability in Space Science Research
Alan Stern and Dave Slater

Introduction

One of the great strengths of our
Institute
is its Internal Research (IR) Program, and this article is to a very great
degree a story of one of the IR Program's successes.

The story begins in 1991. At that time,
our Division's space experimentation expertise was based in mass spectroscopy and measurements of plasmas and magnetic fields. These so called
situ measurements are the space equivalent of the body's sense of smell.
These techniques (reference to the Cassini Tech Today article) provide an
extremely powerful tool for studying planetary atmospheres and
ionospheres, planetary magnetospheres, and the interplanetary medium.
However, they require that the instruments be brought to the place they
are investigating.

By contrast, space remote sensing, through the
use of telescopes and photon detectors, allows one to study objects
across great distances. Well, as we just noted, in 1991 our Division was
known for its expertise in in situ measurement techniques but was
lacking in space remote sensing experience. Because remote sensing
offered to open up whole new avenues of scientific research, and research
work, we made a deliberate decision to gain experience in and become a
center of excellence in space remote sensing. The particular branch of
remote sensing we chose was optical and ultraviolet (UV) imaging and
spectroscopy, which provides powerful tools for studying planetary
atmospheres, comets, and a wide array of astrophysical environments.
Because this was already a mature field with many well-established
research groups, we knew we would have to demonstrate our abilities
internally before we could hope to win projects.

So, with those goals in mind, we put together an
internal research proposal to develop a versatile, low-cost UV/visible
imager that could serve as a prototype miniature astronomical laboratory
aboard the Space Shuttle. We called our instrument SWUIS, or the
Southwest Ultraviolet Imaging System.

Our proposal was accepted in mid-1991, and within a
little over a year, we had put together a working version of our concept.
We proved out the instrument's capabilities in the lab, and then with a
field trip to McDonald Observatory in 1992. We then proposed and were
accepted to fly SWUIS on a pair of high altitude demonstration missions
aboard
NASA SR-71/Blackbird
aircraft in early 1993 (see photo below). These missions took SWUIS to altitudes of almost 90,000 feet and
speeds exceeding Mach 3.2.

With the experience we gained from the Internal
Research funded-laboratory demonstration, and the high performance
aircraft flights, we felt we had a good basis to propose to build up and
fly a SWUIS instrument aboard the Shuttle, and to use it to tackle some
difficult kinds of remote sensing observations in space.

Fortunately, the Universe cooperated, by providing us
with
Hale-Bopp, the "Comet of the Century." Comets are relics from the
formation epoch of the solar system, and planetary scientists relish the
opportunity to study a bright one. So, when Hale-Bopp was discovered in
mid-1995 and it was recognized that when it approached the Sun in
mid-1997, it would become one of the brightest comets ever recorded, NASA
requested proposals to conduct space observations of this unique comet.

Our group proposed to fly SWUIS aboard the Space
Shuttle, in order to image the comet from space when the comet was too
close to the Sun to risk the Hubble Space Telescope. Scientifically, our
goals were to obtain a long, time-lapse series of wide-field images of
the comet so its behavior and morphology could be studied in detail.

The SWUIS Instrument

SWUIS presently has two hardware configurations for Space
Shuttle missions: 1) telescope science mode (TSM); and 2) camera science
mode (CSM). TSM utilizes a telescope for high-spatial resolution imaging of
faint object targets such as planets, comets, and space debris. CSM
utilizes a wide-field camera lens for imaging bright targets that occupy
larger swaths of the sky such as aurora and lightning sprites. Both TSM and
CSM hardware are sensitive to ultraviolet (UV), visible (VIS), and infrared
(IR) wavelengths. The SWUIS TSM hardware is composed of three major
elements: the telescope; the intensified charge-coupled device (ICCD)
camera; and the electronics that provides power and control of the ICCD
camera. In addition to these major components, SWUIS utilizes a custom
built mounting bracket that couples the telescope to the Space Shuttle
side-hatch window for UV observations; a telescope optical coupling assembly
(TOCA) that physically and optically couples the ICCD camera to the
telescope, and which can hold up to three imaging filters in the optical
path; a filter caddy that holds the filters and lenses used in the TOCA; and
associated power and data cables. The data from the ICCD camera is an
analog video signal that is recorded on-board the Shuttle with a portable
camcorder, and which can be downlinked from the Shuttle to the ground for
real-time assessment. The photo at left shows a schematic of the SWUIS TSM hardware
components configured for Space Shuttle flights. Soon, we plan to add a
new mode and new capabilities to SWUIS with a spectrograph now in design.

The telescope (see photo at left), built by Questar Corporation, is a custom
7-inch (18 cm) diameter Maksutov-Cassegrain design ruggedized for space
flight use. It incorporates a UV transmissive front end corrector lens made
of magnesium fluoride, and mirror optical coatings composed of aluminum
overcoated with magnesium fluoride for enhanced sensitivity at UV/VIS/IR
wavelengths (200--1000 nm). The telescope incorporates a small 6x30 mm
finder telescope which allows the Shuttle mission specialist to make fine
pointing adjustments to the telescope during target acquisition. The
telescope is hard mounted to the side-hatch window in the Shuttle mid-deck
area via a custom two-axis mounting bracket with manual slow motion controls
for fine-pointing. A light shield made of Pyrell foam is placed between the
window and the telescope to block unwanted cabin light from entering the
telescope. The telescope and mounting bracket weigh approximately 30 lbs.

We have available a variety of ruggedized ICCD cameras, built
by Xybion Inc., that can fly
as part of the SWUIS hardware complement, and which are sensitive to UV, VIS,
and near-IR (NIR) wavelengths. The wavelength sensitivity of each ICCD
camera is determined by the type of photocathode material used in the
camera's design. The UV/VIS version utilizes a Generation II photocathode with a
sensitivity in the 180--820 nm wavelength range. A second VIS version
utilizes an extended blue Generation III photocathode with high sensitivity between
450 and 910 nm. The NIR version has high sensitivity between 600 and 1000
nm. The output of the ICCD camera is a standard RS-170 video signal at an
interlaced frame rate of 60 Hz with 370 lines of horizontal resolution. The
camera weighs 2.75 pounds and draws about 5 Watts.

The TOCA is a mechanical interface between the telescope
and the ICCD camera. It is designed to hold both imaging filters and
lenses. The effective focal length of the SWUIS TSM system can be varied
between 105 and 257 cm for a field-of-view range between 0.3 and 0.6 deg
(full cone). The electronics that control the ICCD camera were custom built
at SwRI. The Power Interface Box (PIB) provides power conditioning from the
Space Shuttle Orbiter's video interface unit to the
ICCD camera. The PIB also has manual adjustment controls of the ICCD
camera's internal sensitivity (gain) and video output signal. The video
output signal is buffered by the PIB to allow multiple data paths to
camcorders, monitors, and to the Shuttle's video downlink system. During
Space Shuttle missions, SWUIS data is recorded on board with a portable
camcorder, and can also be sent to the ground via satellite link.

SWUIS TSM mode provides astronomers and planetary
scientists with a small but highly capable space telescope. Although
far less sensitive than the Hubble Space Telescope, SWUIS has its own advantages.
These include a far wider field of view, and the capability to study objects
that are much closer to the Sun, such as the inner planets, and comets.

The SWUIS CSM configuration is very similar to the TSM
mode except the ICCD camera is used with a UV transmissive wide-field
lens, instead of the main telescope. A mini-TOCA is used to hold filter
combinations. The CSM can be
mounted to any of the Shuttle windows including the side-hatch window, and
the nine flight deck windows using a Bogan bracket camera mount.
The wide-field lens assembly provides a FOV of approximately 12.5 deg (full
cone). SWUIS CSM allows us to extended SWUIS capabilities into studies of
the Earth's atmosphere, aurora, and the ozone layer; it's also
useful for certain types of stellar astronomy.

Maiden Voyage: Imaging Hale-Bopp

When in mid-1996 NASA announced its selection of
payloads to study comet Hale-Bopp, the roster included a host of
suborbital rocket missions, high-altitude aircraft flights, and one
Shuttle experiment: SWUIS.

Between that point and the June 1997 delivery of SWUIS
to JSC before launch, we (1) designed, fabricated, and certified all of
the flight interface hardware for SWUIS to fly on Shuttle; (2) completed
all of the pre-mission Shuttle crew training; (3) developed all of the
necessary documentation, safety certification, and hardware tests
required to fly on Shuttle; and (4) calibrated the instrument both in the
lab and in the field, using star fields and comet Hale-Bopp itself.

9 Aug 1997. A portrait in the shuttle middeck of SWUIS and MS Robinson after a job well done!

The mission we were assigned to was
STS-85, which was
launched on August 8, 1997 (see photo above). The mission, flown by the orbiter
Discovery, lasted ten days. Some of the other payloads flown were a
Japanese robotic arm, a deployable satellite with an IR telescope, the
TAS technology applications payload, and a Hitchhiker payload called
UVSTAR. SWUIS was operated on nine separate orbits, one more than had been
planned pre-flight. The instrument performed well, and some nine hours of
data were recorded (see Table 1 below). The instrument performed flawlessly, recording over
430,000 images of the comet in a variety of key emission bands.

Table 1: STS-85 Data Collection Log

SWUIS Orbit

Date

Imaging Filters

STS-85-1

09 Aug 1997

Green continuum, OH, H2O+, CO+

STS-85-2

09 Aug 1997

CS, UV, continuum, C2, CN, OH

STS-85-3

12 Aug 1997

OH, CN, C2, broadband Vis continuum

STS-85-4

12 Aug 1997

Broadband UV, H2O+

STS-85-5

14 Aug 1997

OH, UV continuum, broadband Vis continuum

STS-85-6

14 Aug 1997

OH, UV continuum, broadband Vis continuum

STS-95-7

14 Aug 1997

OH, CN, C2, broadband Vis continuum

STS-85-8

15 Aug 1997

OH, UV continuum, broadband Vis continuum

STS-85-9

15 Aug 1997

OH, UV continuum, broadband Vis continuum

The data collected by SWUIS (see images below) provide the only
wide-field UV images of Hale-Bopp, the first-ever UV time-lapse series of a
comet, tens of thousands of images of broad band UV and OH fluorescence, as
well as images in the visible and the near-UV using the standard,
NASA-provided set of Hale-Bopp Watch filters were collected. Comparative
data on Hale-Bopp's coma and tail morphology on three fundamental timescales
are contained in these data: minutes (i.e., during a given filter image
sequence), hours (i.e., on successive orbits on a given date), and days
(i.e., over the 6 day span of the dataset). Further, the SWUIS dataset
contains the only OH (and therefore water-production) rate data that we are
aware of prior to the resumption of HST observations in September 1997, the
only UV dust/continuum images of the whole molecular coma, and
high-frequency photometry of Hale-Bopp making it possible to search for
pulsations and other phenomena of scientific interest.

Sample SWUIS Comet Images:

9 Aug 1997: Two computer enhanced versions of a portion of
the same visible+UV acquisition sequence. Integration time of 83.3s.

Moving Forward: Flights 2 and 3

SWUIS is scheduled to fly its second Space Shuttle
mission in July 1999 aboard the Columbia Orbiter during
STS-93. This mission
will focus on obtaining ultraviolet imagery of an array of planetary and astrophysical
targets. The specific
objectives of SWUIS during this flight are to obtain: (i) the mid-UV albedo
of Mercury for the first time and to search for spatial variations across
the planet; (ii) mid-UV dynamic movies of the upper atmospheres of Venus and
Jupiter; (iii) to establish the morphological appearance and phase curve
of the Moon in the mid-UV for the first time; (iv) to search for Vulcanoids, a
putative population of small, asteroid-like bodies residing interior to Mercury's
orbit; and (v) to obtain
mid-UV dynamic movies of the airglow along the earth's limb (in CSM).

For the third SWUIS flight slated for 2000 or 2001, we are
planning to add a new capability to the SWUIS experiment: a spectrograph
that will attach to the SWUIS telescope. This spectrograph will allow SWUIS
to pursue mid-UV spectroscopy of the inferior planets (Mercury and Venus)
something that no other facility presently can offer. The spectrograph will
also provide a valuable capability for cometary and asteroidal studies in
the mid-UV as well as the VIS and NIR wavelengths.

This new spectrograph, which is being designed and built
at SwRI with NASA funding, will have a wavelength coverage that spans the mid-UV from 200 nm
out to NIR wavelengths beyond 1100 nm. It will allow us to record the
spectrum from astronomical targets with moderately high spectral resolution
(approximately 0.1--0.5 nm). The spectrum will be recorded by the Xybion ICCD
camera (or equivalent detector) which will attach to the focal plane end of
the spectrograph. The ICCD camera will be able to sample approximately 23.0 nm
of spectral bandpass during any single observation. The grating can be
manually rotated to change the bandpass anywhere within the 200--1100 nm
wavelength coverage of the instrument.

With these new capabilities, SWUIS will become a
versatile tool indeed. This unique facility has grown from a camera
flying aboard high altitude aircraft to a sophisticated and highly
reconfigurable astronomical and Earth-remote sensing laboratory, which
has applications ranging from studies of celestial objects, to the ozone hole,
to space debris. With so much going for it, we are hopeful that SWUIS will be
selected for flight as a remote sensing facility aboard the International Space
Station when it becomes operational.

Coda

SWUIS's maiden flight aboard the Shuttle last summer
was a wonderful capstone to the first phase of this project. Now, with
two more flights on the books, and plans for more being proposed, we are
entering a vigorous operational phase for this versatile space
observatory. Already, we see on the horizon the possibility of using
SWUIS to detect and track space debris that is a hazard to the Space
Station and Space Shuttle, the expansion of SWUIS capabilities into the
infrared, and the application of SWUIS to a wide variety of terrestrial
applications, including lightning, aurora, and ozone studies. In future
years we expect to see more SWUIS flight on the Shuttle, an expanding
array of airborne applications (see sidebar), and the potential to make
SWUIS a Space Station facility for remote sensing. None of this would
have been possible without the kindling that Internal Research funding
provided at the project's outset.