December 1982:the Space Telescope program

Development of the Space Telescope

Herman Oberth is reported to have proposed the establishment of an orbital observatory as early as 1943 and occasional references to such a facility may be found in both science
and science fiction after that date. Once the reality of orbital
flight had been established, NASA commissioned two summer
studies of a space telescope, held in 1962 and 1966. It was at
these studies that the first detailed proposals for a space
telescope and its scientific mission were made. In 1967 and
1968 an ad hoc committee of the National Academy of Sciences
promoted a series of seminars on how such a spacecraft could
be used, and focused the attention of the scientific community
on the project.

NASA officially began the project as an advanced study in
1971 and 1972, the steering committee including several leading astronomers and engineers. At this time, the project was known as the Large Space
Telescope and envisaged a mirror 3 m (10 ft) in diameter.

The scientific definition, led by Dr. C. R. O’Dell, was carried
out between 1973 and 1976 when the mission objectives, modes
of operation and preferred instruments were defined. It was
during this study that a decision was made to reduce the size of the primary mirror to 2.4 m (8 ft), a compromise between cost and the need to have as large a mirror as possible. At this
point in the programme the word “Large” was dropped, the
spacecraft being simply known as the Space Telescope.

With European activity in space increasing steadily throughout the 1970’s and the needs of European astronomers in mind,
the ESA proposed to NASA that they would take a 15 percent share in the project. This decision was made on 6 October 1976 by ESA’s Science Programme Committee. The final
agreement was that ESA would provide the spacecraft’s solar
panels and faint object camera. In return, European observers
would receive 15 per cent of the observing time. The memorandum of understanding was signed in Paris on 7 October 1977 by Mr. Robert Frosch, Administrator of NASA and Mr.
Roy Gibson, Director General of the ESA.

Formal confirmation of the project was announced on 14 December 1976 and authorised by the US Congress in
1977. Final design and development activities began in 1977
and will continue essentially until launch. Competitions were
held for contracts to construct various parts of the telescope
and support systems. Kodak and Perkin-Elmer bid for .the
telescope itself and Lockheed, Boeing and Martin Marietta for
the support module. The final awards were to Perkin-Elmer
and Lockheed. Work on the primary mirror began in October
1977 with the casting at the Corning Glass Works
in New York State. The mirror was delivered to Perkin-Elmer
in December 1978 when rough grinding began. Fine polishing
began in August 1980 and was completed by May 1981.
Remaining project milestones are the final coating of the primary mirror,
completion of the secondary mirror, and completion of the
scientific instruments. All of these components will be delivered to Lockheed at Sunnyvale in California for assembly and
check-out. The telescope will then be transported to the
Kennedy Space Center in late 1984 for integration with the
Space Transportation System and launch, scheduled for February 1985.

Spacecraft Description

The Space Telescope consists of three main elements, the
telescope itself, its support systems module and the scientific
instruments.

1. The Optical Telescope Assembly

The heart of the spacecraft is the telescope itself, a Cassegrain reflecting telescope of Ritchey-Chretien form, Light
is reflected from the 2.4 m diameter main mirror to a 0.3 m
secondary mirror. This secondary sends the light back down
the tube and through the centre of the primary to a focus 1.5 m behind the primary. The tube contains a series of light baffles to suppress any unwanted light from other bright objects, such
as the Sun or the Earth.

The primary mirror, 2.4 m in diameter and made of Ultra
Low Expansion Glass, was prepared by the Perkin-Elmer
Corporation, The mirror blank, weighing 907 kg (2000 lb), took
three and a half years to grind and polish to the required
accuracy. On completion in mid-1981, it was estimated to
deviate no "more than 0.000025 mm (one millionth of an inch)
from the ideal surface. This is believed to be the most accurate
telescope mirror ever produced.

After completion of the basic mirror it was then necessary
to coat the surface with a very thin reflective coating. This
operation was carried out in a special stainless steel vacuum
chamber, the largest of its type in the world. First a coating
of highly reflective pure aluminium 0.00051 mm (0.00002 inch)
thick was applied. This was followed by a protective coating
of magnesium fluoride a mere 0.00015 mm (0.000006 inch)
thick. The magnesium fluoride will protect the aluminium
from oxidising and losing its reflectivity.
The small secondary mirror is also being ground and coated
to the same exacting specifications to achieve the best possible
optical performance.

To ensure that the two mirrors remain in the correct relative
positions, despite the varying thermal environment, the telescope’s main internal structure is fabricated from a graphite
epoxy composite. Special optical-control sensors, located at the
telescope’s focal plane, will permit controllers to examine "the
optical condition of the telescope. Telemetered data will enable
ground controllers to check the curvature of the primary mirror
and the alignment of the secondary.
If errors are detected, the surface of the primary may be
corrected by 24 force actuators mounted behind the back
surface of the mirror. Six precision motors move the supporting
structure of the secondary mirror to eliminate any mirror
misalignments.
The combination of precision optics, adjustable mirrors and
perfect observing conditions will make the Space Telescope
a powerful observing tool. Almost every photon arriving at the
main mirror will eventually be focussed at a point one quarter
of the thickness of a human hair.

2. The Support Systems Module (SSM)

The SSM will supply the services required to operate the
spacecraft. Basically it is a collar which fits around the telescope
structure about level with the primary mirror. It contains the
electrical power distributors and communications equipment,
including the telemetry system and the attitude control
gyroscopes.

The solar power array, provided by British Aerospace, consists of two identical deployable and retractable solar panels
11.82 m long with a total area of about 50 sq.m. The arrays,
mounted on a double roll-out flexible substrate, will deliver
over 4 KW of power at over 34V after two years in orbit. The
panels will meet stringent requirements of reliability, both
electrical and mechanical, and dynamic stability since they will
be attached to a spacecraft with very accurate pointing
requirements.

Attitude control is provided by a set of reaction wheels
which receive their attitude information from rate gyroscopes,
star trackers and fine guidance sensors. The wheels will allow
the telescope to be moved through 90 degrees in no more than
20 minutes, including acceleration and deceleration. The star
trackers will use bright stars to determine the pointing accuracy
to within 1 arcmin which is sufficient to place the required
guide stars in the fields of the fine sensors. These three sensors,
located around the primary mirror, use interferometry to provide error signals to the attitude control system. The overall
accuracy of this system will allow pointing and stability to
within 0.007 arcsec.

3. The Scientific Instnrments

The Space Telescope will carry five scientific instruments
behind the primary mirror.each mounted in its own Scientific Instrument Module. These modules will be of standard
sizes, completely independent and capable of operation without mutual interference. This arrangement allows any individual instrument to be removed completely or replaced with the
minimum of effort. Replacement by Shuttle astronauts will
allow the telescope’s equipment to always be the most up-to-date available. The five instruments chosen as the payload for
the first few years of operation are described below.

The Wide Field/Planetary Camera (WF/PC)

This instrument can be operated in two ways, characterised
loosely by the two names: Wide Field Camera (WFC) and
Planetary Camera (PC). The WFC mode, with a field of view
of 2.7 arcmin square will be used for deep sky surveys. The
PC mode is intended for high resolution imaging of faint
sources over a smaller field of view.
The light entering the instrument is directed by a moveable,
pyramid mirror to either the WFC or PC detectors. Both
detectors consist jof four sets of 800 x 800 charge-
coupled devices (CCD), cooled to —95 degrees C, and coated
with the organic phosphor, coronene. The coronene, which
converts ultraviolet photons to visible photons, is used to
increase the wavelength coverage of the detectors. Each CCD
array overlaps its neighbours, allowing the four separate imagesto be accurately recombined to a single large picture.

This instrument will provide a sensitive detector over the
wavelength range from 1,150A to 1.1 and a visual magnitude range of 8 to 28. Minimum exposure time will be 0.1 sec
and typical long exposure times will be about 3000 seconds,
corresponding to half an orbital period. It will be equipped
with a large number of filters, transmission gratings and polarisers for specific observing needs.

The WF/PC will be located in a radial bay and will be
cooled by an thermal radiator which will form part of the
exterior surface of the spacecraft.

The Faint Object Camera (FOC)

The FOC will use the full optical performance of the
telescope to study very faint objects at high angular resolution.
The FOC and WF/PC are complementary, since the FOC
provides high spatial resolution and the WF/PC has a larger
field of view.

The FOC consists of two independent camera systems
operating at f/96 and f/48. When operated in the f/48 mode
the field of view is four times that at f/96, but at lower
resolution. At short wavelengths, very high resolution can be
achieved by inserting a compact Cassegrain assembly into the
f/96 optical path, providing an f/288 mode.

The detectors for the FOC are based on the Image Photon
Counting System developed by Boksenberg. Essentially a very
high performance image intensifier, the IPCS is able to count
individual photons, allowing the study of very faint objects. By
using integration times of up to ten hours, it should be possible
to achieve a signal to noise ratio of at least 4 for stellar objects
as faint as visual magnitude 28. Like the WF/PC the FOC
will be equipped with a variety of filters and polarisers. In the
f/96 mode a coronographic facility is included to suppress the light from bright objects when observing faint sources in the
same field. The f/48 system provides a long slit spectrographic
capability for observing extended objects such as galaxies,
comets and nebulae.

ESA will provide the FOC as part of the European contribution to the project. Located in an axial bay the FOC has
dimensions of 0.9X0.9X2.2 m, weighs 318 kg and will consume
about 140 W over a typical 95 minute orbit.

The Faint Object Spectrograph (FOSl

The Faint Object Spectrograph will be used for moderate
and low resolution spectroscopy at both visible and ultraviolet
wavelengths. It can also be used for spectropolarimetry and
time-resolved spectroscopy.

The FOS uses two magnetically focussed, photon counting
Digicon sensors, one covering the ultraviolet and visible wavelengths, the other the visible and near infrared. The Digicon
operates by re-imaging the detected photoelectrons onto an
array of 512 silicon diodes. Exposure times will vary from a
minimum of 50 microseconds to 10,000 seconds or more. A
continuous set of exposures, each of duration 50 Msec to
10 msec, can be made at rates of up to 100 exposures per
second, allowing accurate, time-resolved spectroscopy.
The faintest stars visible (the limiting magnitude) to the
FOS will vary with the wavelengths studied and the resolution
selected, but is expected to be about 21st magnitude at high
resolution and 25th magnitude at low resolution, for a 10,000
second exposure.

The High Resolution Spectrograph (HRSl

The HRS will provide high quality spectra at ultraviolet
wavelengths between 1100 and 2300A. The instrument may
be operated in one of three resolution modes, but only the
high and moderate ranges will normally be used. In the highest
resolution mode the resolving power will be of the order of
10 s . The moderate resolution mode will be used for targetacquisition, estimating exposure times for high resolution spectra and coverage of the short wavelength region, where high
resolution spectra are impractical because of the low efficiency
of the OTA.

Like the FOS, the HRS uses two Digicon detectors each
with 512 diodes. Minimum exposure time will be 25 msec
when data is transmitted to the ground directly and 50 msec
when the data is stored on board. Limiting visual magnitudes
will vary with wavelength and resolution mode, but for a 2,000
second integration, will be about 11 at high resolution, 14 at
medium resolution and 17 at low resolution.

The High Speed Photometer (HSP)

The High Speed Photometer will be used to study the
time-dependent brightness fluctuations of a variety of objects
over a wide wavelength range. It is capable of resolving two
events a mere 16 Msec apart. This level of resolution is impossible from Earth because of fluctuations in the atmosphere.

The HSP uses four image dissectors (essentially photomultipliers with spatial resolution) and a red sensitive photomultiplier. Two of the image dissectors operate at ultraviolet
wavelengths, the other two in the visible and near ultraviolet.
This array of detectors allows a wide wavelength coverage
from about 1200 to 8000A. Unlike a conventional photometer,
which uses a stepping motor to position various filters in the
optical path of the instrument, the HSP contains no moving
parts. The choice of filter and aperture combination will be
made by positioning of the optical image within the HSP by
small movements of the telescope.

The HSP will operate in one of three modes. The first is for photometry with a single filter. This allows the brightness of the local starfield and nebulosity to be subtracted from
the stellar magnitude to improve accuracy. The second is for
photometry or polarimetry with several filters used sequentially
with small motions. The third is for wide field phbtometry
over a 10 arcsec diameter area without a filter, requiring no
special spacecraft motion.
Limiting visual magnitude with a 1,000 second integration
time and signal to noise ratio of 10 will be 24. Photometric
accuracy is expected to be about 0.2 per cent.

Astrometry with the Fine Guidance System

During normal operations, two of the three Fine Guidance
Sensors will be used for accurate telescope pointing. The third
sensor will be available for astrometric measurements, which
will be both accurate and short.
With the aid of neutral density filters, stars with magnitudes
between 4 and 20 should be measurable, with positions of ten
stars being determined in about ten minutes. Possible targets
include both distant stars and the satellites of the outer planets.

Mission Operations

In 1985 the Space Telescope will be carried into orbit by
Shuttle mission STS-23 from Cape Canaveral. Once in orbit, the
Shuttle will use its orbital manoeuvring system to raise itself
to the required 500 km circular orbit. The telescope will then
be removed from the Orbiter’s cargo bay and positioned in
space by the remote manipulator arm. Before release, it will
receive a preliminary check-out by ground controllers.

Once released, the solar panels will be deployed and the
main cover opened. With the Orbiter still standing by, the
telescope will be given a thorough system check lasting several
days. Only when ground controllers are satisfied that the
spacecraft is fully operational will the Shuttle return to Earth.
Should a problem occur which cannot be cured in situ, the
telescope will be reloaded into the Orbiter cargo bay and
returned to Earth for repairs. Once the telescope has been
declared operational, control will be handed over to the Space
Telescope Science Institute which will conduct routine operations and co-ordinate research workers from both the USA
and other countries. Although in orbit, it will be operated like
a large ground based observatory. Astronomers will be allocated
observing time based on both the importance of their observations and the availability of suitable instruments on the
telescope. Astronomers will probably travel to the Institute to
carry out their observations, allowing them to make on-the-spot judgements during their allocated observing session.

After about two and a half years in orbit, the Shuttle will
again visit the telescope. It will be grappled by the remote
manipulator, its solar panels folded and slowly positioned back
in the cargo bay. Shuttle astronauts will “space walk” to the
telescope and replace some, or all, of the scientific instruments.
Other routine servicing tasks will be carried out while it is
within the cargo bay. Once these operations are complete, it
will be released again and returned to ground control.

After another two or three years in orbit the telescope will
be collected by a Shuttle and returned to Earth complete.
After landing at the Kennedy Space Center, it will be taken
to another establishment, probably the G. C. Marshall Space
Flight Center, for a complete overhaul. The mirrors and solar
cell arrays will be removed and returned to their manufacturers
for refurbishment and new scientific instruments will be
installed in the main structure.
After reassembly and checkout, the telescope will be
returned to Cape Canaveral for relaunch and another five year
period in space. By regular servicing and periodic returns to
Earth, the Space Telescope is expected to remain operational
until into the next century.