Overview Of The Hubble Space Telescope

[Guest Freedom2005]

The Hubble Space Telescope is a coooperative program of the European

Space Agency (ESA) and the National Aeronautics and Space

Administration (NASA) to operate a long-lived space-based observatory

for the benefit of the international astronomical community. HST is an

observatory first dreamt of in the 1940s, designed and built in the

1970s and 80s, and operational only in the 1990s. Since its

preliminary inception, HST was designed to be a different type of

mission for NASA -- a permanent space- based observatory. To

accomplish this goal and protect the spacecraft against instrument and

equipment failures, NASA had always planned on regular servicing

missions. Hubble has special grapple fixtures, 76 handholds, and

stabilized in all three axes. HST is a 2.4-meter reflecting telescope

which was deployed in low-Earth orbit (600 kilometers) by the crew of

the space shuttle Discovery (STS-31) on 25 April 1990.

 

Responsibility for conducting and coordinating the science operations

of the Hubble Space Telescope rests with the Space Telescope Science

Institute (STScI) on the Johns Hopkins University Homewood Campus in

Baltimore, Maryland. STScI is operated for NASA by the Association of

University for Research in Astronomy, Incorporated (AURA).

 

HST's current complement of science instruments include two cameras,

two spectrographs, and fine guidance sensors (primarily used for

astrometric observations). Because of HST's location above the Earth's

atmosphere, these science instruments can produce high resolution

images of astronomical objects. Ground-based telescopes can seldom

provide resolution better than 1.0 arc-seconds, except momentarily

under the very best observing conditions. HST's resolution is about 10

times better, or 0.1 arc-seconds.

 

When originally planned in 1979, the Large Space Telescope program

called for return to Earth, refurbishment, and relaunch every 5 years,

with on-orbit servicing every 2.5 years. Hardware lifetime and

reliability requirements were based on that 2.5-year interval between

servicing missions. In 1985, contamination and structural loading

concerns associated with return to Earth aboard the shuttle eliminated

the concept of ground return from the program. NASA decided that

on-orbit servicing might be adequate to maintain HST for its 15- year

design life. A three year cycle of on-orbit servicing was adopted.

The first HST servicing mission in December 1993 was an enormous

success. Future servicing missions are tentatively planned for March

1997, mid-1999, and mid-2002. Contingency flights could still be added

to the shuttle manifest to perform specific tasks that cannot wait for

the next regularly scheduled servicing mission (and/or required tasks

that were not completed on a given servicing mission).

 

The five years since the launch of HST in 1990 have been momentous,

with the discovery of spherical aberration and the search for a

practical solution. The STS-61 (Endeavour) mission of December 1993

fully obviated the effects of spherical aberration and fully restored

the functionality of HST.

 

 

The Science Instruments

 

Wide Field/Planetary Camera 2

 

The original Wide Field/Planetary Camera (WF/PC1) was changed out and

displaced by WF/PC2 on the STS-61 shuttle mission in December 1993.

WF/PC2 was a spare instrument developed in 1985 by the Jet Propulsion

Laboratory in Pasadena, California.

 

WF/PC2 is actually four cameras. The relay mirrors in WF/PC2 are

spherically aberrated to correct for the spherically aberrated primary

mirror of the observatory. (HST's primary mirror is 2 microns too flat

at the edge, so the corrective optics within WF/PC2 are too high by

that same amount.)

 

The "heart" of WF/PC2 consists of an L-shaped trio of wide-field

sensors and a smaller, high resolution ("planetary") camera tucked in

the square's remaining corner.

 

 

Corrective Optics Space Telescope Axial Replacement

 

COSTAR is not a science instrument; it is a corrective optics package

that displaced the High Speed Photometer during the first servicing

mission to HST. COSTAR is designed to optically correct the effects of

the primary mirror's aberration on the three remaining scientific

instruments: Faint Object Camera (FOC), Faint Object Spectrograph

(FOS), and the Goddard High Resolution Spectrograph (GHRS).

 

 

Faint Object Camera

 

The Faint Object Camera is built by the European Space Agency. It is

the only instrument to utilize the full spatial resolving power of

HST.

 

There are two complete detector system of the FOC. Each uses an image

intensifier tube to produce an image on a phosphor screen that is

100,000 times brighter than the light received. This phosphor image is

then scanned by a sensitive electron-bombarded silicon (EBS) television

camera. This system is so sensitive that objects brighter than 21st

magnitude must be dimmed by the camera's filter systems to avoid

saturating the detectors. Even with abroad-band filter, the brightest

object which can be accurately measured is 20th magnitude.

 

The FOC offers three different focal ratios: f/48, f/96, and f/288 on a

standard television picture format. The f/48 image measures 22 X 22

arc-seconds and yields resolution (pixel size) of 0.043 arc-seconds.

The f/96 mode provides an image of 11 X 11 arc-seconds on each side and

a resolution of 0.022 arc-seconds. The f/288 field of view is 3.6 X

3.6 arc- seconds square, with resolution down to 0.0072 arc-seconds.

 

 

Faint Object Spectrograph

 

A spectrograph spreads out the light gathered by a telescope so that it

can be analyzed to determine such properties of celestial objects as

chemical composition and abundances, temperature, radial velocity,

rotational velocity, and magnetic fields. The Faint Object

Spectrograph (FOS) exmaines fainter objects than the HRS, and can study

these objects across a much wider spectral range -- from the UV (1150

Angstroms) through the visible red and the near-IR (8000 Angstroms).

 

The FOS uses two 512-element Digicon sensors (light intensifiers) to

light. The "blue" tube is sensitive from 1150 to 5500 Angstroms (UV to

yellow). The "red" tube is sensitive from 1800 to 8000 Angstroms

(longer UV through red). Light can enter the FOS through any of 11

different apertures from 0.1 to about 1.0 arc-seconds in diameter.

There are also two occulting devices to block out light from the center

of an object while allowing the light from just outside the center to

pass on through. This could allow analysis of the shells of gas around

red giant stars of the faint galaxies around a quasar.

 

The FOS has two modes of operation PP low resolution and high

resolution. At low resolution, it can reach 26th magnitude in one hour

with a resolving power of 250. At high resolution, the FOS can reach

only 22nd magnitude in an hour (before S/N becomes a problem), but the

resolving power is increased to 1300.

 

 

Goddard High Resolution Spectrograph

 

The High Resolution Spectrograph also separates incoming light into its

spectral components so that the composition, temperature, motion, and

other chemical and physical properties of the objects can be analyzed.

The HRS contrasts with the FOS in that it concentrates entirely on UV

spectroscopy and trades the extremely faint objects for the ability to

analyze very fine spectral detail. Like the FOS, the HRS uses two

521-channel Digicon electronic light detectors, but the detectors of

the HRS are deliberately blind to visible light. One tube is sensitive

from 1050 to 1700 Angstroms; while the other is sensitive from 1150 to

3200 Angstroms.

 

The HRS also has three resolution modes: low, medium, and high. "Low

resolution" for the HRS is 2000 -- higher than the best resolution

available on the FOS. Examining a feature at 1200 Angstroms, the HRS

can resolve detail of 0.6 Angstroms and can examine objects down to

19th magnitude. At medium resolution of 20,000; that same spectral

feature at 1200 Angstroms can be seen in detail down to 0.06 Angstroms,

but the object must be brighter than 16th magnitude to be studied.

High resolution for the HRS is 100,000; allowing a spectral line at

1200 Angstroms to be resolved down to 0.012 Angstroms. However, "high

resolution" can be applied only to objects of 14th magnitude or

brighter. The HRS can also discriminate between variation in light

from ojbects as rapid as 100 milliseconds apart.

 

 

Mission Operations and Observations

 

Although HST operates around the clock, not all of its time is spent

observing. Each orbit lasts about 95 minutes, with time allocated for

housekeeping functions and for observations. "Housekeeping" functions

includes turning the telescope to acquire a new target, or avoid the

Sun or Moon, switching communications antennas and data transmission

modes, receiving command loads and downlinking data, calibrating and

similar activities.

 

When STScI completes its master observing plan, the schedule is

forwarded to Goddard's Space Telescope Operations Control Center

(STOCC), where the science and housekeeping plans are merged into a

detailed operations schedule. Each event is translated into a series

of commands to be sent to the onboard computers. Computer loads are

uplinked several times a day to keep the telescope operating

efficiently.

 

When possible two scientific instruments are used simultaneously to

observe adjacent target regions of the sky. For example, while a

spectrograph is focused on a chosen star or nebula, the WF/PC

(pronounced "wiff-pik") can image a sky region offset slightly from the

main viewing target. During observations the Fine Guidance Sensors

(FGS) track their respective guide stars to keep the telescope pointed

steadily at the right target.

 

If an astronomer desires to be present during the observation, there is

a console at STScI and another at the STOCC, where monitors display

images or other data as the observations occurs. Some limited

real-time commanding for target acquisition or filter changing is

performed at these stations, if the observation program has been set up

to allow for it, but spontaneous control is not possible.

 

Engineering and scientific data from HST, as well as uplinked

operational commands, are transmitted through the Tracking Data Relay

Satellite (TDRS) system and its companion ground station at White

Sands, New Mexico. Up to 24 hours of commands can be stored in the

onboard computers. Data can be broadcast from HST to the ground

stations immediately or stored on tape and downlinked later.

 

The observer on the ground can examine the "raw" images and other data

within a few minutes for a quick-look analysis. Within 24 hours, GSFC

formats the data for delivery to the STScI. STScI is responsible for

data processing (calibration, editing, distribution, and maintenance of

the data for the scientific community).

 

Competition is keen for HST observing time. Only one of every ten

proposals is accepted. This unique space-based observatory is operated

as an international research center; as a resource for astronomers

world-wide.