The Hubble Space Telescope Second Servicing Mission (SM-2)

Space Telescope Imaging Spectrograph (STIS)

The Hubble Space Telescope has provided breathtaking views of our universe -- pictures of remarkable clarity and detail. To understand how the universe really works, astrophysicists need sophisticated and powerful diagnostic tools. Just as a medical doctor requires tools, such as x-ray machines or blood analyzers, to diagnose the physical condition of a patient, the astrophysicist uses a spectrograph to diagnose the physical properties of planets, stars, galaxies and quasars.

Observations with a spectrograph reveal the chemical composition of celestial objects and how their composition changed as the universe evolved. They can tell us how hot a source of light is, how dense it is, and how fast it is moving. For example, a spectrograph can reveal the presence of monster black holes at the centers of galaxies by showing how rapidly stars and gas are moving around a galactic nucleus.

A new, high-tech spectrograph will be installed on the Hubble Space Telescope during the Second Servicing Mission (SM-2) in early 1997. This Space Telescope Imaging Spectrograph (STIS) will give scientists a far more sensitive and efficient diagnostic tool than they have ever had before in space.

What is STIS?

STIS is a two-dimensional imaging spectrograph that spans ultraviolet, visible, and near-infrared wavelengths. A spectrograph is an instrument that uses optical elements called gratings or prisms to separate the light gathered by the telescope into its component colors. This is similar to how raindrops spread out the component colors of sunlight into a rainbow. The detailed changes in brightness of the light from color to color (or wavelength to wavelength) give scientists information about the composition and other physical properties of the celestial source emitting the light or of the intervening gas that absorbs the light.

What kind of science can STIS do?

STIS can search for massive black holes by studying the star and gas dynamics around the centers of galaxies.

STIS can measure the distribution of matter in the universe by studying quasar absorption lines.

STIS can use its high sensitivity and spatial resolution (or ability to detect fine detail) to study stars forming in distant galaxies.

STIS can perform spectroscopic mapping -- giving fine details of planets, nebulae, galaxies and other objects.

STIS can provide physical diagnotics, such as chemical composition, temperature, density, velocity of rotation or internal mass motions in planets, comets, stars, interstellar gas, nebulae, stellar ejecta, galaxies and quasars.

Why use STIS ?

STIS provides unique and powerful spectroscopic capabilities for the HST. It includes all the major capabilities of both the current spectrographs, the Goddard High Resolution Spectrograph (GHRS) and the Faint Object Spectrograph (FOS), and adds new technological capability. The STIS optical design features internal corrective optics to compensate for the HST primary mirror spherical aberration.

The Quick and Efficient STIS

The main advance in STIS is its capability for two-dimensional rather than one-dimensional spectroscopy. For example, it is possible to record the spectrum of many locations in a galaxy simultaneously, rather than observing one location at a time. This means that many regions in a planet's atmosphere or many stars within a galaxy can be recorded in one exposure. STIS can also record a broader span of wavelengths in the spectrum of a star at one time. As a result, STIS is much more efficient at obtaining scientific data than the earlier HST spectrographs.

STIS supports spectroscopy with resolving powers from ~100 to 200,000 from the ultraviolet to the visible (115 to 1000 nanometers (nm)). The resolving power of a spectrograph is the wavelength observed divided by the smallest wavelength difference that can be measured. At its highest resolving power of 200,000, STIS operating at 200 nm, can resolve spectral features 0.001 nm apart.

STIS can simultaneously record the spectra of up to 512 spatially resolved locations within an extended object, such as a galaxy or nebula.

STIS's wavelength coverage in a single exposure is 15 to 35 times that of the GHRS.

STIS's basic operating modes support time integrated spectroscopy and imaging, time resolved spectroscopy, and imaging and target acquisition.

STIS can accomplish both ultraviolet (UV) and optical imaging (180 nm to 500 nm) with a small complement of narrow and broad band filters. The human eye is sensitive to light in the violet (400 nm) to red (700 nm) range, for comparison.

Very high resolution data in the ultraviolet can be obtained by recording each photon (particle of radiation/light) detection event and transmitting it to the ground with location and an event time with accuracy's of up to 150 microseconds.

STIS uses three detectors:

A cesium iodide photocathode Multi-Anode Microchannel Array (MAMA) for 115 to 170 nm

A cesium telluride MAMA for 165 to 310 nm

A Charge Coupled Device (CCD) for 305 to 1,000 nm

All three detectors have a 1,024 x 1,024 pixel format

Who does what?

The Laboratory for Astronomy and Solar Physics at the Goddard Space Flight Center in Greenbelt, Md., heads the development of STIS. The principal investigator is Dr. Bruce E. Woodgate. The prime contractor is Ball Aerospace Systems Group, Boulder, Colo. Following its installation on the HST and calibration, observing time on STIS will be allocated on a competitive basis to scientists throughout the world. The operation of STIS will be managed by the Space Telescope Science Institute in Baltimore, Md.

STIS Physical Characteristics

Weight approx. 700 lbs (318 kg)

Dimensions 7.1 x 2.9 x 2.9 ft (2.2 x 0.89 x 0.89 m)

Field of View

MAMA 25 x 25 arcsec

CCD 50 x 50 arcsec

Pixel format 1024 x 1024

Wavelength range 115 - 1000 nm

For Additional Information Contact:

Tammy Jones

Goddard Space Flight Center

Office of Public Affairs

(301) 286-5566

Internet: http://www.gsfc.nasa.gov

Space Telescope Science Institute

Office of Public Outreach

(410) 338-4707

Internet: http://www.stsci.edu

STIS homepage

http://www.ball.com/aerospace/hst.html

November 1996