Astronomical Instrumentation

Astronomers use a wide variety of instruments to detect photons with energies from GeV (gamma rays) to micro-eV (radio). Here we will mention a few of the instruments most relevant to this course. Much of this consists of links to other websites.

The instruments discussed here are:

The purpose of this page is not to provide full technical details of the instruments, but rather to provide overviews and links to relevant sites. General background on how instruments work is available from a number of sources, such as

The Hubble Space Telescope

The Hubble Space Telescope (HST) is a 2.4m Cassegrain telescope in low Earth orbit. The HST is operated by the Space Telescope Science Institute (STScI). An overview of the telescope is given here, and a more detailed description is also available. The telescope optics are diffraction-limited. The mirrors are coated with MgF, which has no reflectivity shortward of 1150Å. The HST is designed to be serviced in orbit; to date 2 very successful servicing missions (SM) have been used to replace old instruments with improved (or at least different) instruments, refurbish failing hardware, and correct the notorious problem with the spherical aberration of the primary mirror.

The HST is the final result of the vision of Lyman Spitzer, who described, in 1946, the benefits of putting a telescope in space (see the 1991 August 10 issue of the Astrophysical Journal Letters, 377, i, which is dedicated to Lyman Spitzer). A telescope on the ground suffers from three major problems:

By getting above the atmosphere, one can observe all wavelengths, realize diffraction-limited imaging, and reduce the background noise. The first has obvious benefits. Diffraction-limited imaging not only lets one resolve fine details, but also lets one see faint objects because the sky background is resolved out. The reduced background in space also lets one detect fainter objects than can be detected from the ground.

The HST carries a number of scientific instruments. These are:

Most data from the HST remains proprietary for a 52 week period from the time the proposer gets the data (some data become public more quickly). After that time, the data are available through the HST archives.

The International Ultraviolet Explorer

The International Ultraviolet Explorer (IUE) was the little satellite that could. It operated for nearly 20 years before being turned off for lack of funding. Go here for a good description of the satellite and its cababilities (scroll about half way down the page to the Additional Information section).

The IUE database forma a large and uniform spectroscopic archive with over 70,000 images. All the data are available to the public. The data can be downloaded either from NDADS, the National Data Archival and Distribution System, or through MAST, the Multimission Archive at STScI.


The Extreme UltraViolet Explorer is a spectroscopic and photometric satellite operating in the 70-760Å band. This region has been little explored, both because of technical difficulties with detectors in this region, and because interstellar extinction is large at these wavelengths, and the number of expected EUV sources was small.

For first 6 months of its mission, EUVE scanned the sky, and produced a EUVE Bright Source List. All the EUVE catalogs and data are available via the HEASARC EUVE archive or the MAST EUVE archive.

EUVE spectroscopic data, from the DS/S instrument, is the first true X-ray spectroscopy in the 70-400Å range. The DS/S uses grazing incidence diffraction gratings to achieve a resolving power E/dE of 300. EUVE spectra clearly show the coronal emission lines of Fe IX through Fe XXIV is active stars, as well as the continuum emission of cataclysmic variables and hot stars.

EUVE data become public one year after receipt by the original observer, and are available through MAST, the Multimission Archive at STScI.


ROSAT, short for Röntgensatellit, which is German for X-ray satellite, is just that, an astronomical satellite sensitive to soft X-rays (0.1-2.4 keV). A detailed overview of the mission is available from the ROSAT center at the Max Planck Institut für Extraterrestrische Physik (MPE). Another source of information is the ROSAT Guest Observer Facility at the Goddard Space Flight Center.

ROSAT features two X-ray instruments, a position-sensitive proportional counter (PSPC) and a High Resolution Imager (HRI). Both of these utilize the focal plane of a grazing-incidence Wolter-Type I telescope.

The PSPC has a 2o circular field of view, with about 10 arcsec spatial resolution on-axis. There is severe vignetting off-axis. It has about 60% energy resolution, near the theoretical maximum for a high voltage proportional counter.

The HRI is a microchannel plate array. It has almost no intrinsic energy resolution, but has higher spatial resolution (about 3 arcsec) than does the PSPC, but over a 38 arcmin square field of view.

In addition, there is a wide field camera (WFC) which operates at EUV wavelengths. It has its own telescope.

ROSAT PSPC and HRI data become public one year after receipt by the original observer, and are available via the ROSAT Archive at the HEASARC, or from the MPE archive.

For the first six months of its mission, ROSAT performed an all-sky survey (the RASS) using the PSPC. The first of its data products is the RASS Bright Source Catalog. The sky survey data remain proprietary to MPE.

Since to the survey, ROSAT has been used in pointed mode. It is these pointings that are archived and available to the public. A summary list of sources detected in the PSPC pointings is given in the WGA Catalog of ROSAT Point Soutces. Further derivative data are available from the ROSAT Results Archive. These catalogs are also maintained at the MPE site - check the options under Databases.


The Advanced Satellite for Cosmology and Astrophysics is an imaging X-ray telescope with an emphasis on intermediate-resolution spectroscopy over the 0.4-10 keV energy range. ASCA is a cooperative program between the US and Japan.

ASCA has 4 thin-foil telescopes, designed for medium spatial resolution but large effective area at low cost. Two of the telescopes feed Solid State Imaging Spectrographs (SIS); the others feed the Gas Imaging Spectrometers (GIS). The SIS detectors are front-illuminated CCDs. These achieve 2% energy resolution at 6 keV, with about 1 arcmin spatial resolution over a 22 arcmin square field. The GIS detectors are imaging gas scintillation proportional counters. These can achieve an energy resolution of 8% at 6 keV, and are the more sensitive of the two sets of detectors at energies above about 5 keV. The field of view of the GIS is 50 arcmin.

ASCA data become public one year after receipt by the original observer, and are available through the ASCA Archive at the HEASARC.

An ASCA SIS Source catalog is available at the HEASARC.


The Rossi X-Ray Timing Explorer (RXTE) is an X-ray light bucket, with an enormous collecting area but very limited spatial resolution (See here for another introduction to the RXTE). The RXTE uses proportional counters and scintillators to obtain high-time-resolution data on X-ray sources in the 2-250 keV range. The instruments are the

Note that RXTE data are currently archived in a complicated data format (yes, it's FITS format, but it is not straightforward to extract the useful data). If you plan to work witn RXTE data, you should plan to use the FTOOLS software package.


See the Compton Gamma Ray Observatory(CGRO) Science Support Center web page. The CGRO contains 4 scientific instruments to investigate the universe at gamma-ray energies (greater than about 100 keV). These are the

These data are not easy to analyze, for technical reasons, and novices are dissuaded from trying to use then during the short (3.5 week) lab session. However, feel free to try, but read the documentation at these sites first!


The CHANDRA X-ray observatory (formerly AXAF, the Advanced X-ray Astronomy Facility) was successfully placed into orbit in July 1999. CHANDRA is the third in NASA's series of Great Observatories, and is the X-ray equivalent of the Hubble Space Telescope. It will return images with 0.5 arcsec spatial resolution, and has a transmission grating spectrometer to get spectra with resolutions up to 1000. The detectors are ACIS, the AXAF CCD Imaging Spectrograph, and HRC, the High Resolution Camera.

Orbital checkout/science verification is now underway. There are no data in the archives as of September 1999 - but come back next year.


FUSE, the Far Ultraviolet Spectroscopic Explorer, was placed into orbit on June 24 1999. FUSE is designed to obtain high resolution (R = 30,000) spectra in the astrophysically-important, but obervationally-difficult 912-1200 Angstrom region.

FUSE is currently undergoing orbital checkout and science verification. There are no data in the archives as of September 1999 - but come back next year.

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