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
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
- Kitchin, C.R., Astrophysical Techniques (Bristol:Adam Hilger). A
comprehensive general introduction to astronomical instrumentation.
- Longair, M.S., High Energy Astrophysics (Cambridge). Chapters
6 to 8 give a slightly dated description of instrumentation, with an emphasis
on X-ray and gamma-ray astronomy techniques.
- Lena, P., Observational Astrophysics (Springer-Verlag). Chapter
5 is a good, but terse, description of astronomical detectors and how they
- The best overall reference for astronomical CCD detectors, how they work,
and how to
use them, is Astronomical CCD Observing and Reduction Techniques,
Volume 23 of the ASP Conference Series, edited by S.B. Howell
(San Francisco: Astronomical Societry of the Pacific).
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
- The atmosphere absorbs much of the infra-red radiation, and all of the
ultraviolet, X-ray, and gamma-ray radiation. There is significant
attenuation even at optical wavelengths.
See here for details.
- Atmospheric seeing, the blurring of an image due to atmospheric turbulence,
is the main limit on the resolution of a telescope. For an ideal telescope,
the angular resolution, given by the Rayleigh criterion, is
1.22 lambda/D radians, where lambda is the wavelength of the
light and D is the diameter of the telescope. Thus for a 2.4m telescope
like the HST, operating at 5000Å, the angular resolution is
0.05 arcsec. Atmospheric seeing limits the
resolution to about 0.3 arcsec at the very best sites with active
controls, but more typically the seeing is of order 1 arcsec.
- The night sky is not completely dark, because of reflected light from
aerosols and dust in the atmosphere, as well as thermal emission in the
infrared. While the sky above the atmosphere
isn't completely dark either (due to scattered light from interplanetary
dust, for example), it is a few magnitudes fainter than you can get from
the surface of the Earth.
Details are here.
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:
- WFPC2, the
Field and Planetary Camera 2, is a CCD camera which operated between 1150 and
11,000Å. Spatial resolution is 0.05 arcsec in the PC chip. There are a
number of filters.
- STIS, the Space Telescope Imaging
Spectrograph, combines CCD detectors for high sensitivity in the
optical and photon-counting MultiAnode Microchannel Array (MAMA)
detectors in the ultraviolet. STIS offers spectral resolutions between 1000
- NICMOS, the
Infrared Camera and MultiObject Spectrograph, is the only instrument
currently operating in the near-IR, at wavelengths between 1.1 and 2.2 microns.
- FOC, the
Object Camera, is a high spatial resolution, UV sensitive photon counting
- FGS, the
Guidance Sensors. Although these are primarily used to maintain pointing
telescope, they can also be used for astrometric observations, to measure the
precise positions of stars and separations between binary stars.
- GHRS, the Goddard
High Resolution Spectrograph was a high resolution UV spectrograph. It was
removed from the HST during SM2.
- FOS, the Faint Object
Spectrograph, was a low resolution UV-optical spectrograph. It was
removed from the HST during SM2.
- HSP, the High Speed
Photometer was a very sensitive photon-counting photometer which was
removed during SM1.
- WF/PC, theWide
Field and Planetary Camera was the predecessor to the WFPC2. It was affected
by the spherical aberration. It was replaced by WFPC2 in SM1.
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
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
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.
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
EUVE archive or the MAST EUVE
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
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
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
Observer Facility at the Goddard Space Flight Center.
ROSAT features two X-ray instruments, a position-sensitive
counter (PSPC) and a High Resolution Imager (HRI).
Both of these utilize the
focal plane of a grazing-incidence Wolter-Type I telescope.
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
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
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
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
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
Imaging Spectrographs (SIS);
the others feed the
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.
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
Archive at the HEASARC.
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
Counter Array (PCA), which has a 6500 cm2 collecting area
for 2-60 keV photons. The PCA has 1 microsecond time resolution.
- High Energy
X-ray Timing Experiment (HEXTE), which provides 8 microsecond time
resolution between 12 and 250 keV, using NaI/CsI scintillators.
Monitor (ASM), which scans 80% of the sky every 90 minutes, to provide
a long term history of the brightness of strong X-ray sources, and to
locate transient X-ray sources. Light curves from the
ASM are available at the
ASM Light Curves Page.
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
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.
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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