ASTRONOMY IN SPACE
by Peter V. Mason, retired, Jet Propulsion Laboratory, and Visiting Associate, California Institute of Technology. Pmason@alumni.caltech.edu
In thinking about the reasons to perform astronomy in space, we first consider the effect of the earth’s atmosphere. On a scale of decreasing energy, gamma rays, cosmic rays, X-rays and the ultraviolet are so energetic that they interact strongly with the elements in the atmosphere, producing lower energy particles that are all we can detect. If we wish to see the primary radiation, we must go to space. In the optical region, the atmosphere is largely transparent, but there is enough interference that the Hubble telescope can see much that is hidden from even the large telescopes on the high peaks of Hawaii and Chile.
As we go into the infrared, the atmosphere becomes increasingly opaque because of interaction with the molecule in the atmosphere. Below 20 micrometers, we are essentially blind until we reach the submillimeter range. Even here, unscrambling the astronomical data from the background requires detection in at least two bands, and preferably three or four.
What are the kinds of instruments that are used in space? Gamma rays are typically captured in cooled solids, and the cooling has largely been in the range of the solid cryogens, i.e. methane, xenon, ammonia, etc. Next we have X-rays, which are also captured in cooled solids, but the detector signal-to-noise ratios benefits greatly from lower temperatures. In fact, the X-ray the detectors aboard the ASTRO-E2 mission to be launched in 2005 are at 65 milliKelvin, which will be the lowest yet in space.
Ultra-violet and optical detectors are not crucially dependent on low temperatures, but benefit from some cooling. If you have friend who is an ardent amateur astronomer (and either well-to-do or willing to sacrifice creature comforts), he will have charge coupled detectors cooled to dry ice temperatures. As we move into the infrared, cooling again becomes crucial. Below 20 microns and into the millimeter range, bolometers are the detector of choice. Their signal-to-noise ratio improves as T-2.5, so halving the temperature improves the S/N by a factor of 5.6. Below 1 mm (300 GHz, for radio types) various semiconductor detectors, coupled to semiconductor amplifiers such as HEMT’s (High Electron Mobility Transistors) have the lowest signal-to-noise ratio. Typically, their performance does not improve below 20 K, so they can be cooled by heat exchange if a lower temperature is also required..
