Ecological Niches in Infrared and Sub-Millimeter Space Astronomy: Expected Sensitivity as a Function of Observatory Parameters

Details Ecological Niches in Infrared and Sub-Millimeter Space Astronomy: Expected Sensitivity as a Function of Observatory Parameters Ecological Niches in Infrared and Sub-Millimeter Space Astronomy: Expected Sensitivity as a Function of Observatory Parameters

Nov 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995pasp..107.1099t&link_type=abstract

Publications of the Astronomical Society of the Pacific, v.107, p.1099

Astronomy and Astrophysics

Astronomy

3

Telescopes, Instrumentation: Miscellaneous

Scientific paper

Using current estimates of the celestial and atmospheric background emission, infrared source crowding, and detector performance, we estimate expected point source sensitivities for possible future infrared and sub-millimeter observatories on the ground, in the air, and in space. Our goal is to evaluate the effects of variations in basic observatory system parameters for typical operation over the wavelength range labmda ~3 - 1000 microns. For the first time in a general astronomical journal, we evaluate mission design goals for the wavelength range identified as the premier for this decade. Here we emphasize the effects of telescope temperature, aperture, and emissivity upon detectable point source signal level using diffraction-limited instrumentation, along with approximations to improvements in sensitivities using the new generation of array detectors. We find that [1] for broadband imaging, a larger aperture is more important than extremely low optical system temperature in detecting weak sources in two situations: (1) at short wavelengths, where a telescope need only be cooled to several tens of kelvins for the optical system emission to fall below that of the celestial background, and (2) at far-infrared and sub-millimeter wavelengths, where interstellar "cirrus" and extragalactic confusion eventually determine the limits to sensitivity, which, below some critical temperature, can only be improved upon by increasing the effective angular resolution. [2] for moderate-resolution spectroscopy (lambda/delta-lambda ~30 - 2000), the faintest signal level is always achieved when the telescope temperature is low enough so that the optical system contributes less than the celestial background. This temperature declines as the wavelength increases, so, for example, T <~ 10 K for lbmad >~ 100 microns. However, for sub-millimeter wavelengths, on or near the Rayleigh-Jeans side of the optical system emission, sensitivity increases less strongly with declining temperature. Consequently, large, warm (ground-based and airborne) sub-millimeter telescopes compete favorably with small cold ones in space and those windows accessible from within the Earth's atmosphere. [3] for high-resolution spectroscopy {lambda/delta-lambda >~ 10^5+), the thermal background from sky and optical system can be low enough that detector noise limits achievable sensitivity. If so, the telescope temperature need not be extremely low and light-gathering aperature is relatively more important. However, to take full advantage of reduced thermal background via high spectral resolution, detector performance may need to be near optimum. [4] conversely, in many circumstances, current and near-future detectors are over-specified in key aspects of operation compared to the high background of the far-infrared. Under such circumstances, detectors may be operated in non-optimum conditions without significant adverse effects upon overall system sensitivity. (SECTION: Astronomical Instrumentation)

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