ECOLOGICAL NICHES IN INFRARED AND SUBMILLIMETER SPACE ASTRONOMY - EXPECTED SENSITIVITY AS A FUNCTION OF OBSERVATORY PARAMETERS

Using current estimates of the celestial and atmospheric background em ission, infrared source crowding, and detector performance, we estimat e expected point-source sensitivities for possible future infrared and submillimeter 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 lam bda similar to 3-1000 mu m. For the first time in a general astronomic al journal, we evaluate mission design goals for the wavelength range identified as the premier for this decade. Here we emphasize the effec ts of telescope temperature, aperture, and emissivity upon detectable point-source signal level using diffraction-limited instrumentation, a long with approximations to improvements in sensitivities using the ne w generation of array detectors. We find that: [1] For broad-band imag ing, a larger aperture is more important than extremely low optical-sy stem temperature in detecting weak sources in two situations: (i) at s hort wavelengths, where a telescope need only be cooled to several ten s of kelvins for the optical-system emission to fall below that of the celestial background, and (ii) at far-infrared and submillimeter wave lengths, where interstellar ''cirrus'' and extragalactic confusion eve ntually determine the limits to sensitivity, which, below some critica l temperature, can only be improved upon by increasing the effective a ngular resolution. [2] For moderate-resolution spectroscopy (lambda/De lta lambda similar to 30-2000), the faintest signal level is always ac hieved when the telescope temperature is low enough so that the optica l system contributes less than the celestial background. This temperat ure declines as the wavelength increases, so, for example, T less than or similar to 10 K for lambda greater than or similar to 100 mu m. Ho wever, for submillimeter wavelengths, on or near the Rayleigh-Jeans si de of the optical system emission, sensitivity increases less strongly with declining temperature. Consequently, large, warm (ground-based a nd airborne) submillimeter telescopes compete favorably with small col d ones in space in those windows accessible from within the Earth's at mosphere. [3] For high-resolution spectroscopy (lambda/Delta lambda gr eater than or similar to 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 l ow and light-gathering aperture is relatively more important. However, to take full advantage of reduced thermal background via high spectra l resolution, detector performance may need to be near optimum. [4] Co nversely, in many circumstances, current and near-future detectors are over-specified in key aspects of operation compared to the high backg round of the far infrared. Under such circumstances, detectors may be operated in non-optimum conditions without significant adverse effects upon overall system sensitivity.