Ha. Thronson et al., ECOLOGICAL NICHES IN INFRARED AND SUBMILLIMETER SPACE ASTRONOMY - EXPECTED SENSITIVITY AS A FUNCTION OF OBSERVATORY PARAMETERS, Publications of the Astronomical Society of the Pacific, 107(717), 1995, pp. 1099-1118
Citations number
41
Categorie Soggetti
Astronomy & Astrophysics
Journal title
Publications of the Astronomical Society of the Pacific
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.