The estimate of sensitivity for large infrared telescopes based on measured sky brightness and atmospheric extinction

Abstract

In order to evaluate the ground-based infrared telescope sensitivity affected by the noise from the atmosphere, instruments and detectors, we construct a sensitivity model that can calculate limiting magnitudes and signal-to-noise ratio ($S/N$). The model is tested with tentative measurements of ${\mathrm{M}}^{\prime }$-band sky brightness and atmospheric extinction obtained at the Ali and Daocheng sites. We find that the noise caused by an excellent scientific detector and instruments at $-135^{\circ }\mathrm{C}$ can be ignored compared to the ${\mathrm{M}}^{\prime }$-band sky background noise. Thus, when $S/N=3$ and total exposure time is 1 second for 10 m telescopes, the magnitude limited by the atmosphere is $13.01^{\mathrm{m}}$ at Ali and $12.96^{\mathrm{m}}$ at Daocheng. Even under less-than-ideal circumstances, i.e., the readout noise of a deep cryogenic detector is less than $200e^{-}$ and the instruments are cooled to below $-87.2^{\circ }\mathrm{C}$, the above magnitudes decrease by $0.056^{\mathrm{m}}$ at most. Therefore, according to observational requirements with a large telescope in a given infrared band, astronomers can use this sensitivity model as a tool for guiding site surveys, detector selection and instrumental thermal-control.

13 pages, 12 figures. Accepted for publication in Research in Astronomy and Astrophysics

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