This summary presents estimates of the sensitivities of the most commonly-used AAT instruments, and aims to provide the basic data needed to calculate exposure times for different observations. The information may be useful when deciding how many nights of telescope time to apply for, or when planning an observing run. AAT staff members also use these numbers when providing the technical assessments of observing requests required by ATAC and PATT. Further details for most instruments are given in the relevant user manuals.
The numbers are intended only as a guide, and the actual count rates measured by observers at the telescope will depend on the exact details of the instrumental setup and atmospheric conditions. A series of plots showing the transmission through a rectangular slit as a function of seeing FWHM can be found in Diego (1987, PASP, 97, 1209). While no table of the atmospheric extinction as a function of wavelength appropriate to Siding Spring Observatory has been published, the values measured at CTIO and given in Table 3 of Stone and Baldwin (1983, MNRAS, 204, 347) are adequate for the present purpose. Typical values for the sky brightness at Siding Spring Observatory are as follows (in magnitudes/sq.arcsec):
The moonlit sky will be brighter than the above values
at small angular
distances from the moon and in cloudy conditions.
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