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A useful figure of merit for UES is that at 4000 Å an object with AB = 17.0 (defined as usual above the atmosphere), observed through a wide slit (no light losses at the slit), will produce 1 electron/s/Å when observed with the Tektronix CCD. This figure can be converted to a more useful count rate by adopting: i) 67% transmission for the atmosphere at ZD = 30 degrees; ii) 65% transmission through a 1 arcsec wide slit in 1 arcsec FWHM seeing; iii) a pixel size of 0.043 Å\ in the spectral direction. With these parameters, an object with AB = 17.0 will give a count rate of electrons per wavelength bin per 1000 s integration, adding all the counts along the slit. From these guidelines, approximate exposure times for objects of interest can be estimated. Obviously, the final S/N will include contributions from sky subtraction and systematic sources of noise such as read-out, dark current and cosmic rays.

The count rates obtained with the IPCS at 4000 Å are approximately 6 times lower than the above values.

Table gif in Appendix gif lists values of AB magnitude giving 1 electron/s/Å\ for the combination WHT+UES+Tektronix CCD over the full optical range, from 3500 to 8500 Å (the estimated errors are mag).

Tue Aug 15 16:42:46 BST 1995