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Overall performance of the system

Table 8 gives sensitivity values for ISIS and FOS-II measured from wide slit observations of spectrophotometric standards, in the case of ISIS with the low dispersion (158 line/mm) gratings. These measurements give the overall performance for the entire system, including atmosphere, telescope, spectrograph and detectors. The performance is expressed as a magnitude (AB magnitude in Oke's system; see for instance Oke, Astrophys. J. suppl. 27, 21, 1974) which gives a measured count rate of 1 photon/second/Å at the detector. When comparing these values with observed count rates measured from CCD data it is important to convert to detected photons using the correct ``gain'' (electrons/ADU) factor, and to subtract the correct bias value. When comparing with data measured of the DMS display using the X-CUT and Y-CUT functions in the STATISTICS menu it is also important to remember that the DMS averages CCD data, it does not sum it. On the other hand for IPCS data the DMS does sum.

Fri Jan 7 15:34:48 GMT 1994