Table 6 gives sensitivity values for Prime and Auxiliary focus determined from observations of standard stars taken at low airmass on photometric nights. These are expressed as the magnitude for a count rate of one photon/second in a given standard broad band. These numbers can be compared with measured count rates as determined with, for instance, the iraf routine imexamine running on the SPARCstation. Figure 7 shows: A) the S/N achieved in 900 sec as a function of V magnitude, and B) the time required to reach a S/N of 5 (a detection) as a function of V magnitude; the 3 curves in each Figure show the effects of varying the FWHM between 0.5 and 2.0 arcsec. These calculations assume the TEK2 chip, with a pixel scale of 0.42 arcsec/pixel and a R/N of 5 electrons, and a dark sky of V=20.8 mag/arcsec . For example, assuming a FWHM of 1 arcsec, a S/N of 5 at V=25 can be reached in 500 sec of integration. See Appendix C for more details of the La Palma sky.
Table 6: Sensitivity, expressed as broad band magnitude for 1 photon/s, of WHT prime and auxiliary imaging systems
Figure 7: SN Calculations