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

Wed Sep 17 12:36:20 BST 1997