This form allows you to calculate ULTRACAM count rates and signal-to-noise ratios for the WHT, VLT and NTT. The WHT calculation is based on measured zero points for the u', g', r', i' and z' filters and should therefore be accurate to approximately 5% in object counts. The VLT and NTT calculations are based on the WHT values, but scaled to take into account the different telescope apertures, the extra reflection at Nasmyth and the different pixel scales. The VLT and NTT sky brightness, extinction and telescope throughput are therefore assumed to be identical to the WHT, reducing the accuracy of the results.

If you don't know the SDSS magnitude of your target, you can use the formulae in Table 7 of Smith et al. (2002, AJ, 123, 2121; astro-ph/0201143) or this little f77 program to convert from UBVRI to u'g'r'i'z' magnitudes.

filter   u'         g'         r'         i'         z'      
object magnitude
exposure time  secs
sky brightness   dark         grey         bright      
seeing  arcsecs
binning factor
readout speed   fast (fdd)        slow (cdd)