This form allows you to calculate ULTRASPEC object counts, sky counts and signal-to-noise ratios when imaging on the TNT and NTT. The calculation is based on measured zero points for the u', g', r', i' and z' filters. The form also tells you when you are in danger of saturating the avalanche register of the EMCCD, which can significantly shorten the lifetime of the detector.

The following assumptions are made:
  • Avalanche gain = 9 (i.e. the highest possible value) is used whenever the avalanche output is selected.
  • Binning factors in x and y are always the same when imaging.
  • Proportional mode, rather than photon-counting mode, is assumed when calculating the signal-to-noise ratio. In theory, a factor of 1.4 improvement in the signal-to-noise ratio is achievable when photon counting, but in reality the improvement is more marginal than this.
  • A single electron entering the avalanche register results in a distribution of electrons at the output with mean value ~1200. The distribution is an exponential, hence the probability of obtaining an amplification n times higher than the mean is given by e-n. A value of 5 for n is adopted for safety, which will occur once in every ~100 amplifications.
  • The peak signal per pixel in a seeing disc is calculated assuming a two-dimensional Gaussian profile.
  • The number of pixels covered by the seeing disc is calculated assuming a circle of radius 1.5 times the seeing.
  • Extinction and sky bightness values have been taken from the ING ETC: signal

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.

A note on gain in avalanche mode. I use the following definition:

system gain (e-/ADU) = electronic gain (e-/ADU) / avalanche gain

  • electronic gain (e-/ADU). Measured from avalanche-output flats taken with no avalanche gain. The number of ADU that each electron measured at the output is converted into. This is the same as gain measured in a conventional CCD.
  • avalanche gain (no units). The factor by which an electron entering the avalanche register is multiplied by when it gets to the output.
  • system gain (e-/ADU). Measured from bias frames with avalanche gain. The number of ADU that each photo-electron is converted into after passing through the avalanche register.

telescope   TNT         NTT      
filter   u'         g'         r'         i'         z'      
object magnitude
exposure time  secs
sky brightness   dark         grey         bright      
seeing  arcsecs
binning factor
readout mode   normal         avalanche      
readout speed   slow         medium         fast