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:
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.
system gain (e-/ADU) = electronic gain (e-/ADU) / avalanche gain
A note on gain in avalanche mode. I use the following definition:
- 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
- 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.