This form allows you to calculate ULTRASPEC object counts, sky
counts and signal-to-noise ratios when performing
spectroscopy with EFOSC2 on the NTT. The calculation is
based on measured zero points in u', g', r', i' and z'. 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:
- No order-sorting filters are used.
- Avalanche gain = 9 (i.e. the highest possible value) is used whenever
the avalanche output is selected.
- The stellar magnitude given is appropriate for the
central wavelength of the grism being used.
- Proportional mode, rather than photon-counting mode, is used 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 ~1750. 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 amount of light entering the slit is calculated assuming
a two-dimensional Gaussian profile and takes into account the
wavelength-dependent change in profile as a function of airmass and
angle from parallactic. Uses Chris Benn's
- The peak signal per pixel in the spatial direction is calculated
assuming a one-dimensional Gaussian profile.
- The number of pixels covered by the seeing profile is calculated
by assuming it has a FWZI of 3 times the seeing.
- Extinction and sky bightness values have been taken from the ING ETC:
Details of the available grisms are given here.
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.