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Exposure Time Calculator 2.9
Grism / Fiber Band Bandwidth Slitwidth / Fiber diameter "
Source Magnitude FWHM " Single Exp. Time (sec)     Number of Exposures     Binning
Sky Conditions
Airmass Extinction Sky Brightness
         (D, G or B for typical dark, grey or bright)
Graphical output None S/N vs. Exptime S/N vs. Magnitude Peak vs. Exptime
  • The ETC does not yet support 'extended sources' for FIES
  • ALFOSC seeing measurements
  • The ETC uses the following default values for atmospheric extinction: U=0.55, B=0.25, V=0.15, R=0.09, I=0.06, Z=0.05, Y=0.07, J=0.09, H=0.07, Ks=0.08 mag/airmass.
    Use extinction=0 for the ETC to use these values.
  • Darkest La Palma sky: U=22, B=22.7, V=21.9, R=21.0, I=20.
    Use sky-brightness=0 for the ETC to use these values.
  • Moonlight brightens the sky by about 1 mag (first/last quarter) to 4 mag (full moon) in UBV.
  • In I the full moon brightens the sky about 2 mag.
  • The sky level in the near-IR is temporally and spatially variable due to airglow and thermal emission The ETC uses these standard magnitudes: J=15.6, H=13.9, Ks=13.2
  • The calculation is based on the SIGNAL program, kindly provided by Chris Benn (ING). Information and guidelines of usage can be found here
  • S/N estimate includes the object photon noise, sky photon noise and the readout noise. In the case of multiple frames, average combining is assumed for the readout noise.
  • Peak counts in the imaging mode for a point source are calculated assuming a 2-D gaussian point spread function for the point source.
  • Peak counts in the spectroscopy mode are calculated assuming a gaussian profile for the spectrum.
Back to top Last modified: November 29 2022