UCL Echelle Spectrograph Signal-to-Noise Calculator V3.0

Points to note:

• The EEV 2K x 4K CCD was commissioned on UCLES in July 2001, and should now be the detector of choice for most proposals when working blueward of H-alpha. For proposals concentrating on wavelengths redward of H-alpha, or requiring complete coverage from blue to red, the MITLL3A 2K x 4K CCD is to be preferred. The MITLL2A CCD has been withdrawn from service. The Tek 1K CCD is still included in this calculator, but it has been effectively superseded by the EEV CCD.
• XTRASLOW readout speed is not available for the EEV CCD.
• Magnitude is the magnitude of your source at the wavelength of observation. For a wavelength of 3500 Å, use the U magnitude; for wavelengths of 4000 or 4500 Å, use the B magnitude; for wavelengths of 5000 or 5500 Å, use the V magnitude; and for wavelengths of 6000 or 6500 Å, use the R magnitude.
• Binning indicates whether on-chip binning in the spatial direction will be used. Binning on-chip will decrease the contribution of Nrn. Note that the MITLL3 cannot be binned.
• Npix is the number of binned spatial pixels that will be summed over. Increasing Npix will generally increase Nobj, but the signal-to-noise may not necessarily improve, due to the increase in Nsky. Note that the MITLL3 cannot be binned.
• Nobj is the number of ADU per spatially-binned pixel from the object, summed over all the exposures, at the blaze peak nearest the wavelength of interest. To work out the equivalent number of electrons expected, multiply by the gain appropriate for the CCD and readout speed in use.
• Nsky is the number of ADUs per binned pixel from the sky (above comments for Nobj apply also to Nsky).
• Nrn is the detector noise in ADUs.
• Please use the AAT median seeing of 1.5" for proposals. If your project requires you to assume 1.0" seeing (or better), you must justify this in your proposal, and provide a backup program for poor seeing.
• Sensitivities are not currently well-determined for wavelengths outside the range adopted here. In any case, the actual signal-to-noise ratio will be quite heavily influenced by atmospheric absorption (below ~3600 Å), and by airglow (above ~7000 Å).
• The signal-to-noise ratio is calculated per spatially-binned pixel. Since the EEV CCD has pixels which are 10% smaller than the MITLL CCDs (13.5 microns instead of 15), then you may still come out ahead using the EEV CCD if the S/N ratio per pixel is comparable with the MITLL3.
• A 1" slit projects to 54 microns at the detector in the dispersion direction (without the focal modifier). This equates to 3.6 MITLL3 pixels, and 4 EEV pixels. Higher resolution (up to R=100,000 or more) is now achievable with the MITLL3 and EEV detectors, but only by closing the slit down to 0.5" (which in typical seeing at Siding Spring costs you an awful lot of light!).
• Single integrations in excess of 1800 seconds are not recommended, due to the increasing covering factor of cosmic rays. When you break up an integration of several thousand seconds into a number of shorter exposures, the final signal-to-noise ratio will always be less than for the equivalent single exposure, owing to the fixed readout noise contribution from each exposure.
• If the detector is centered on a different wavelength or order, then the counts may be reduced by camera vignetting. Additionally, use of the beam rotator will further reduce the throughput by ~5%.
• The reduction in throughput with the Iodine cell in place is ~30% (~0.4 mag) and should be taken into account when determining your exposure times.