GMOS Imaging Sensitivity Estimates

This page presents results from using the GMOS Integration Time Calculator (ITC). The table lists the estimated brightnesses of point sources and uniform surface brightness sources that give a signal-to-noise ratio (S/N) of 5 in a one-hour integration.

Important caveat: these estimates make unverified assumptions about the telescope, camera, and detector performance. GMOS lab tests have been used to guide these estimates, but on-sky commissioning tests will be needed to verify sensitivities.

ITC help pages include information on the filter zero points, calculation methods, and guidelines and approximations specific to the ITC for GMOS.

The ITC includes adjustments for observing conditions in its calculations. For the table presented here, image quality was assumed to be 70%-ile and the other conditions were 50%-ile (median). Specifically, the estimates are for dark time and photometric conditions. The meanings of the observing condition criteria are explained in detail in the Observing Conditions pages. Further, an airmass of less than 1.2 was assumed. Those applying for time on Gemini should use the ITC to make calculations using the most flexible set of criteria possible, as the joint probability of all observing conditions being 50% percentile or better is only 6.2%.

All magnitudes listed below result in a S/N of 5 in a total integration time of 1 hour. The ITC calculates the S/N in an aperture that maximizes S/N given the predicted image quality for the observing conditions and wavelength requested. The sensitivity values in the table below use the optimum ITC aperture. For uniform surface brightness sources, an aperture with an area of 1 square arcsec is used. No binning of the detectors were used for these estimates.

The input magnitudes used for the ITC are in the passband given in the column "ITC standard passband". A spectrum of an A0V star was assumed.

The estimates assume that a 1 hour total exposure time was divided in 4 individual exposures in order to clean the images for bad pixels, cosmic-ray-events and to cover the areas in the gaps between the CCDs.

The sky subtraction aperture is assumed to the 5 times larger than the object aperture.

This table was derived for GMOS North although GMOS South gives similar results (with the latter having slightly better sensitivity in the UV and blue). The table was based on aluminium primary and secondary mirror coatings. Please use the appropriate integration time calculator for specific results (the ITC defaults to the current silver coatings). 

Last update March 2, 2005; Phil Puxley
Previous version March 2, 2001; Inger Jørgensen