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Sensitivity Table (Imaging)

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.

The ITC help pages include information on the filter zero points, calculation methods, and there is also information about 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 was 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 into 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 be 5 times larger than the object aperture.

This table was derived for GMOS North with the original e2v detectors. GMOS South gives similar results (with the latter having slightly better sensitivity in the UV and blue). The table was based on aluminum primary and secondary mirror coatings. Please use the appropriate integration time calculator for specific results (the ITC defaults to the current silver coatings). 

Filter Central wavelength
[nm]
ITC standard
passband
Point Sources Extended Sources Exposure time
(mag) (mag/arcsec2) (sec)
g' 475 V 26.5 27.0 4 x 900
r' 630 R 26.1 26.6 4 x 900
i' 780 I 25.5 25.9 4 x 900
z' 950 I 24.3 24.6 4 x 900

New! for semester 2011B: This second table was derived for GMOS North as expected after the focal plane is upgraded to Hamamatsu CCDs. Sensitivities have been calculated for both the Hamamatsu Red and Blue CCD types; the single Hamamatsu Blue CCD covers the rightmost 1.37 arcmin x 5.5 arcmin portion of the imaging field of view. Note - the apparent slight improvement in g' and r' for the Hamamatsu Red CCDs is an artifact of the silver mirror coatings now assumed by the ITC, as the Hamamatsu Red and ev2 detectors for GMOS-N have essentially equivalent QE blueward of 650 nm.

Filter Central wavelength
[nm]
ITC standard
passband
Point Sources
[mag]
Extended Sources
[mag/arcsec2]
Exposure time
[sec]
Hamamatsu Red Hamamatsu Blue Hamamatsu Red Hamamatsu Blue
g' 475 V 26.59 26.72 27.04 27.17 4 x 900
r' 630 R 26.19 26.22 26.62 26.65 4 x 900
i' 780 I 25.76 25.74 26.16 26.15 4 x 900
Z 875 I 24.87 24.83 25.25 25.22 4 x 900
Y 1010 I 23.42 23.35 23.69 23.62 4 x 900