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GMOS Spectroscopic Sensitivity Estimates

Brightnesses of point sources and extended sources giving signal-to-noise ratios (S/N) of 5 in a one-hour integration with GMOS are given in the table below. They were estimated using the the GMOS Integration Time Calculator (ITC) and apply to the grating and filter combinations given in the table.

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 per (binned) pixel in the spectral direction in a total integration time of 1 hour, at the wavelengths given in the column "Wavelength". Spectral binning was chosen to give 3-4 pixels per resolution element. A spatial binning factor of 2 was used, which gives sufficient sampling for the median image quality. The ITC calculates the S/N in a spatial 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 spatial aperture. For uniform surface brightness sources, an aperture with an area of 1 square arcsec is used.

The 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 and cosmic-ray-events.

The sky subtraction aperture is assumed to the 5 times larger than the object aperture. It is assumed that the GMOS slit length is sufficient to give derive the sky from the on-source exposures.

The long-pass filters GG455 and OG515 may be used to eliminate 2nd order contamination for gratings B600_G5303 and R150_G5306. The table below shows the effect in throughput for R150_G5306 used with OG515. The SDSS filters g', r', i' and z' may be used to limit the spectral range and make it possible to use multiple "banks" of slit-lets in MOS mode. The table below shows the effect in throughput for R400_G5305 used with the r' filter.

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).

Grating Filter	ITC standard passband	Wavelength (nm)	Binning (spectral x spatial)	(Binned) Spectra pixel size (nm)	Spectra resolution (nm)	Slit width (arcsec)	Point Sources	Extended Sources
Grating Filter	ITC standard passband	Wavelength (nm)	Binning (spectral x spatial)	(Binned) Spectra pixel size (nm)	Spectra resolution (nm)	Slit width (arcsec)	(mag)	(mag/arcsec²)
R150_G5306 none	R	400	2 x 2	0.35	1.14	0.50	21.8	21.8
		600	2 x 2	0.35	1.14	0.50	22.8	22.8
		800	2 x 2	0.35	1.14	0.50	21.9	21.9
		400	4 x 2	0.70	2.28	1.00	22.4	22.4
		600	4 x 2	0.70	2.28	1.00	23.2	23.2
		800	4 x 2	0.70	2.28	1.00	22.4	22.4
R150_G5306 OG515_G0306	R	600	2 x 2	0.35	1.14	0.50	22.7	22.7
R150_G5306 OG515_G0306	R	800	2 x 2	0.35	1.14	0.50	21.7	21.7
R400_G5305 none	R	600	1 x 2	0.085	0.40	0.50	21.4	21.4
		800	1 x 2	0.085	0.40	0.50	20.8	20.8
		600	2 x 2	0.17	0.80	1.00	22.1	22.1
		800	2 x 2	0.17	0.80	1.00	21.5	21.5
R400_G5305 r_G0303	R	600	1 x 2	0.085	0.40	0.50	21.3	21.3
B600_G5303 none	R	400	2 x 2	0.09	0.27	0.50	21.4	21.4
		500	2 x 2	0.09	0.27	0.50	22.2	22.2
		600	2 x 2	0.09	0.27	0.50	21.8	21.8
		400	4 x 2	0.18	0.54	1.00	22.1	22.1
		500	4 x 2	0.18	0.54	1.00	22.8	22.8
		600	4 x 2	0.18	0.54	1.00	22.5	22.5

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