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The available slits are shown in the following table. The slit widths are given in pixels and arcsec projected on the sky. The spectral resolution is lower in a wider slit (see the grism table), even for point sources since under most atmospheric conditions the seeing wings of pointlike objects fill the wider slits. Note that for each f/6 slit width, 2 slits are available, one of which is offset from the optical axis and causes the spectrum to be shifted to shorter wavelengths on the array. Note also that the f/32 10 pixel wide slit and the f/6 2 pixel wide slit are the same slit.

Slit name
Gemini ID Slit width
Slit width
Slit length
f/6 2-pixel centered G5211 1.94 0.226 50
f/6 4-pixel centered G5212 4.02 0.470 110
f/6 6-pixel centered G5213 6.42 0.750 110
f/6 2-pixel blue G5214 1.94 0.226 50
f/6 4-pixel blue G5215 3.5 0.409 50
f/6 6-pixel blue G5216 6.0 0.696 50
f/32 4-pixel centered* G5229 3.2-4.4* 0.07-0.10 22
f/32 7-pixel centered G5230 6.6 0.144 22
f/32 10-pixel centered** G5211 10 0.22 22
* Width varies monotonically across slit, narrowest width at top.
** This is the f/6 2-pixel wide slit used with the f/32 camera.

Slit throughputs for the three different f/6 slit widths are presented in the following table as a function of image quality. Note that these throughputs are for an S/N-optimized software aperture of 1.4 times the image FWHM and are not based on the total signal within the slit. The shape of the model tip-tilt corrected PSF does not vary signficantly across the 1-5um range and so this table is independent of wavelength. (Of course the delivered image quality does vary with wavelength and is described as part of the observing condition constraints). The values in this table were derived using the NIRI Integration Time Calculator.

Slit width
Throughput as function of image quality (50% EED in arcsec from 0.3-0.9 arcsec)
0.3 0.4 0.5 0.6 0.7 0.8 0.9
0.232 0.58 0.45 0.36 0.30 0.26 0.23 0.19
0.464 0.86 0.73 0.64 0.55 0.49 0.45 0.40
0.696 0.87 0.83 0.80 0.71 0.65 0.62 0.56

Slitless spectroscopy, sometimes used for flux calibration instead of or in addition to imaging, is available with NIRI. The focal plane wheel containing the slits is controlled independently of the wheel containing the grisms. One can therefore image a source with or without a grism and with or without a slit. Sensitivities for slitless spectroscopy are much lower than when slits are used, and thus the technique is not useful on very faint sources.

Positioning of the slits and grisms appears to be repeatable at the pixel level, but are not perfectly repeatable at the sub-pixel level. During long integrations (>30 minutes) it is necessary to move the grism (and possibly the slit as well) out of beam to check the pointing. This may mean that the wavelength coverages before and after reacquiring the target are not precisely the same.

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