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GNIRS Slit Properties

This page outlines GNIRS'

The spectrograph entrance slit is defined by two mechanisms. The width of the slit is defined by the one of several slits in a photo-etched mask located in the slit slide; the length of the slit is defined by one of several openings in the decker slide.

The slit mask is located at the re-imaged focal plane, while the decker apertures are slightly ahead of it, and therefore somewhat out of focus (by a few pixels). The decker sizes are matched to the full width of the array in long slit mode, or to the minimum spacing between adjacent spectra when the cross-dispersing prisms are used.

The slit widths currently available are listed below:

Slit name Slit width (pixels)
[nominal values]
short camera (0.15"/pix) long camera (0.05"/pix)
0.10 arcsec n/a 2
0.15 arcsec n/a 3
0.20 arcsec n/a 4
0.30 arcsec 2 6
0.45 arcsec 3 9
0.675 arcsec 4.5 13.5
1.0 arcsec 6.7 20

 

The slit (decker) lengths in the long slit and crossed-dispersed modes are:

Configuration Slit length
short camera long camera
Long-slit 99 arcsec 49 arcsec
Cross-dispersion 7.0 arcseca 5.1 arcseca or
7.0 arcsecb

a 0.9 - 2.5 µm; see the prisms and XD page
b 1.2 - 2.5 µm; see the prisms and XD page

 

Slit throughputs for several different 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 (i.e. integrated along the slit) of 1.4 times the image FWHM and are not based on the total signal within the slit. The shape of the model PSF does not vary significantly 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 calculated with the GNIRS Integration Time Calculator.

Slit width
(arcsec)
Image quality (50% EED in arcsec)
0.33 0.39 0.50 0.61 0.81 0.89 1.56
0.10 0.23 0.19 0.15 0.12 0.09 0.08 0.04
0.15 0.34 0.30 0.24 0.19 0.14 0.13 0.07
0.20 0.45 0.40 0.32 0.26 0.19 0.17 0.09
0.30 0.62 0.56 0.46 0.38 0.29 0.27 0.14
0.45 0.78 0.72 0.63 0.54 0.42 0.39 0.23
0.675 0.82 0.81 0.73 0.66 0.54 0.50 0.31
1.0 0.90 0.87 0.83 0.81 0.74 0.71 0.49

 

Slit orientation: With GNIRS mounted on a side port of Gemini North and for a slit position angle of 90 degrees, further east corresponds to lower column numbers. For a slit position angle of 0 further north corresponds to lower column numbers.