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Prisms and XD Spectroscopy

GNIRS contains two prisms designed to enable cross-dispersed spectroscopy in the 0.9-2.5 µm region. One, known as SXD, was designed for use with the short blue camera (0.15"/pix), and the other, known as LXD, for use with the long blue camera (0.05"/pix). The prisms disperse incident light in the direction orthogonal to that of the gratings, which allow grating orders 3-8 to appear at different locations on the array, as shown here. The prisms are made of SF57 glass and have excellent transmission across almost the whole wavelength range but attenuate slightly at the long wavelength end of the K-window.

Because the prisms spread the orders along the slit direction, it is necessary to use a much shorter slit than can be used in the single order / long slit mode. The slit in use with the prism for the short blue camera prism has a length of 7.0 arcsec; that for the long blue camera has a length of 5.0 arcsec. The long blue camera can also be used with the SXD prism, and the 7.0" slits, but then can only cover orders 3-5 (J, H, and K - but may miss the very shortest part of the J band). If the science does not require the shorter wavelength orders, the user can make use of this configuration which gives increased spatial coverage and greater flexibility (e.g. in nodding along the slit), with only a small degradation in spectral resolution.

Atmospheric refraction, which causes a wavelength dependent smearing of the light from the target along the elevation angle (sometimes called the parallactic angle), is always an issue at zenith angles greater than 45 degrees for the long camera and can be an issue for the short camera, particularly when the narrowest slits are used. The effect of differential refraction is shown in this table. Note that roughly 2/3 of the differential refraction in the 1.0-2.5 µm region occurs between 1.0 µm and 1.5 µm, and that the effects of flexure between the guider and GNIRS also need to be considered when selecting the position angle of the slit.

The prisms degrade the spectral resolution when used with the long blue camera at the highest spectral resolutions (i.e., with the narrowest slit - 0.10 arcsec). The degradation has been measured to be 15% with the LXD prism and 25% with the SXD prism. With wider slits the degradation is much less.

The following table summarizes the properties of each cross-dispersed mode described above.

GNIRS in XD Mode
Slit Length
orders observed wavelength
spectral resolution
with 2-pix wide slit
SB (0.15") + SXD 7.0 3-8 0.9-2.5µm 2.0 pix
LB(0.05") + LXD 5.0 3-8 0.9-2.5µm 2.3 pix
LB(0.05") + SXD 7.0 3-5 1.2-2.5µm 2.5 pix