The Instrument uses a fixed width slit, three prisms to cross disperse the light into at least 5 orders across the science detector, and a mid resolution grating. In addition, the first fold mirror is mounted on a piezo tip-tilt stage (S-340) to minimize the effects of flexure (FCS).

Spectrograph schematics from Wilson et al. 2004

3D representation showing the tip-tilt mirror.


The instrument has a fixed slit of 0.55" width and 18" in length. It is made of a reflective material.

In early 2018 we detected an imperfection in the slit. The effect of this imperfection is to show a trace on the dome flats. The follwing drawing (all dimensions are in units of arcseconds) shows the position of the imperfection (shown as a triangle). If possible, we suggest to avoid the compromised section of the slit.


Two first two prisms are made of ZnSe with a 22 degree apex. The third prism is made of Infrasil with 50 degrees apex.


[l mm-1]
[pix per resolution element]
Blaze Wavelength
Blaze Angle
Spectral Resolution
Peak Efficiency within each order Notes
110 110 2.7 6.98130 22 2700
See plot below
55% at 825nm, 52% at 930nm, 55% at 1100nm, 60% at 1350nm, and 79% at 1725nm. made by Newport

Spectral resolution as a function of wavelength and order for a 0.55 arcsec slit.

Final Spectrum

The dispersed light is focused by a 7-element camera onto the detector. The following is a 700 seconds exposure obtained at CIT. The spectrum shows five cross-dispersed orders that almost (small gap from 1.85 to 1.88 microns) continously cover the near-IR spectrum from the Y band (order 7 at the bottom) to the K band (order 3 at the top).

NIRES spectra

Plot of the cross dispersed orders (Wilson et al. 2004).

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