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2.2 4-m RC Spectrograph

The 4-m Ritchey-Chretien Focus Spectrograph is the primary spectroscopic work-horse for optical spectroscopy at Kitt Peak. There are a dozen diffraction gratings available, which provide resolutions from 300 to 5000. It uses the ``UV Fast Camera" to focus the spectrum onto a CCD detector. The CCD currently in use with the RC Spectrograph is a Tektronix 2048 detector with excellent cosmetics. The total system throughput (telescope + spectrograph + CCD) is 11-14% under typical seeing conditions with a nominal slit width (18% peak under excellent seeing conditions). The spectrograph is normally used in long-slit mode (slit length=5.2) for either two-dimensional spectroscopy of extended sources or for spectroscopy of point sources with excellent sky subtraction. In this (normal) mode, a slit-viewing TV (field of view 2.5') provides the means for acquisition. However, the spectrograph can also be used with multi-slit masks. The instrument rotator at the 4-m allows remote control of the position angle of the slit on the sky, although such rotations need to be done with the telescope within 30 of zenith. Thus the spectrograph can be rotated to the parallactic angle (to keep all of the light down the slit), or, if you need to use a particular position angle, you can use the atmospheric dispersion corrector (``Risley prisms"); see Sec. 4.4.1.

The focus of the RC Spectrograph begins to degrade on the red side (large pixel numbers) past pixel 1500, with poor resolution and substantial vignetting past pixel 1700. A similar focus degradation occurs on the blue side between pixels 0 and 200.

A list of the available gratings with the resulting wavelength coverage, dispersion, and resolution, is given below. We give the wavelength coverage both for 1500 pixels, to which people doing demanding work may want to restrict themselves, and for 1700 pixels, which may be suitable for some qualitative applications. The laboratory measurements of the grating efficiences are shown in Fig. 2 and Fig. 3.

Notes: (a) Based on 2.5 pixels FHWM corresponding to 300 m slit (2 arcsec). Better resolution can be achieved with smaller slit widths. (b) Spectral coverage limited by overlapping orders. (c) Spectral coverage limited by grating efficiency and atmospheric cut-off.

 
Figure 2:   The efficiencies of the various RC gratings are shown here (continued on the next figure).

 
Figure 3:   The efficiencies of the various RC gratings are shown here (continued from the previous figure).