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Argus Spectroscopy Update (1Mar96) (from CTIO, NOAO Newsletter No. 45, March 1996) The bench spectrograph that is used with the Argus fiber positioning instrument at the CTIO 4-m telescope is now set up with the facility Blue Air Schmidt camera (229 mm focal length) and the Loral 3K x 1K CCD. This camera/CCD is the general-use camera for the 4-m telescope R-C spectrograph, the 4-m telescope echelle camera, and the Argus bench spectrograph. The properties of the camera/CCD are described in the June 1995 NOAO Newsletter No. 42, p.29. Since the Argus bench spectrograph uses the same gratings and camera as the R-C spectrograph, the dispersion and wavelength coverages are the same for the two spectrographs. A table of gratings, dispersions, and wavelength coverages for the R-C mode are given in the table in NOAO Newsletter No. 42, p.30. As with the R-C and echelle spectrographs, there is no longer any "red" camera. The Blue Air Schmidt with the Loral CCD is our only camera for Argus. The two-layer AR coating on the Loral CCD will fringe longward of 7500 Angstroms. The fringing properties of the camera are given in the December 1995 NOAO Newsletter No. 44, p.25. At present, we have no engineering data about the specific effects of the fringing in the Argus setup, but it is not expected to be very much different than the data given in the cited Newsletter. The fiber separation at the focus of the collimator has been increased to take advantage of the large format CCD. The fiber separation projects to a 20.6 pixel separation on the CCD, and the 48 spectra now have no overlap perpendicular to the dispersion: the inter-orders go "black." The 100 um fibers de-project by a factor of 2.2 to about a 3.1 pixel spot perpendicular to the dispersion. For faint objects where modest S/N is expected, most observers will want to bin the CCD by a factor of 2 perpendicular to the dispersion to reduce the effect of the read noise. The typical resolution along the dispersion is about 3.3 pixels. The actual spectral resolution for a given setup can be calculated from the grating dispersion listed in the table in NOAO Newsletter No. 42, p.30 where the dispersions in Angstrom/pixel are given for all the gratings. The Argus echelle mode is still available. In this mode, single orders from the finely-ruled echelle (316 groove/mm) are isolated using order-separating filters. The grating is used in order 56120/Angstroms, e.g. 10th order near 5600 Angstroms. The 3 pixel resolution is about 17,000. The wavelength coverage is about Delta Lambda = Lambda/50 which is about 30% of the bandpass of the typical order-separating filter and is also much smaller than the free spectral range. The wavelength coverage of the Argus/echelle mode is effectively limited by the CCD size. A summary of the Argus echelle mode can be found in the CTIO WWW pages or the March 1991 NOAO Newsletter No. 25 p.23. The larger beam size, due to the increased fiber separation in the spectrograph, means that the echelle mode (where the collimator/camera angle is only 15 degrees) will have vignetting perpendicular to the dispersion. The first and last fibers have about 60% the intensity of the central fibers. A new spectrograph camera, which will operate in quasi-Littrow mode, is presently being designed for the Argus spectrograph (and the future CTIO Hydra fiber positioning instrument), which will reduce the vignetting problem. The WWW pages (http://www.ctio.noao.edu/argus/argus.html) for Argus, which include the Argus technical and user's manuals, have been completely updated. N. Suntzeff, T. Ingerson, R. Schommer
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