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Scattered Light Analysis of the Coude Echelle/Grism System (1Dec94) (from KPNO, NOAO Newsletter No. 40, 1 December 1994) Use of the 31.6 g/mm echelle and grism cross-dispersers has become quite popular, comprising roughly half the usage of the Coude Feed during the current semester. This combination allows resolutions of about 100,000 on camera 5, and about 2.5 times higher with camera 6. As with all cross- dispersed echelle setups, there is some scattered light, the characterization of which is important for proper subtraction. A recent observer, Jason Cardelli, has done an analysis of some aspects of the scattered light which forms the basis of this report. Scattered light was found to originate primarily from two sources: echelle grating scattering along its dispersion direction, and grism scattering in the cross dispersion direction. Echelle scattering was determined by analyzing a very high S/N Th-Ar spectrum formed from the average of 90 exposures. The line profiles could be well represented by the linear sum of a Gaussian core and power law wings: I (x) = e^-(x-xo)^2/1.44 + 0.038 (x - x[SUB o])^-1.6 (x x[SUB o]) where x[SUB o] (the line center) and x are measured in pixels. While the data used in this analysis cover a limited wavelength interval (3700-4500 ), in general the relative intensity of the power law wings should show a decrease with increasing wavelength. Grism scattering probably also includes the usual Gaussian core and power law wings, but the data used for this study were obtained with a slit too long to effectively characterize this component. However, analysis of flat lamp data does reveal the main component of scattered light in this system to be "ghost" orders in the interorder space on either side of the parent order. These weak "ghosts" are probably the result of internal reflections in the grism wedge or in the 3^o fused silica wedge used for tuning the grism central wavelength. For point sources, the average per pixel intensity of this background ranges between about 3%-8% of the integrated on-order intensity for the wavelength interval studied here. The ratio of background to spectral order intensity increases towards shorter wavelengths. Analyses of the profiles of strong interstellar Ca II K absorption lines indicate that background underneath the spectral orders appears to be generally consistent with a simple average of the interorder background from above and below the order of interest. On-order data corrected using such a simple interorder background extraction scheme result in net spectra with a conservative accuracy of about 1-2% of the average net on-order intensity. Copies of Jason's report may be obtained from the undersigned or found at the coude spectrograph. IRAF routines for echelle data are described in "A Users' Guide to Reducing Echelle Spectra With IRAF," which includes a description of background subtraction algorithms. This document is available via anonymous ftp as described in the September 1994 NOAO Newsletter. [Figure not included] The observed profiles for a pair of Th-Ar lamp lines (filled and open symbols) plotted in pixel space and normalized to their peak intensity. Daryl Willmarth We are pleased to include contributions from KPNO users in the Newsletter and are grateful to Jason Cardelli for his careful analysis of scattered light in the coude spectrograph. Caty Pilachowski
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