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Making Reasonable Choices for High Dispersion...(1Jun92) Spectroscopy at the Coude (from KPNO, NOAO Newsletter No. 30, 1 June 1992) With a variety of improvements and additions over the last decade, the instrumentation available at the Coude Feed telescope is now quite diverse, allowing a broad range of resolution and spectral coverage. A review of the choices available may help observers to plan their observing programs. The traditional Coude Spectrograph is still the choice of most observers. The Coude Spectrograph provides two cameras for use with CCD detectors, a long focal length camera for very high spectral resolution (Camera 6) and a fast, short focal length camera (Camera 5) for moderate/high spectral resolution. The spectrograph can be used with a wide range of gratings, from low angle R-C Spectrograph gratings to conventional coud-type gratings, to even an echelle grating for very high spectral resolution. The orders of the echelle grating can be separated using grisms borrowed from the 4-m CryoCam. The slit can be replaced with an optical fiber feed to reduce guiding errors for radial velocity measurements. In addition to the Coude Spectrograph, the Fiber Optic Echelle (FOE) spectrograph is also available with the Coude Feed telescope. The FOE can only be used with an optical fiber. The FOE operates in two modes, the "quasi-Littrow" mode and the "high dispersion" mode. In quasi-Littrow mode, the beam from the echelle grating is cross-dispersed with a prism in double pass; a fast camera is used to image the spectrum. The spectrum format is fixed to provide nearly complete spectral coverage from the calcium H and K lines at 3900 A to the calcium triplet at 8500 A. The resolving power is 5,000 to 10,000, depending on the detector. In the FOE's high dispersion mode, the beam from the echelle grating is fed to a grating for cross dispersion, and then to a longer focal length camera. Both the echelle grating and the cross dispersing grating can be adjusted to center the wavelengths of interest on the detector. A slit at the output of the optical fiber is needed to obtain high spectral resolution, from 30,000 to about 80,000, again depending on the detector. The spectral coverage is limited to a few orders of the echelle grating and is incomplete within the wavelength region chosen. How does an observer choose the best configuration for a particular observing program? For very high resolution work (say 250,000), the best choice is the echelle grating on Camera 6 of the Coude Spectrograph. For merely high resolution (about 100,000), the best choice is the echelle grating on Camera 5. With conventional gratings, Camera 5 works well for resolutions in the range 10,000-30,000. With R-C gratings, resolutions of a few thousand are possible for faint stars. The FOE is the instrument of choice if broad spectral coverage is needed to monitor variable stars or if ultra-precise radial velocities are needed. Unless the stability of the FOE is needed for velocities, we recommend the echelle grating on Camera 5 with the Coude Spectrograph for high resolution spectroscopy. The table below identifies what combinations of camera and grating are good for particular spectral resolutions. Resolv. Disp. Wavelength Instrument Power A/mm Camera Grating Coverage Coude 250,000 0.4 6 Echelle 6 A Coude 100,000 1.5 5 Echelle 24Ax20 orders Coude 30,000 6 5 A 100 A Coude 10,000 15 5 KPC-007 185 A Coude 3,000 60 5 R-C 250 700 A FOE 10,000 20 QLM Echelle 3900-8600 A FOE 60,000 2 High Echelle 32Ax5 orders For further information and advice, consult thee Coude Feed instrumentation manuals or call one of us! Daryl Willmarth, Sam Barden, Caty Pilachowski, Phil Massey
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