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NICMASS at the Feed - Update (1Sep95) (from KPNO, NOAO Newsletter No. 43, September 1995) The NICMASS HgCdTe system, described in the NOAO Newsletter No. 41 (March 1995), has been used successfully for a number of observing runs at the Coude Feed Spectrograph during the last semester. As we reported in the article, the performance of the system is limited largely by photon noise from infrared background within (and outside of) the bandpass of the blocking filters. We determined empirically since then that significant background was due to scattered light completely bypassing the blocking filter. This last source of background has been virtually eliminated by mechanical baffling and modifications to the filter mounting fixture. In addition, we installed a well-blocked filter at 1.624um, particularly for use with the echelle grating, where a narrow blocking filter is required for order separation. This resulted in a substantial reduction in background, from 180 to 20 e-/s, the latter value being close to that expected from the ambient thermal background through the filter. Integration times up to 6000 s are now possible. The figure illustrates the spectrum of the late-type star AS 289 (H = 5.5) after 1 hour of integration; the estimated S/N~ 100 represents an improvement of a factor of 5 over the performance reported in the previous Newsletter article. The CO lines in this bandpass are a useful grid for radial velocity measurements of late-type stars with this instrument configuration. A preliminary analysis by Frank Fekel (Tenn. State) of a number of such stars indicates night-to-night scatter in determined radial velocities to be less than 0.3 km/s. [Figure not included] Spectrum of the late-type star AS 289 (H = 5.5) using the echelle grating at a resolving power of 43000. This is the average of two 1800 sec integrations. The spectrum of the bright radial velocity standard HR 6056 is plotted at the same scale to illustrate the high S/N of the spectra. We are in the process of purchasing a well-blocked filter at 1.0830um to replace the present filter, which appears to suffer significant extraband leakage. This should improve the performance of the system at that wavelength region with both the echelle and grating B, since the ambient thermal background should be negligible. We expect to evaluate this filter during the next scheduled observing in October and will report the results in the next Newsletter. We will continue to offer this system for shared-risk observing at the Coude Feed Spectrograph during the spring 1996 semester. Prospective observers should familiarize themselves with the system and its operation by reference to the description on the World Wide Web at http://scruffy.phast.umass.edu/Irlab/NICMASS/nicuser.html, or by contacting the undersigned (rjoyce@noao.edu; khinkle@noao.edu) for information regarding a specific application. Dick Joyce, Ken Hinkle
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