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Atmospheric Dispersion Correction at the 4-m: ...(1Dec95) The Risleys Are Back! (from KPNO, NOAO Newsletter No. 44, December 1995) Alexei Filippenko deserves credit for having reminded us all of the significance of differential refraction in doing spectroscopy (1982 PASP 94, 715). Even at a modest 1.5 airmasses, an image at 4000 AŹ is displaced by 1.1" from the image at 6000 AŹ. If you are trying to observe over this wavelength range using a 2" slit, you will be suffering a large amount of light-loss unless the slit happens to be aligned at the parallactic angle; i.e., the position angle on the sky that results in the slit being perpendicular to the horizon. At the 4-m it is relatively easy to set the slit to the parallactic angle, although such rotation requires moving near the zenith. Sometimes this is inconvenient and, at other times, impossible--such as in the case of multi-slit slitlet masks, which are designed for a specific position angle. However, a little-known fact is that the 4-m is also equiped with atmospheric dispersion correctors, which can be moved into the beam at f/8 or f/15: the so-called Risley prisms. Previous tests of this ancient device (NOAO Newsletter No. 32) were not encouraging, but this summer we had a chance to both clean and adjust the prisms mechanically, as well as to install new software, which should do a more accurate job of controlling the prisms. Tests conducted this summer showed that the ADCs are now doing their job correctly. The Risleys have now been used sucessfully for three staff runs with both the R-C spectrograph and CryoCam. In the accompanying figure we show how well the Risleys do their atmospheric compensation. The boxes denote the standard star fluxes for a spectrophotometric standard. We then show two observations of this star, both obtained at airmass of 1.8, and both obtained in the worst possible scenario of the slit being 90d to the parallactic angle (parallel to the horizon). We see that the observation obtained with the Risley prisms follows that of the known flux distribution of the star, while that obtained without the Risleys was down by a factor of 3 in the blue. We note that if you are doing spectrophotometry with the Risleys, you must observe your standards with the Risleys as well, to properly calibrate the small but wavelength-dependent light losses of the Risleys themselves. Of course, everything comes with some price, and indeed there are some light losses with the Risleys (5-8% at wavelengths > 4000 A). But this hit is considerably smaller than the losses you may experence due to differential refraction! [Figure not included] Phil Massey, Jim DeVeny, Buell Jannuzi
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