AAS 196th Meeting, June 2000
Session 52. Ground Based Instrumentations
Display, Thursday, June 8, 2000, 9:20am-4:00pm, Empire Hall South

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[52.08] WOCS: Geometry of the Mayall KPNO 4m Telescope CCD Mosaic

V. Kozhurina-Platais (Yale), I. Platais (Yale/USRA), T.M. Girard, W.F. van Altena (Yale), T.E. Armandroff, I. Dell'Antonio, B.T. Jannuzi, K. Mighell (NOAO), E. Falco (SAO), J.R. Stauffer (CfA), A. Sarajedini (Wesleyan U.), A. Klemola (UCO/Lick Obs.), WOCS Collaboration

The WIYN Open Cluster Study (WOCS) makes use of the Mayall KPNO 4m telescope CCD Mosaic Imager to obtain second-epoch positions of selected open clusters. The ultimate goal is to produce accurate proper motions for stars in these clusters. This requires an astrometric calibration of the CCD Mosaic Imager, which consists of eight individual chips. To achieve this, we have created a secondary astrometric reference system in the area of the open cluster NGC 188. This standard system is based upon numerous Lick astrograph plates taken in 1963-1999 and the Tycho-2 Catalog.

We present absolute coordinates and rotation angles for each chip in the unified CCD ``plate'' coordinate system and examine their stability with time. In addition, empirical distortion coefficients are obtained for the new prime focus corrector. A major problem in the process of calculating the various constants is the presence of unpredictable, systematic offsets in the short exposures, caused by correlated motions of stars due to atmospheric turbulence. This somewhat limits the final accuracy of the derived CCD coordinate system.

This study is supported in part by NSF grant AST9819777 to Yale University and NSF grant AST9530632 to UCO/Lick Observatory.

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