Copyright © 1981 by the University of Washington Press

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The Coordinate Markings

For the convenience of users in finding and identifying objects in M31, markings have been added at the sides of the charts giving approximate 1950 coordinates. These are not astrometrically useful for two reasons: first, the field of the 4-meter telescope with its correcting lens in place is not flat and therefore the scale of the plates varies from the center outward, making coordinate markings such as given here only approximate; and, second, the coordinates were not determined from a sufficiently large number of stars on each chart to give astrometric accuracy.

The coordinates were established by noting the positions of SAO Catalog stars on each of the 4m charts, approximately five or six stars per chart. The scale of the charts was then figured, and the right ascension scale determined as a function of declination. Next, three overlays were constructed for use in declination zones ~ 1 1/4° wide. The appropriate overlay was moved in both right ascension and declination independently until the best fit was found to the SAO star positions on each chart. Preliminary coordinates were then marked on the chart. Globular clusters were identified from the list of Sargent et al. (1977) and their published positions were used as secondary positional standards to check the preliminary chart coordinates, especially for those charts which had few SAO stars on them. This check led to the revision of two charts' coordinates. Final positions are accurate to within 2 seconds of time in right ascension and 20-arc seconds in declination. The coordinates from the small-scale charts were transferred to the large-scale charts by eye.