We have re-reduced the Southern Polar Zone of the Yale Photographic Star Catalogue (-70 ^circ to -90^ circ declination) using an overlapping plate technique. This region was photographed in 1955-56 with 64 overlapping plates and the reduction of these plates (i.e. the determination of the equatorial coordinates of the star images on the plates) was completed in 1971. Because of the scarcity of observations from the Southern Hemisphere prior to the 1970's, this data set is particularly valuable. The plates where photographed so that they overlapped 50% in right ascension and 50% in declination. Thus every region of the sky was covered by at least 2 plates and sometimes as many as 20 different plates. Originally these plates were individually reduced. Thus after the reduction, some of the stars had as many as 20 different position estimates (i.e. right ascension and declination). Clearly a star occupies one position at one time, but this can not be obtained without mathematically enforcing this constraint on a multiple plate reduction process. Instead, the separately computed values were averaged to give the "best" (in the mathematical statistics sense) estimate for the stellar position. Taking advantage of the high degree of overlap of these plates, we used a more powerful reduction method for overlapping plates called the overlapping plate technique. This method enforces the simple fact that, at a given instant of time, a star is at only one position. When this constraint is enforced, all the reduction parameters on all plates are obtained simultaneously by solving one large system of equations. The application of these improved reduction procedures and resources to the original measurements have yielded star positions which are a 20% improvement over those originally published. These new estimates of star positions are capable of yielding improved proper motions, which are invaluable data for many aspects of astronomy.
- Pub Date:
- January 1995
- Physics: Astronomy and Astrophysics