The Method Extractions from the scan database are typically done over small regions, no larger than on a side, so that distortions are minimized. Objects that are classified as stars in both frames are matched using a nearest-neighbor algorithm in both spatial directions. The number of reference stars remaining is typically 25 to 30. After matches are made and accepted, we then determine a solution for an affine transformation from one frame to the other using the least-squares, singular value decomposition routines from Press et al. In this mode, the position of a specific star is centered in an extraction window for each epoch, and the motion is determined for that star.
Arrows indicate the magnitude and direction of the proper motion for the stars.
Of the remaining 17, six are near our faint or bright limit. However, upon inspecting the actual photograph, also in Figure 2, we see that two stars of the right separation and position angle are present in each position. We are pleased to acknowledge the support of the Space Telescope Science Institute and its Catalogues and Surveys Branch, and in particular, B. Lasker and B. McLean.