Obtaining accurate comet positions despite some inaccurate catalog stars

From an astrographic negative a grid is determined from measurement of all the reference stars and using their catalog positions, and other grids from selections of reference stars. These grids are determined from many stars, and individual stars will have errors with respect to the grid. There are ways to identify stars X that are not at their cataloged position: (1) determine the focal length of the telescope from the measurements of each pair of stars and look for discordant results; (2) stars X are several arcseconds off the grid, and the other stars fit better when stars X do not help determine the grid; and (3) the grid is stretched or distorted to include the stars X. Fifteen to twenty percent of the stars in the S.A.O. catalog are 1.5 seconds or more off their catalog positions. An interactive session with a computer can find and eliminate these errant stars and result in more accurate comet positions.