Long-Focus Photographic Astrometry. Telescope; Measuring Machine

Abstract

We shall now consider the precision branch of astrometry established in the beginning of the twentieth century, namely photography with long-focus telescopes portraying a small, virtually plane field of the celestial sphere. This technique introduced a marked advance in positional precision. We shall be primarily concerned with the astrometry of a central star, or stars, relative to a background of a small number of (distant) reference stars within the limited field of less than one degree. The techniques of observing measuring and calculating for this differential method were first developed by Frank Schle-singer with the long-focus visual refractor of Yerkes Observatory (focal length 19.37 m, aperture 102 cm) for the purpose of determining precise annual parallaxes (1910–11, 1924).