The StarScan plate measuring machine: overview and calibrations

TL;DR

The paper presents an overview of the StarScan machine, its calibration, and its application in measuring photographic astrograph plates with high precision for astronomical catalogs.

Contribution

It introduces the StarScan measuring machine, details its calibration process, and demonstrates its capability to measure photographic plates with sub-micrometer accuracy.

Findings

Repeatability of 0.2 micrometer in measures

Overall accuracy of 0.5 micrometer in x,y data

Position information accuracy of at least 0.65 micrometer

Abstract

The StarScan machine at the U.S. Naval Observatory (USNO) completed measuring photographic astrograph plates to allow determination of proper motions for the USNO CCD Astrograph Catalog (UCAC) program. All applicable 1940 AGK2 plates, about 2200 Hamburg Zone Astrograph plates, 900 Black Birch (USNO Twin Astrograph) plates, and 300 Lick Astrograph plates have been measured. StarScan comprises of a CCD camera, telecentric lens, air-bearing granite table, stepper motor screws, and Heidenhain scales to operate in a step-stare mode. The repeatability of StarScan measures is about 0.2 micrometer. The CCD mapping as well as the global table coordinate system has been calibrated using a special dot calibration plate and the overall accuracy of StarScan x,y data is derived to be 0.5 micrometer. Application to real photographic plate data shows that position information of at least 0.65 micrometer accuracy can be extracted from course grain 103a-type emulsion astrometric plates. Transformations between "direct" and "reverse" measures of fine grain emulsion plate measures are obtained on the 0.3 micrometer level per well exposed stellar image and coordinate, which is at the limit of the StarScan machine.