An astronegatives digitization method in the Pulkovo observatory glass library

This paper presents an overview of the methodology for digitizing 877 plates from the collection of the Pulkovo glass library and the process of obtaining the positions of the objects observed on them. The astronegatives used in this work were taken with the Normal astrograph at the Pulkovo Observatory in the period from 1893 to 1941. For digitization, the Pulkovo Mobile Digitizing Device (MDD) was used, which provides an internal measurement accuracy of astronegatives of 1 μm. The scale of the resulting scans was 1.0 arcsec/pix. In order to compensate for camera distortions and to reliably link the positions of objects determined on the scans to the Gaia system, calibration scans of template plates with a large number of objects were carried out at the NAROO high-precision astronomical digitization center (Paris Observatory). The results of the measurements of the astronegatives contained Uranian and Neptunian images are presented. The accuracy of obtained coordinates is about 50 mas in Gaia CRF. The typical values of the (O-C) relative to the EPM2017 ephemerides are less than 100 mas. These are the most robust measurements of Uranus and Neptune coordinates regarded to the epochs 1890s - 1940s in the modern reference system. A database was constructed from the resulting fits-files. The corresponding web-resource has been made for open access to the digitized astronegatives of the Pulkovo Observatory in the adopted Virtual Observatory format.