Astrometric properties of the Tautenburg Plate Scanner


Jens Brunzendorf, Helmut Meusinger, Tautenburg


Complete text (abstract only):

The Tautenburg Plate Scanner (TPS) is an advanced plate-measuring machine run by the Thüringer Landessternwarte Tautenburg (Karl Schwarzschild Observatory), where the machine is housed. It is capable of digitising photographic plates up to 30 cm × 30 cm in size. In our poster, we reported on tests and preliminary results of its astrometric properties.

The essential components of the TPS consist of an x-y table movable between an illumination system and a direct imaging system. A telecentric lens images the light transmitted through the photographic emulsion onto a CCD line of 6000 pixels of 10 µm square size each. All components are mounted on a massive air-bearing table.

Scanning is performed in lanes of up to 55 mm width by moving the x-y table in a continuous drift-scan mode perpendicular to the CCD line. The analogue output from the CCD is digitised to 12 bit with a total signal/noise ratio of 1000 : 1, corresponding to a photographic density range of three. The pixel map is produced as a series of optionally overlapping lane scans. The pixel data are stored onto CD-ROM or DAT. A Tautenburg Schmidt plate 24 cm × 24 cm in size is digitised within 2.5 hours resulting in 1.3 GB of data. Subsequent high-level data processing is performed off-line on other computers.

During the scanning process, the geometry of the optical components is kept fixed. The optimal focussing of the optics is performed prior to the scan. Due to the telecentric lens refocussing is not required. Therefore, the main source of astrometric errors (beside the emulsion itself) are mechanical imperfections in the drive system, which have to be divided into random and systematic ones.

The r.m.s. repeatability over the whole plate as measured by repeated scans of the same plate is about 0.5 µm for each axis. The mean plate-to-plate accuracy of the object positions on two plates with the same epoch and the same plate centre has been determined to be about 1 µm. This accuracy is comparable to results obtained with established measuring machines used for astrometric purposes and is mainly limited by the emulsion itself.

The mechanical design of the x-y table introduces low-frequency systematic errors of up to 5 µm on both axes. Because of the high stability of the machine it is expected that these deviations from a perfectly uniform coordinate system will remain systematic on a long timescale. Such systematic errors can be corrected either directly once they have been determined or in the course of the general astrometric reduction process.

The TPS is well suited for accurate relative measurements like proper motions on plates with the same scale and plate centre. The systematic errors of the x-y table can be determined by interferometric means, and there are plans for this in the next future.

Bibliographical details:

Jens Brunzendorf, Helmut Meusinger: Astrometric properties of the Tautenburg Plate Scanner. In: Peter Brosche, Wolfgang R. Dick, Oliver Schwarz, Roland Wielen (Eds.): The Message of the Angles - Astrometry from 1798 to 1998. Proceedings of the International Spring Meeting of the Astronomische Gesellschaft, Gotha, May 11-15, 1998. (Acta Historica Astronomiae ; 3). Thun ; Frankfurt am Main : Deutsch, 1998, p. 148-149.