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Astron. Astrophys. 333, 531-539 (1998)

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2. Observations and measurements

In order to get a better accuracy in the proper motion determination of Pal 5 we used more deep plates from the UKST archive in addition to the plates earlier measured with the APM facility in Cambridge (two POSS 1, one UKST and two Tautenburg plates). Altogether, 10 plates were measured with the APM facility (see Table 1). The POSS 1 plates and the UKST plate j5193 had already been used in the determination of the absolute proper motion of the globular cluster M 5 (Scholz et al. 1996) which is located near to the northern edge of the UKST plates listed in Table 1. All measured objects were classified into stars, nonstellar objects, noise images and merged objects using the standard APM software.


Table 1. Plate material

On all plates listed in Table 1 the globular cluster Pal 5 is more than 1 degree from the plate edges so that in principle a large, e.g. [FORMULA] field could be used in the reduction. Nevertheless, we selected a smaller, 1 square degree region centered on Pal 5 ([FORMULA]), in order to reduce the systematic effects in combining the plates of different Schmidt telescopes. However, the removal of systematic errors is the most extensive part of the proper motion reduction, described in Sect.  3.1.

In addition to the plates measured with the APM facility in Cambridge, the data of the second epoch Palomar survey plate were received from the Digitised Sky Survey. For the image detection and determination of image parameters in this 1 square degree FITS frame we used the software developed for the Münster Redshift Project (Horstmann et al. 1989).

The internally calibrated instrumental APM magnitudes (Bunclark & Irwin 1983) measured on the blue reference plate j5193 were transformed to approximated B magnitudes with photoelectric standards in the globular cluster M 5 from Arp (1962). From 21 standards identified only 15 with stellar classification on the plate j5193 were included in the external magnitude calibration using a linear polynomial fit.

From several runs of the plate matching with different reference plates and the number counts of matched objects plate j5193 was found to be the deepest plate with a limiting [FORMULA] and was therefore used as the reference plate for the proper motion study.

The Tautenburg plates have a better scale for astrometric work (and for the classification of merged objects) but their limiting [FORMULA] is about 1-2 magnitudes below the limiting magnitude of the other Schmidt plates used in this study.

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© European Southern Observatory (ESO) 1998

Online publication: April 20, 1998