2 Observations and measurements

 

2.1 Plate material

  For this study we used exclusively Schmidt plates from the Palomar Schmidt telescope (scale 67.2 arcsec/mm), i.e. from the Palomar Observatory Sky Surveys (POSS) and from the Tautenburg Schmidt telescope (scale 51.4 arcsec/mm). The plate material of Palomar (P) and Tautenburg (T) plates is described in Table 1.

  

Table 1: Plate material

• [$^{\mathrm{a}}$] Measurements taken from the APM sky catalogues.
• [$^{\mathrm{b}}$] FITS frame from the STScI Digitized Sky Survey (DSS).

For the proper motion study we selected a one square degree field centered on the cluster IC348 ($\alpha_{2000} = 3^{\rm h} 44^{\rm m} 36\hbox{$.\!\!^{\rm s}$}5, \delta_{2000} = 32\hbox{$^\circ$}8\hbox{$^\prime$}59\hbox{$.\!\!^{\prime\prime}$}8$). On all Palomar plates the cluster was located at about 1.5 to 2 degrees from the plate centre in each coordinate. The last two columns in Table 1 show the coordinate differences (cluster centre minus plate centre) on the plates. With a one square degree field around the cluster we do not use the plate edge zones which would be problematic for both astrometry and photometry. The Tautenburg plates were taken with the cluster in the plate centre. Due to rather poor weather conditions these plates do not go as deep as the Palomar plates.

Whereas for the older Palomar plates (POSS1 and Quick-Survey) as well as for the Tautenburg plates Kodak 103a-O or the similar ORWO ZU-21 emulsions were used, these are replaced by fine grain emulsions IIIa-J and IIIa-F in the POSS2 survey (Reid et al. 1991). According to Totten & Irwin (1998) the POSS1 E plate covers with  6200 Å - 6900 Å  a similar wavelength region as the POSS2 R plate (6300 Å - 6900 Å). On the other hand, the POSS1 O plate (3400 Å - 4900 Å) covers a passband comparable to U+B and the POSS2 B_j plate (3900 Å - 5400 Å) passband is also different from the standard B passband. The Tautenburg plates were taken in the U and B passbands. Zero point shifts and colour terms in the transformation relations between U and B magnitudes of the Tautenburg system and the standard system are very small (van den Bergh 1964; Börngen & Chatschikjan 1967). In the present study, the Tautenburg plates were therefore envisaged mainly for photometry. The transformation of the Palomar B_j and E magnitudes, respectively to B and R magnitudes will be subject of Sect. 3.

2.2 Measurements and determination of image parameters

  All Palomar Schmidt plates, except o1457 were measured with PDS microdensitometers at the Space Telescope Science Institute. The POSS1 plate e1457 and the Quick-Survey plate st277 were measured with 25 $\mu$m pixel size, whereas the POSS2 plates sj01520 and sf04859 were scanned with 15 $\mu$m pixel size.

The POSS1 plates were also measured with the APM facility at Cambridge, UK (Kibblewhite 1984). In our study these APM measurements were mainly used for image identification and classification. We did not use them for the astrometry since these measurements were taken from the APM catalogues (Irwin et al. 1994) providing only one set of coordinates for both the E and O plates. This set of coordinates was produced using the E plate as a reference plate so that for all objects measured on both plates only the coordinates as measured on the E plate were preserved. Moreover, the blue POSS1 plate o1457 seemed to be very noisy with thousands of spurious images in the one square degree field detected by the APM.

The Tautenburg plates, which we used for photometry only, were scanned with the Tautenburg Plate Scanner (TPS) described by Brunzendorf & Meusinger (1998). The TPS essentially consists of a rigid CCD line camera system with telecentric projection and an x-y movable plate holder. Plates are digitised with 10 $\mu$m resolution, corresponding to 0.5arcsec/pixel on a Tautenburg plate.

For the object search and the determination of image parameters in the FITS frames we used the Münster Redshift Project (MRSP) software package (Horstmann et al. 1989). In its original version, this software executes a linear transformation between photographic densities and the logarithm of the corresponding intensities. For its application to Tautenburg plates a nonlinear transformation is used, based on either the characteristic curve of the individual plate or an average characteristic curve, for the transformation to intensities. The individual characteristic curve is derived from the calibration wedge exposed onto each plate. The use of such a nonlinear transformation ensures that the intensity profiles of stellar images are well-approximated by a Gaussian even when the central densities are outside the linear part of the characteristic curve. From all detected objects on each plate we selected for further reduction only those fitted by a two-dimensional Gaussian profile. Measurements on a large number of plates of a different field have shown that the MRSP Gaussian fitting procedure (programme "profil'', Horstmann et al. 1989) works well for stars over a magnitude interval of at least 13mag (Brunzendorf & Meusinger 1999). In the present study we used only an 8 mag interval (10.5 < R < 18.5).

The data from the APM catalogues include a classification of all measured objects into stars, objects with non-stellar image shape, merged objects and noise. The APM classification of stellar images is reliable until about two magnitudes above the plate limit. In order to exclude unreliably measured stars, we used this APM image classification on the reference plate (see Sect. 2.3) in addition to the condition of a Gaussian profile as obtained by the MRSP software from the FITS file of the same plate.

Unreliable measurements were mainly due to strong image crowding of the stars with nebulosities and the internal reflection halo of oPersei on the Schmidt plates. Therefore, many bright stars in the central region of IC348 had to be excluded from further reduction.

2.3 Plate matching

  For the determination of proper motions and of a UBVR photographic photometry we compiled a catalogue of measurements from all comparison plates with respect to one reference plate. The plate matching is done iteratively starting with bright objects and large search radii (up to several mm), finally iterating down to faint objects within a target search radius of about $75~\mu$m, corresponding to 5 arcsec in the scale of the Palomar Schmidt plates. In the matching procedure only simple coordinate transformations were applied, i.e. an affine model ($2 \times 6$ plate constants) allowing for shift, rotation and scale factor.

The first epoch Palomar plate e1457 measured on both APM and PDS was chosen as the reference plate in the plate matching. This plate was selected because it was one of the deepest plates and the only first epoch plate which could be used in the proper motion determination. From about 2300 objects detected by the MRSP software in the one square degree FITS frame of the reference plate e1457, about 2000 stars could be fitted by a Gaussian. About 1800 of these stars were identified with Gaussian image shape objects on at least two other plates. The additional condition of stellar image classification on the reference plate as measured on the APM further restricted the number of objects in the final sample to 1431 stars.

The number of identifications of the stars of our final sample on the comparison plates depended on the limiting magnitude and the different passbands of the plates. Whereas nearly all of the 1431 stars in our sample were identified with stellar objects on the POSS2 R and B_j plates, about 77% were found on the POSS Quick-Survey V plate. In contrast, only about 38% could be identified on the Tautenburg B plates and even only 18% on the U plate owing to the higher extinction at shorter wavelengths and the poor weather conditions during exposure.