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Astron. Astrophys. 331, 934-948 (1998)

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2. The new catalog of homogeneous RGU data

As summarized in Table 1, the catalog comprises new photographic three-color data for more than 18,000 stars remeasured with the Iris photometer from the Basel Palomar-Schmidt survey plates in 14 original intermediate- and high-latitude fields. While the original catalog data obtained from the same plates (Becker & Fenkart 1976 (I), Becker et al. 1976 (II), Becker et al. 1978 (V), Becker et al. 1988 (XI), Becker et al. 1989 (XII)) were derived based on a preliminary standard of the RGU photometric system and methodology which allowed to pioneer the exploration of the space density distributions of the field-halo stars down to the faintest observed magnitude limits, in the present work we have rather attempted to exploit the significantly improved calibration and standardization of the RGU photometric system which have been established in recent years and which allow us to provide photometric data of high systematic accuracy and homogeneity - an indispensable requirement for the more ambitious goal of a physically consistent determination of the density and the metallicity distributions of the stars. Thus, the following basic principles were adopted for the construction of the present new catalog:


Table 1. The new Basel high-latitude Palomar-Schmidt survey field data1

  1. All the data have been derived following the definition and standard of the photographic RGU system based on synthetic photometry, as summarized in Buser & Fenkart (1990). In particular, calibration of the RGU plates has been done employing available photoelectric UBV standard sequences exclusively. From these, equivalent standard RGU data were derived via UBV-RGU transformations which account for the systematic differences of the stellar spectral energy distributions due to their different physical parameters, such as effective temperatures, luminosities, and metallicities (Buser 1978, 1988; Güngör 1996, Buser et al. 1997c). Thus, the RGU calibration standard magnitudes and colors are comprehensively and consistently tied with the standard UBV data throughout the observed ranges in physical properties of the stars - which eliminates the serious limitations of the preliminary RGU standard and its associated systematic errors propagating into the original catalog data (Buser 1988, Buser & Rong 1995).
  2. Stars have been included in the catalog only down to the magnitude limits defined by the available photoelectric UBV standard sequences used for the plate calibrations. Extrapolations of the characteristic curves were strictly limited to less than about 0.5 magnitudes fainter than the faintest photoelectric data point. In particular, all extensions employing additional independent electronographic and/or photographic data - a method often employed to expand the magnitude ranges in the original catalogs - were deliberately excluded from the calibration process, because in no case did such data allow to reconstruct the corresponding photometric systems with sufficient accuracy and confidence (Buser 1988).
  3. Photometric standardization of the photographic program stars has been based strictly and uniformly on the evaluation of color equations defined by the magnitude residuals (as functions of observed colors) for the calibration standard stars. Because the preliminary RGU standard was ill-defined - or not defined at all - outside its limited color (or physical parameter) range of application, a systematic color correction based on the "final" two-color distributions of the program stars often had to be substituted for this fundamental step as part of the data interpretation - implying significant loss of control of the real significance of the data in the original catalogs.
  4. As in the original field survey, the probability of field errors has been minimized by selecting relatively small-sized fields near the plate centers and standard star sequences. In fact, the usual zonal checks revealed field errors in a few cases only which then were accounted for by standard methods.

While the full details on the error budget in the catalog data will be given in Papers III and IV, it may suffice here to mention that the above procedure applied to an average of 5 plates per color (i.e., a total of 15 plates per field) provides G magnitudes and G-R and U-G colors with typical internal errors of [FORMULA] and [FORMULA] mag. For each field, the resulting catalog of star counts as functions of observed apparent magnitude and colors, [FORMULA] and [FORMULA] then constitutes the data base used for testing the interpretive models.

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© European Southern Observatory (ESO) 1998

Online publication: March 3, 1998