2 Observations, reduction, and standardization

In a field of area 2.0 square degrees centered on $(\alpha,\delta(1950.0)) = (14^{\rm h} 01^{\rm m} 24^{\rm s}, +54^{\circ} 35')$ = (l,b) = (101$^\circ$, +60$^\circ$), a total of 1806 stars were measured with the Basel Iris photometer by one of us (S.K.) from 15 48'' Palomar-Schmidt plates - i.e., 5 plates for each of three RGU colors - taken by Purgathofer and Tammann in 1972. These measurements were reduced to RGU magnitudes using a photoelectric sequence which was established by Sandage and Tammann (1974) and which extends down to limiting UBV magnitudes of 19.85, 19.49, and 18.76 magnitudes, respectively. The UBV data for the calibration stars were converted to the standard RGU system via the transformation equations calculated from a comprehensive grid of detailed flux distributions of spectrophotometric standard stars by Buser (1978a,b).

Calibration curves could then be defined accurately down to limiting RGU magnitudes of 18.07, 19.50, and 21.65 magnitudes, respectively, whence instrumental RGU magnitudes and colors were derived for both the calibration standard and the program stars. Although the number of calibration standard stars is small and the residuals between their instrumental and standard magnitudes as functions of instrumental color show considerable scatter, the resulting color equations in Fig. 1 show the typical behavior expected as resulting from minor mismatches of the photometric passbands behind the instrumental and the standard systems. While the residuals for the two longer-wavelength photometric passbands (G, R) can be well approximated by linear equations, those for the short-wavelength U-magnitude feature a characteristic nonlinearity which is due to this near-UV passband's high sensitivity to the strength of the Balmer lines and discontinuity (Buser 1978a, 1988). Consequently, the corresponding color equation has been approximated by a saw-tooth-like curve defined by three contiguous linear segments of different slopes. Thus, the following color equations were adopted and applied in generating the final catalog of standard RGU data:

$$G_{\rm s} = G_{\rm obs} + 0.05(G{-}R)_{\rm obs} - 0.06,$$ (1)

$$R_{\rm s} = R_{\rm obs} + 0.17(G{-}R)_{\rm obs} - 0.17,$$ (2)

$$U_{\rm s} = U_{\rm obs} + 0.28(U{-}G)_{\rm obs} - 0.35, (U{-}G)_{\rm obs} \leq 1.66,$$ (3)

$$\begin{eqnarray} U_{\rm s} = U_{\rm obs} {-} 0.36(U{-}G)_{\rm obs} {+} 0.60, 1.66 &{<}& (U{-}G)_{\rm obs} \nonumber\\ &\leq& 2.49,\end{eqnarray}$$ (4)

$$U_{\rm s} = U_{\rm obs} + 1.09(U{-}G)_{\rm obs} - 3.05, 2.49 < (U{-}G)_{\rm obs}.$$ (5)

Full details on the plate journal, the identification charts, and the final photometric data will be published separately in the all-survey catalogs for 14 fields of the Basel high-latitude program (Buser et al. 1998b).

  

Figure 1: Residuals between instrumental and standard magnitudes defining the color Eqs. (1) through (5) which have been applied to standardize the present catalog of field data