2. Selection of secondary standards

  As the CCD observations for the MC projects mentioned above are carried out at the Danish 1.5 m telescope at La Silla, it was decided to establish the set of secondary standards through simultaneous observations at the small fully automatic Danish 50 cm photometric telescope (Strömgren Automatic Telescope or SAT), also located at La Silla. The procedure has the additional advantage of saving observing time at the 1.5 m telescope, since the nightly extinction can be measured both more accurately and easier at SAT. For our telescope combination stars in the magnitude interval 8.5 - 11 are optimal as secondary standards; see also Jønch-Sørensen (1993) for further comments.

In order to cover a wide range of the HR diagram we have select candidates from several lists of stars with published indices. The number of previous observations per stars is often low, and as different transformation procedure have furthermore been applied, it was decided to re-observe all candidates together with an extensive net of primary standards, and thereby both increase the internal accuracy and also secure homogeneity in the transformations.

B stars were selected mainly from Knude (1992), who presented photometry for 528 stars in the V range 8 to 9 based on at least two (but always few) observations per object. Knude used B, A, F and G standards for the transformation without any separation in groups like those defined by Olsen (1983).

A and F stars were selected from the partly overlapping studies of E-region stars by Jønch-Sørensen (1993), which includes results for 64 stars, and by Kilkenny & Laing (1992), which contains uvby data for 201 stars. E-region G and K giants were taken also from Jønch-Sørensen (1993). 28 stars are included in both lists, and although different approaches have been applied in the transformation of the observations to the standard system, the uvby results are reported to agree well (Jønch-Sørensen 1993).

Finally a limited number of G and K dwarfs were selected from the extensive catalogues by Olsen (1993, 1994a).

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Figure 1: Residuals (uvby) for the primary standard stars. The transformation regions are indicated by circles (BAF), triangles (GK V) and boxes (GK III), respectively