3. Observation

As mentioned above the photoelectric observations were carried out with the fully automatic Danish 50 cm telescope (SAT) at La Silla (Florentin Nielsen et al. 1987; Sterken & Manfroid 1991; Sterken 1991). This telescope is permanently equipped with a six-channel spectrograph photometer and photon counting system for simultaneous measurements in uvby or , respectively (Florentin Nielsen 1987). The pre-programmed observing procedure includes accurate centring at a target before (and often also between) the 2-3 individual 10-120 seconds integrations. Sky measurements of adequate duration, calculated from the angular distance to the Moon and from its phase, were included near each object. All data were obtained through a circular diaphragm of diameter. Except for the u measurements of the faintest and reddest stars the photon shot noise (statistics of star plus sky) was kept at or below 0.5% per measurement.

The observations were carried out during November 1993, October 1994 and November 1995 (see Table 1 (click here)). The 1993 and 1994 periods were devoted to uvby and observations of B, A and F stars covering the b-y range from -0.08 to 0.40 and the range 2.58 - 2.90, thereby providing secondary standards for the uvby "BAF region" (Olsen 1983) and the "B and A regions" (Grønbech & Olsen 1977). In 1995 additional uvby photometry, but no measurements, were obtained for some F stars in the b-y range 0.25-0.40, but the main effort was placed on the two transformation regions for G and K dwarfs and giants, respectively (Olsen 1983). This procedure was chosen in order to be able to include a large number of primary standards (and extinctions stars) for the relevant transformation regions every night. Typically 50-70 observations of primary standards were done per night compared to about 40-50 measurements on program star. The primary standard stars were selected from the references mentioned in Sect. 1 (click here).

  [IMAGE ]
Figure 2: Residuals () for the primary standard stars with error bars indicating the rms errors of one observation. The transformation regions are shown as circles (A) and boxes (B), respectively

  [IMAGE ]
Figure 3: Comparison (uvby) with the photometry of Knude (1992). The error bars indicate the rms errors of one observation (this paper)

  [IMAGE ]
Figure 4: Comparison (uvby) with the photometry of Kilkenny & Laing (1992). The error bars indicate the rms errors of one observation (this paper)

  [IMAGE ]
Figure 5: Comparison (uvby) with the photometry of Jønch-Sørensen (1993). The error bars indicate the rms errors of one observation (this paper)

  [IMAGE ]
Figure 6: Comparison (uvby) with the photometry of Olsen (1993, 1994). The error bars indicate the rms errors of one observation (this paper)

  [IMAGE ]
Figure 7: Comparison () with the photometry of Knude (1992) (upper) and Jønch-Sørensen (1993) (lower). The error bars indicate the rms errors of one observation (this paper)