Astron. Astrophys. 352, 600-604 (1999)

3. Observations and reductions

3.1. Observing procedures

All observations in the present catalogue were made by the SAT in its fully-automatic mode during ten nights in December 1997 - January 1998. Details about the spectrometer, the observing procedure, and the fully-automatic mode are given by Olsen (1993, 1994).

A circular diaphragm of 17" was used. The number of photo-electrons counted was 100,000 in the y-channel and 70,000 in the -channel, except for a few of the faintest stars. The background was measured at each program star at a fixed offset.

3.2. Instrumental systems

Before reductions, several sky measurements from a small area of the sky, and contiguous in time, were combined and then used on all stars in the area. Sky measurements contaminated by faint stars were eliminated in this process.

The instrumental systems of the SAT were computed, following the procedure outlined by Olsen (1993). The uvby system is based on 22 nights and the system is based on 13 nights in the period mentioned above. For all nights, second-order night corrections have been applied.

3.3. Transformations to the standard systems

For the 30 secondary standards, the photometry from the sources referred to by Olsen (1988) was adopted as standard, i.e. it essentially defines the uvby system for reddened OB-stars and the system for values below 2.6.

Olsen (1983) discussed a probable systematic error in the photometry of the so-called SH stars among the primary uvby standards. He suggested certain small corrections to this subset of the primary standards. Most of the 30 secondary standards used here have photometry from Crawford et al. (1971) and probably suffer from the same systematic error. Therefore, the corrections suggested by Olsen (1983) were also applied to the photometry of these stars.

Two of the secondary standards (HD 152236 and 154090A) were only used as standards (cf. their D residuals in Table 1).

Table 1. Catalogue of 53 standard stars (primary and secondary) on the Crawford-Barnes standard uvby system. Secondary standard stars are marked by an asterisk. Column 2 gives the photometry transformed to the standard Johnson V magnitude. The internal rms errors of one observation (weight 1) are given in Cols. 3, 5, 7, and 9, respectively, for V, , , and . NN is the number of nights on which the star was observed. W is the weight of the four-colour indices, and VW the weight of the V magnitude. The last four columns give the differences D = transformed value - standard value. Units: 1 mag.

All standard stars were given equal weights in the least-squares solutions, which determine the following transformation equations:

where subscript i refers to the instrumental systems and is the standard deviation. A comparison with the transformation coefficients determined earlier for B, A, and F-type main-sequence stars (Olsen 1993, Table 7) shows several significant differences. This is not surprising, considering the luminous and partly reddened nature of the OB standards used here.

© European Southern Observatory (ESO) 1999

Online publication: December 2, 1999
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