Photometric reduction consists of three parts: (a) the reduction in the instrumental photometric system (b) the data correction for the atmospheric extinction and (c) the transformation to the standard photometric system. For every new step of the reduction new errors are introduced and therefore the final accuracy of the results will decrease inevitably.
Two two-dimensional Moffat profiles were fitted simultaneously to the double star components on each CCD frame according to the classical least square technique (Cuypers 1994).
The components' magnitude difference resulted from this on chip instrumental photometry had always an accuracy of a few millimagnitudes. On the other hand, all nights proved to be of good photometric quality as well. These two factors enabled us to get nightly instrumental magnitudes of the components of the observed double stars with an accuracy of 0.004 mags or better.
The atmospheric extinction correction
was derived according to the classical formulas
where the ``0" index refers to the mangitude and the color indices outside the atmosphere. We should remind the v mangitude and the b-v and u-b color indices refer to the instrumental values.
For the determination of the first order extinction coefficients we used the
A0 - type E408 standard star from the E regions (Graham
1982), which was observed each night, many times per night at
different airmasses smaller than or equal to two.
The first order extinction coefficients that we obtained are the
This list shows as well which filter was used each night
for carrying out the observations.
Second-order extinction coefficients of the UBV system are fairly constant and can be calculated only twice per year for each observatory. For this reason, we used the values given by Gocherman et al. (1993) as updated by Grothues (1995).
For the transformation in the standard system, we used the average and
the standard deviation values
of the V extinction corrected magnitude and of the (b-v) extinction
corrected colour. These were calculated from the two V and the two
(b-v) extinction corrected values,
which were obtained in the corresponding two pairs of two
different nights of observations with the same filter.
As we observed only one night in U filter, we calculated the error of the
(u-b) extinction corrected colour
according to the corresponding error equation.
The three standard deviation values of these extinction corrected magnitudes and colours, were around 0.01 mags.
The V magnitude and the B-V and U-B color indices, which refer to the
standard system, can be obtained from the extinction
corrected values v, (b-v)
and (u-b) according to the following transformations:
For the estimation of and we used
improved photometric values of 18 photometric
standard stars from the E regions (Grenon 1991)
observed in the night of the to of
April. The values obtained for the transformation coefficients are the
Figure 1: The two colour diagram
|No||ADS or DM|
Table 2 contains the photometric results of the reduction that was performed on 40 visual binaries. Two lines per binary are given. The first corresponds to V magnitude and colours of the primary while the second one of the secondary. As in Table 1, the first column is an enumeration of the binaries, while the second column contains the Durchmusterung designation of the primaries. The next three columns contain the V magnitude and its error and the (B-V) and (U-B) color indices with their errors and respectively.
The error estimations in Table 2 resulted from the standard
deviation values of the extinction corrected magnitudes
and colours and the uncertainties of the transformation coefficients
into the standard UBV system.