The blazars in the fields of which photometric sequences were obtained are
listed in Table 1, where their coordinates at the 2000 equinox are
The comparison stars are indicated in the finding charts shown in Figs. 1-10
(they are 10 wide; north is up and east is on the left). Their
calibrated magnitudes are reported in Table 2, together with the
estimated uncertainties () and the number of observations (N) done in
each band. In general, the given magnitude represents the median value among
the observations; when only two observations are available, the weighted mean
is given. The reported uncertainty corresponds to the maximum between the
standard deviation on the weighted mean and the uncertainty of the mean defined
as , where are the
uncertainties of the single data.
This is one of the best measured fields, with 8 observations in the B and R bands, and 4 observations in the V one.
A standard sequence was published by Ghisellini et al. (1997), with errors ranging from 0.04 to 0.08 mag; their Stars A, B, C, and D correspond to our Stars 2, 3, 5, and 6, respectively. By comparing our results with theirs, we see that magnitudes are in agreement within the errors, with the only exception of the R magnitude of Star 2.
Star 5 of the present photometric sequence was also calibrated by Takalo et al. (1994; their Star 2), whose derived magnitudes are in accordance with ours.
A comparison among the results of the different authors is performed in Table
Two comparison stars in the field of MS 0737.9+7441 were already calibrated by Smith et al. (1991); their Stars A and B are our Stars 1 and 4 and the respective magnitudes agree inside the errors.
Notice in Fig. 4 the presence of another AGN (MS 0737.0+7436) to the south-west of the BL Lac.
Star 1 of our photometric sequence corresponds to that calibrated in the UBV bands by Véron & Véron (1976); while the V magnitudes are in agreement, we find a B magnitude which is fainter by .
Four stars were calibrated in the field of Mkn 180; Star 3 is just below the source, and in Fig. 7 is not distinguishable from it. In our frames instead it is well separated from the BL Lac.
Stars 1 and 2 of our photometric sequence have already been calibrated by
Véron & Véron (1976; their Stars 3 and 2) in the UBV filters and
by Smith et al. (1991; their Stars A and B) in the UBVRI bands. We
confirm the results of the above authors in the BVR filters. Standard VRI
magnitudes for Stars 1, 4, and 6 were published by Fiorucci & Tosti
(1996); also in this case there is accordance among their data and ours.
A comparison among calibrations is shown in Table 4.
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