3 Results

The blazars in the fields of which photometric sequences were obtained are listed in Table 1, where their coordinates at the 2000 equinox are given.

  

Table 1: List of the BL Lacs for which photometric sequences are derived

The comparison stars are indicated in the finding charts shown in Figs. 1-10 (they are 10$^\prime$ wide; north is up and east is on the left). Their calibrated magnitudes are reported in Table 2, together with the estimated uncertainties ($\sigma$) 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 $\sigma_\mu = (\sum{1/\sigma_i^2})^{-1/2}$, where $\sigma_i$ are the uncertainties of the single data.

Table 2:BVR magnitudes of the comparison stars

3.1 S5 0716+71

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 3.

  

Table 3: Comparison among different calibrations in the field of S5 0716+71

3.2 MS 0737.9+7441

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.

3.3 Mkn 421

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 $0.10 \, \rm mag$.

3.4 Mkn 180

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.

3.5 Mkn 501

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.

  

Table 4: Comparison among different calibrations in the field of Mkn 501

  

Figure 1: Finding chart of PKS 0048-09

  

Figure 2: Finding chart of 1ES 0502+675

  

Figure 3: Finding chart of S5 0716+71

  

Figure 4: Finding chart of MS 0737.9+7441

  

Figure 5: Finding chart of 1ES 1028+511

  

Figure 6: Finding chart of Mkn 421

  

Figure 7: Finding chart of Mkn 180; Star 3 is just below the BL Lac

  

Figure 8: Finding chart of 1ES 1517+656

  

Figure 9: Finding chart of Mkn 501

  

Figure 10: Finding chart of 1ES 1959+650