other observers, we deliberately choose a symmetric [$\gamma$]{}–ray box obtained as a result of intersection of three stripes, and not only its northern part, which is a common part for all four triangulation stripes, since the symmetry of the error box evidences in favor of a correct burst localization. The total area of the symmetric box is $\approx 1.5\,{\rm \ arcmin^{2}}$.

The log–book is adduced in Table 1. For the GRB790613 field there are: I – the date and UT of observations, II – the area number and the filter, III – the exposure time, IV – the seeing, V – the night sky brightness per one square arcsec, VI – the zenith distance.

The procedure of construction of the GRB790613 field mosaic we obtained is shown in Fig.2. The roman numerals and bold letters correspond to the numbers and letters in column III of the log-book. Small letters indicate seven secondary photometric standards. The numbers correspond to areas with homogeneous photometric limits (see Table 2).

Data reduction and calibration
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Standard data processing involves bias removing, flat fielding and removing of space particles and bad chip pixels. Bias-images were obtained during the night at regular intervals. Flat-field frames were 