further analysis we used only the objects with a $S/N  \geq  4$. The $S/N = 4$ corresponds to the error of 0.25$^m$ and for an object with magnitude determined with such an error the color error is 0.35.

The photometric procedure consisted of the following stages:

1.  Since we were interested first of all in blue ($ B-V   <0$) objects, then by the images obtained in the [*B*]{} filter using MIDAS context INVENTORY and smoothed with a gaussian we obtained coordinates for all detected objects of the whole mosaic.

2.  By the coordinates obtained at the first stage the aperture photometry was done in all three [*BVR*]{} filters.

3.  The obtained values were corrected for the finite radius of the aperture and atmospheric extinction at the given zenith distance, and were reduced to the standard Cousins system.

The results of the photometry are shown in Figs. 3,4,5. The crosses denote 31 objects in Figs. 3,4 with $B-V \ <\ 0$ and $S/N  >  4$. Photometric limits (defined as $ S/N=4$) for the CCD mosaic are given in Table 2.

The $S/N$ was calculated for every object by the formula: $$\frac{S}{N} = \frac{F_{star}}{\sqrt{F_{star}+F_{sky}}},$$ where $F_{star}$ is the flux from a star 