in the given aperture, $F_{sky}$ is the sky background in the same aperture.

Photometric errors were determined as follows:

1.  If we had one measurement only, then the error was calculated using Poisson distribution of accumulated quanta: $$\sigma_{mag} = \sqrt{ \sigma_{star}^{2} +  \sigma_{sky}^{2}},$$ where $$\sigma_{star} = \sqrt{F_{star}},\, \sigma_{sky} = \sqrt{F_{sky}}.$$

2.  If we have several measurements, then in addition to 1) $$\sigma_{mag}
      = \sqrt{\frac{\sum_{i}^{n} (mag_{sr} - mag_{i})^{2}}{n-1}}.$$ And correspondingly, the color errors are obtained as follows: $$\sigma_{(B-V)}
        = \sqrt{ \sigma_{B}^{2} +  \sigma_{V}^{2}},\,  \sigma_{(V-R)} = \sqrt{
        \sigma_{V}^{2} +  \sigma_{R}^{2}}.$$ The color errors for several objects with color indices closest to those of probable black–body objects are shown in Fig.5.

The search for blue stellar–like objects
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The search for a neutron star type object with a black–body surface temperature $T_{bb}\approx  10^{5}\rm \ K$, as a possible potential source of a $\gamma$-ray burst, in the case $\gamma$-ray bursters are of local Galactic origin, was the main purpose of our observations. These sources must be observed as faint ($ V>23^{m}$) blue stellar–like objects of $ B-V \approx -0.37\ {\rm and}\
\ V-R \la -0.1$ and may be soft X-ray sources.

That is why our first step in selection of probable “candidates” was selection 