A test of the photometric calibration accuracy for the guide-star catalog (Master Catalog) of the Spectrum-UV project (World Space Observatory-Ultraviolet) has been performed using CCD observations in a spectral band close to that of the guide sensor system of the T-170M space telescope. The mean photometric uncertainty of the Master Catalog at 14~m-17~m is 0.23~m; variations in this uncertainty over the sky are within a factor of two. About 2% of the stars in the Master Catalog were found to have photometric errors in excess of 2~m. We analyze the correspondence of large photometric errors to flags of the Master Catalog.
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