Deep UBVRI photometric calibration of high-latitude fields: SA 57 (1307+30) and Hercules (1720+50)

We present CCD photometric calibration sequences in the magnitude range V = 17-22 for two fields at high Galactic latitude: SA 57 (at the North Galactic Pole) and Hercules (l = 77, b = 35). Photometry to a precision of about 0.02 mag at V = 20 and, in general, better than 0.10 mag at V = 22 was obtained in the Johnson UBV as well as the Kron-Cousins R and I bands. These data are suitable for setting magnitude zero-points in catalogues of faint stars, galaxies, and QSOs, and we apply them to our own photographic catalogs in these two fields. We also note a significant deviation in the (V-R, R-I) color-color diagram for the locus of faint (V is greater than 20) M dwarfs compared to the locus provided by much brighter M dwarfs. This deviation may indicate differences in spectral properties between Population I and older populations of late dwarfs; however we do not discount the possibility that this locus for the faint stars, which appears as a saturation in V-R color with increasing R-I color, is the result of systematic photometric error.