1994PASP..106.1258M

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 magnitudes at V=20 and, in general, better than 0.10 magnitudes 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 photograhic catalogues 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>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.