Use of DPOSS data to study globular cluster halos: an application to M 92

We exploited the large areal coverage offered by the Digitized Palomar Observatory Sky Survey to analyze the outer most regions of the galactic globular cluster M 92 (NGC 6341). Two independent photometric reduction programs (SKICAT and DAOPHOT) welt used to construct a color-magnitude diagram and a surface density profile for this cluster, based on J- and F-band DPOSS plates. A strong similarity has been found in the performance of the two programs in the low-crowded outermost cluster regions, After removing the background contribution, we obtained the cluster outer surface density profile down to a surface brightness magnitude of mu(V) similar to 31 mag arcsec(-2) and matched it with the inner profile of Trager et al. (1995). The profile shapes match very well: since our data are uncalibrated, the shift in magnitudes between the profiles has been also used to calibrate our profile. The analysis shows that the cluster has an extra tidal halo extending out to similar to 30' from the cluster center at a 3 sigma level over the background noise. This halo is revealed to be almost circular.