Crowded Field Photometry of M15 Using the TRIFFID Camera

High-resolution imaging has been performed for a number of years using TRIFFID, the UCG DIAS High Resolution Camera. Post-exposure image sharpening (PEIS) of TRIFFID data routinely produces resolution improvements up to the theoretical maximum factor of two. HST resolution remains superior but unlike TRIFFID it cannot be used for extended observations of variable objects, and its photometric calibration is poorer. We have combined the complementary qualities of HSTs high astrometric precision with TRIFFIDs extended coverage and photometric precision to perform crowded-field photometry in the innermost region of the post core-collapse globular cluster M15. The formation of close binaries (including eruptive binaries) by tidal interaction in the core of post core-collapse globular clusters was expected theoretically, but investigations have been hampered hitherto by the extreme crowding towards the cluster cores. Our combined technique virtually eliminates this problem, and thereby provides an extension of the HSTs capability. Our photometric study produced evidence of faint variable stars. We stress that it will require longer period photometry to uniquely identify the type of variable: should they be confirmed as eruptive binaries this would represent a vital step in our understanding of globular cluster evolution and dynamics.