TRIFFID PHOTOMETRY OF GLOBULAR-CLUSTER CORES - I - PHOTOMETRIC TECHNIQUES AND VARIABLE-STARS IN M15

The Galway/DIAS Image Sharpening Camera, TRIFFID, has been used to mak e observations in two colours of the centre of the post-core-collapse globular cluster M15. We present here our analysis of the photometry i n B over two seasons. We have combined the complementary qualities of the HST's high astrometric precision and TRIFFID's extended coverage a nd photometric precision, to perform crowded-field photometry in the i nnermost region of M15, Our technique virtually eliminates the problem of extreme crowding which has hitherto hampered studies of the variab le star populations in globular cluster cores, and thereby provides an extension of the HST's capability. Candidate variables detected with the HST have been confirmed and monitored over longer periods. We show that most of these are RR Lyrae stars, and that one is a short-period Type II Cepheid (the third to be discovered in M15), Our photometric study also produced evidence of a similar number of new variables. The se also appear to be RR Lyrae stars, except for a possible eclipsing s ystem. Further data from an upgraded version of TRIFFID have recently been obtained to help to refine the light curves of all these objects.