UBVI photometry of NGC 6864

Specifically, we study the level both of the horizontal (HB) and red giant branches (RGB) relative to the main-sequence turnoff (TO) in the U magnitude. Along with the presented photometry of M75, we use our collection of photometric data on GCs belonging to the metal-poor range, [Fe/H]zw<-1.1dex, obtained from observations with different equipment, but calibrated by standard stars situated in the observed cluster fields.

We confirm our earlier finding, and extend it to a larger magnitude range. We demonstrate that Delta(U_TO_^BHB^) expressing the difference in U magnitude between the TO point and the level of the blue HB, near its red boundary, of the metal-poor GCs observed with the EMMI camera of the NTT/ESO telescope is about 0.4-0.5mag smaller as compared to GCs observed with the 100" telescope and 1.3m Warsaw telescope of the Las Campanas Observatory.