SIMBAD references

A program of four-color photometry of BHB stars in globular clusters has been underway, using the 0.9 m telescopes at Cerro Tololo and Kitt Peak National Observatories. With 2048 times 2048 pixel chips an entire globular cluster can be measured on one frame. The earlier chips allowed only a small section of a cluster to be measured. In M 92, for example, 184 blue stars have been found in comparison to the 32 found in earlier single-channel photometric studies. Two features of CCD photometry contribute to the finding of a greater number of blue stars. One is that the CCD frame, as reduced by the ALLSTAR reduction program, allows one to measure stars in very crowded regions. Another feature is that fainter stars can be measured with the CCD technique. Some problems with CCD photometry are the lack of faint standards and the creation of flatfield frames. The blue stars in a globular cluster CM-diagram have been classified into several different groups (classical BHB stars, stars above the BHB and an interesting group found well below the horizontal branch).