摘要：Context. We show the benefits of using electron-multiplying CCDs and the
shift-and-add technique as a tool to minimise the effects of atmospheric turbulence, such
as blending between stars in crowded fields, and to avoid saturated stars in the fields
observed. We intend to complete, or improve on, the census of the variable star population
in globular cluster NGC 6715.
Aims. Our aim is to obtain high-precision time-series photometry of the
very crowded central region of this stellar system via the collection of better angular
resolution images than has been previously achieved with conventional CCDs on ground-based
telescopes.
Methods. Observations were carried out using the Danish 1.54-m telescope
at the ESO La Silla observatory in Chile. The telescope is equipped with an
electron-multiplying CCD that enables short-exposure-time images to be obtained (ten
images per second) that were stacked using the shift-and-add technique to produce the
normal-exposure-time images (minutes). The high precision photometry was performed via
difference image analysis employing the DanDIA pipeline. We attempted automatic detection
of variable stars in the field.
Results. We statistically analysed the light curves of 1405 stars in the
crowded central region of NGC 6715 to automatically identify the variable stars present in
this cluster. We found light curves for 17 previously known variable stars near the edges
of our reference image (16 RR Lyrae and 1 semi-regular) and we discovered 67 new variables
(30 RR Lyrae, 21 irregular (long-period type), 3 semi-regular, 1 W Virginis, 1 eclipsing
binary, and 11 unclassified). Photometric measurements for these stars are available in
electronic form through the Strasbourg Astronomical Data Centre.
关键词：atmospheric effects;instrumentation: high angular resolution;globular clusters: individual: NGC 6715 (M 54);methods: observational;stars: variables: general;stars: variables: RR Lyrae
