High-resolution optical imaging of the core of the globular cluster M15 with FastCam

Abstract

We present high-resolution I-band imaging of the core of the globular cluster M15 obtained at the 2.5-m Nordic Optical Telescope with FastCam, a low readout noise L3CCD-based instrument. Short exposure times (30ms) were used to record 200 000 images (512×512 pixels each) over a period of 2 h and 43 min. The lucky imaging technique was then applied to generate a final image of the cluster centre with full width at half-maximum ∼0.1 arcsec and 13 × 13 arcsec^2 field of view. We obtained a catalogue of objects in this region with a limiting magnitude of I = 19.5. I-band photometry and astrometry are reported for 1181 stars. This is the deepest I-band observation of the M15 core at this spatial resolution. Simulations show that crowding is limiting the completeness of the catalogue. At shorter wavelengths, a similar number of objects have been reported using Hubble Space Telescope (HST)/Wide Field Planetary Camera observations of the same field. The cross-match with the available HST catalogues allowed us to produce colour–magnitude diagrams where we identify new blue straggler star candidates and previously known stars of this class.

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