On the use of asymmetric PSF on NIR images of crowded stellar fields

We present data collected using the camera PISCES coupled with the Firt Light Adaptive Optics (FLAO) mounted at the Large Binocular Telescope (LBT). The images were collected for two different pointings by using two natural guide stars with an apparent magnitude of R less than or similar to 13 mag. During these observations the seeing was on average similar to 0.9 arcsec. The AO performed very well, in fact the images display a mean FWHM of 0.05 arcsec and of 0.06 arcsec in the J- and in the Ks band, respectively. The Strehl ratio on the quoted images reaches 13-30% (J) and 50-65% (Ks), in the off and in the central pointings respectively. On the basis of this sample we have reached a J-band limiting magnitude of similar to 22.5 mag and the deepest Ks band limiting magnitude ever obtained in a crowded stellar field: Ks similar to 23 mag. J-band images display a complex change in the shape of the PSF when moving at larger radial distances from the natural guide star. In particular, the stellar images become more elongated in approaching the corners of the J-band images whereas the Ks-band images are more uniform. We discuss in detail the strategy used to perform accurate and deep photometry in these very challenging images. In particular we will focus our attention on the use of an updated version of ROMAFOT based on asymmetric and analytical Point Spread Functions. The quality of the photometry allowed us to properly identify a feature that clearly shows up in NIR bands: the main sequence knee (MSK). The MSK is independent of the evolutionary age, therefore the difference in magnitude with the canonical clock to constrain the cluster age, the main sequence turn off (MSTO), provides an estimate of the absolute age of the cluster. The key advantage of this new approach is that the error decreases by a factor of two when compared with the classical one. Combining ground based Ks with space F606W photometry, we estimate the absolute age of M15 to be 13.70 +/- 0.80 Gyr.