  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 using two natural guide stars with an apparent magnitude
of R<13 mag. During these observations the seeing was on average ~0.9". The AO
performed very well: 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 ~22.5 mag and the deepest Ks-band limiting magnitude ever
obtained in a crowded stellar field: Ks~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.
