  Precise photometric and astrometric measurements on astronomical images
require an accurate knowledge of the Point Spread Function (PSF). When the PSF
cannot be modelled directly from the image, PSF-reconstruction techniques
become the only viable solution. So far, however, their performance on real
observations has rarely been quantified. Aims. In this Letter, we test the
performance of a novel hybrid technique, called PRIME, on Adaptive
Optics-assisted SPHERE/ZIMPOL observations of the Galactic globular cluster
NGC6121. Methods. PRIME couples PSF-reconstruction techniques, based on
control-loop data and direct image fitting performed on the only bright
point-like source available in the field of view of the ZIMPOL exposures, with
the aim of building the PSF model. Results. By exploiting this model, the
magnitudes and positions of the stars in the field can be measured with an
unprecedented precision, which surpasses that obtained by more standard methods
by at least a factor of four for on-axis stars and by up to a factor of two on
fainter, off-axis stars. Conclusions. Our results demonstrate the power of
PRIME in recovering precise magnitudes and positions when the information
directly coming from astronomical images is limited to only a few point-like
sources and, thus, paving the way for a proper analysis of future Extremely
Large Telescope observations of sparse stellar fields or individual
extragalactic objects.
