New pointing calibration technique using star signals in the ASTRI Cherenkov camera and the Variance method

The pointing calibration of Imaging Atmospheric Cherenkov Telescopes (IACTs) is often a technological challenge: their cameras are not designed for imaging the stars in the Field of View (FoV) and this prevents from using the standard astrometry of the focal plane for monitoring the pointing of the instrument. A common solution is to adopt auxiliary optical devices aligned with the line-of-sight of the telescope but, in order to avoid systematic errors, a pointing strategy considering also the signal from the Cherenkov camera is desirable, especially when a dual-mirror optical configuration i s a dopted. I n t his c ontribution, we p resent a n ew custom astrometry technique that we developed for the Cherenkov camera of ASTRI telescopes, using the so-called Variance method: an ancillary output data-flow o wning t he p ossibility t o i mage t he s tellar c omponent o f the Night Sky Background with relatively good sensitivity (limiting magnitude similar to 7). Despite the large angular size of Cherenkov camera pixels (similar to 11 '') and their relatively small number (a few thousand), our automatic astrometric routine is able to identify the stars in the FoV with sub-pixel precision, giving the possibility of monitoring the pointing of the telescope in real-time, without any additional hardware. Our technique has been already tested on archive data taken with the ASTRI-Horn prototype telescope, located in Italy, and it will be implemented in the incoming ASTRI Mini-Array: a facility of 9 identical Cherenkov telescopes under construction in Tenerife (Canary Islands). In this contribution we discuss the features of this novel procedure, its potentialities, and how they will enhance the scientific accuracy of future ASTRI telescopes.