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Cherenkov telescope cameras are not suitable to perform astrometricalpointing calibration since they are not designed to produce images of the sky,but rather to detect nanosecond atmospheric flashes due to very high-energycosmic radiation. Indeed, these instruments show only a moderate angularresolution (fractions of degrees) and are almost blind to the steady orslow-varying optical signal of starlight. For this reason, auxiliary opticalinstruments are typically adopted to calibrate the telescope pointing. However,secondary instruments are possible sources of systematic errors. Furthermore,the Cherenkov camera is the only one framing exactly the portion of the skyunder study, and hence its exploitation for pointing calibration purposes wouldbe desirable. In this contribution, we present a procedure to assess thepointing accuracy of the ASTRI-Horn telescope by means of its innovativeCherenkov camera. This instrument is endowed with a statistical method, theso-called Variance method, implemented in the logic board and able to provideimages of the night sky background light as ancillary output. Taking intoaccount the convolution between the optical point spread function and the pixeldistribution, Variance images can be used to evaluate the position of starswith sub-pixel precision. In addition, the rotation of the field of view duringobservations can be exploited to verify the alignment of the Cherenkov camerawith the optical axis of the telescope, with a precision of a few arcminutes,as upper limit. This information is essential to evaluate the effectivepointing of the telescope, enhancing the scientific accuracy of the system.

Effective pointing of the ASTRI-Horn telescope using the Cherenkov camera with the Variance method