SIMBAD references

The DArk Matter Particle Explorer (DAMPE) can measure γ-rays in the energy range from a few GeV to about 10 TeV. The direction of each γ-ray photon is reconstructed in the DAMPE payload reference system. To convert this to celestial coordinates, we require the celestial orientation of the payload system, which, however, may slightly deviate from that of the satellite system provided by the star-tracker. In this paper, we adopt a maximum likelihood method and use the γ-rays centered around several bright point-like sources to measure and correct the angular deviations between the DAMPE payload and satellite system, the so-called "boresight alignment". We also check our method of boresight alignment for some sets of simulation data with artificial orientations and obtain consistent results. The time-dependent boresight alignment analysis does not show evidence for significant variation of the parameters.