A distortion solution for the Bok telescope with four CCD chips

Abstract

The Beijing-Arizona Sky Survey (BASS) is an imaging survey that uses the 2.3-m Bok telescope at Kitt Peak. In order to realize the astrometric potential of the Bok telescope and improve the astrometry of BASS, a distortion solution for the Bok telescope is needed. In the past, we used a single lookup table to correct all the positional errors. However, this method cannot be applied to the reduction of observations from the Bok telescope, which is equipped with four CCD chips. We thus developed a method that is quite different from our previous one, using two third-order polynomials to fit the lookup table. By using the polynomial geometric distortion (GD) correction, the astrometry of BASS is greatly improved. Moreover, an additional lookup-table correction is found to be more effective to obtain the final GD. The results show that the positional measurement precision of the appropriate bright stars is estimated at about 20 mas or even better in each direction. In addition, the relative positions of the chips are measured. The change of the inter-chip gaps in the horizontal or vertical is no more than 5 pixels between 2016 and 2017, and the change of the roll angle is no more than 0.1°.