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The Beijing-Arizona Sky Survey (BASS) is an imaging survey and uses the 2.3 mBok telescope at Kitt Peak. In order to tap the astrometric potential of theBok telescope and improve the astrometry of BASS, a distortion solution for theBok telescope is made. In the past, we used a single lookup table to correctall the positional errors. However, this method can not be applied to thereduction of the observation for the Bok telescope where four CCD chips areequipped with. Then quite different from our previous method, two third-orderpolynomials were used to fit the lookup table. By using the polynomialGeometric Distortion (called GD hereafter) correction the astrometry of BASS isimproved greatly. Moreover, an additional lookup table correction is found tobe more effective to obtain a final GD. The results show that the positionalmeasurement precision of the appropriate bright stars is estimated at about 20mas and even better in each direction. Besides, the relative positions of thechips are measured. The change of the inter-chip gaps in horizontal or verticalis no more than 5 pixel between 2016 and 2017 and the change of the roll angleis no more than 0.1 degree.

A distortion solution for the Bok telescope with four CCD chips