In order to provide the beneficial expression of lens distortion for star trackers, the numerical results of the grid
distortions of four typical star tracker lens systems have been investigated, and then data fitting is done with combined
multinomial of different number of terms and powers of radial radius. The results indicate that the expression of relative
distortion including terms of the one, two, three and four powers of the radial distortion is beneficial in terms of
accuracy, and the fitting function with high-order is not quite helpful in providing a more accurate expansion. In order to
further validate this distortion model, a star tracker camera calibration approach has been simulated with the data
obtained by ray tracing via the Non-Sequential Components of ZEMAX, with imperfect alignment and assembly taken
into account. Simulation results also indicate that the four-order of the radial distortion is beneficial.
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