Distortions were measured at thirty-six positions over the photocathodes of four different types of image orthicon tubes operated in a high precision camera system designed for astronomical use. Rms values of distortion for these tubes were found to be of the order of 0.2% to 0.3% of the picture width. Changes in distortion over 16–26-h periods of operation were measured for three tubes and were found to be of the order of 0.02% to 0.03% of the picture width, making it possible to correct astronomical TV images for distortion to within a few parts in 104.
W. T. Powers and J. Burns, "Experimental Evaluation of Geometric Accuracy and Stability of the Image Orthicon for Astronomical Applications," Appl. Opt. 9, 2172-2175 (1970)