Placement of the corrector plate relative to the mirror on a Schmidt camera is not as critical as the placement of the film holder relative to the mirror. The corrector is generally placed at the radius of the mirror's curvature, or at twice the mirror's focal length, simply by measuring that distance with a ruler. However, the corrector plate has to be accurately centered relative to the optical axis. With the Celestron/Epoch Schmidt cameras, we have little control over the placement of the corrector. We take it on faith that the tube assembly was manufactured square on the end so the corrector cell is perpendicular to the optical axis. The corrector has such a low magnification power that it is fairly insensitive to a slight tilt or sagging from being supported only at its rim. The only real adjustment we have for the corrector is to be sure that the cork shims that center the corrector under its mounting rind are in good shape and are equally spaced around the corrector. If examination of star images reveals comatic stars, this may be corrected by using the corrector plate shims to move the corrector in the direction of the image coma.
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