Use a 100-watt or brighter lamp to illuminate the film holder through the film-loading door. View the film holder through the telescope eyepiece. If there is no sharp image, shift the eyepiece in the diagonal. If you shift the eyepiece out of the telescope to reach a sharp image you have to decrease the distance between the film holder and the mirror, and visa versa (* see note at bottom of paragraph). The eyepiece travel needed to achieve good focus of the film holder in the eyepiece is proportional to the square of the focal lengths of the Schmidt camera and the focusing telescope. When using an 8-inch SCT as a focusing scope, this formula works out to (80 inches/12 inches) squared. If we move the eyepiece a quarter of an inch (.250 inches) to achieve good focus on the target film holder, the film holder must be moved just under .006 inches to shift it to the proper focus point. This gives us a very sensitive tool for checking the camera's focus. The focus at the center of the film holder is checked first, and then the overall focus is examined by looking at three points on the edge of the film holder corresponding to where the Invar rods lie parallel to the edge of the film holder. To see the edges of the holder move the telescope away from the main axis with the holding fixture jackscrews. To avoid any confusion it is helpful to have marked numbers in the different quarters of the holder. All positions on the film holder ought to be seen focused correctly when being viewed at the "infinity" position with the eyepiece. If so, the camera focus is adjusted correctly.

*NOTE: When I researched this procedure, this is the way it was explained to me. But the first time I performed the focus check, the instructions seemed backward. After I refocused the telescope on a star to "rezero" the infinity focus, the procedure seemed to work properly again. My conclusion is that you have to be very careful about what you use as a focusing target in the film holder. It is harder to focus on the surface of the film than you would think.

 When viewing the focal plane with a film emulsion mounted in the film holder, the face of the film may be featureless except for a few random dust specs. To add some depth to the bland film emulsion to aid focusing, lightly dust the film plane with fine talcum powder. Be sure to thoroughly clean the powder off the film holder before doing photography.

To allow navigation across the film plane with the high magnification focusing scope scratch an x in the center and numbers where each Invar rods passes the edge.

 Scratching a mark into the film with a fine pin point shows as deep gouge with the high manification of the focusing scope. Numbering the edge of the film plane where each Invar rod is adjusted allows recording the effect of each focus adjustment.