Adjusting Camera Focus

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.

The camera focus is adjusted by turning the jam nuts that hold the spider to the Invar rods that space the spider assembly and the mirror. It must be emphasized that this is a meticulous operation that will take time. Carefully log all adjustments so that any overcorrections can be undone. When working with the adjusting nuts, a turn of one flat on the nut (one 6th of a full turn) moves it a little less than .008-inch. However, when adjusting one of the three Invar rods by one nut flat, (one 6th of a turn) it actually moves the film holder by .004-inch because the adjustment works across the diameter of the camera. Making minute, precision adjustments to the nuts on the Invar bars is easier than you think. I say this from the experience of an engine mechanic who routinely uses setscrews and jam nuts to adjust cylinder head valve clearances to within .001". The trick is don't think of moving the nuts a full flat. Don't even loosen the opposing jam nut when you initially tighten a nut for an adjustment. Simply apply pressure on the nut being adjusted until you perceive that it has indeed just fractionally moved. The same is true when loosening the opposite jam nut. At no point is either of the adjusting nuts ever completely loosened. You are "creeping" the adjustment by a 1/4 or less of a nut flat at a time and taking up the microscopic slack between the pitch of the nut and bolt threads. It's a slow process, but fine tolerances can be achieved.

When adjusting the Invar rod jam nuts, do not loosen any of the nylon set screws that center the Invar cage assembly in the tube.

The filter film holder is focused the same way as the white light film holder. However, a nearly transparent Wratten filter must be installed on the holder to compensate for the shift in the focus position machined into the holder to accommodate the refracting properties of the filter material. Ideally, both the white light and filter film holders should focus identically if they were both manufactured specifically for the same camera. If used equipment is being checked, there is a possibility that film holders could have been swapped between cameras and they may not be parfocal.

The optical focus method can achieve good accuracy in setting the focus of a Schmidt camera. But there is room for improvement in achieving ultimate perfection in star images. This will require some short star exposure on film. To determine the exact adjustment point where the camera passes from inside focus to outside focus (or vice versa), place a 45-degree pie-shape of dark paper on the corrector with the point at the center of the corrector. Take a test photo and examine the star images at high magnification. Out of focus stars will show the pie shape. Continue to adjust the focus. If you pass the point of best focus, the pie shape in the star images will flip to the opposite side of the star images. This tells you that you now have to back up the focus.

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