Part II: Collimating and Focusing the Celestron/Epoch Schmidt Camera

The Schmidt camera is a rugged instrument being simple in design with no moving parts to wear out or break. The camera's focus is preset at the factory and without interference of some catastrophic event, the Schmidt camera will retain its precise infinity focus virtually forever. However, in the real world, the camera is transported, handled, and subjected to variations in temperature and humidity which all conspire to eventually alter the instrument's focus and collimation.

The term collimation means placing the mirror, corrector plate, and film holder on axis and not tilted relative to each other. Focus means proper placement of the mirror and film holder relative to each other.

If the camera is out of collimation, star images will be comatic, or flared and comet-like in one direction over the entire field. If the camera is out of focus, star images will be fuzzy or swollen, sometimes showing the shadow of the film holder at the center of the bloated star image. The image may not necessarily be out of focus over the entire field. In an extreme case, such as a tilted mirror, the camera can be out of collimation and out of focus at the same time.

A quarter century of use eventually softened the focus on my Schmidt and forced me to research the techniques of servicing the Celestron/Epoch Schmidt camera. Since all other owners of these instruments are also now without factory support, I decided to chronicle my efforts to repair my 8-inch Schmidt as to guide others in the repair of their own Schmidt cameras.

I should say that my experiences are with the original Celestron 8-inch Schmidt camera, not the 5.5-inch or the modified version that has been upgraded by Epoch Instruments with a different mirror cell and spider assembly. References to design or assembly differences in the Epoch Schmidt cameras as opposed to the Celestron models are gathered from anecdotal information provided by owners of these instruments.

There are a number of critical measurements in a Schmidt camera. The curve of the film holder must be actly concentric with the curve of the mirror. The distance from the mirror to the film holder must be equal between the mirror and the center of the film holder and the mirror and the edges of the film holder. The mirror, film holder, and corrector plate must be on-axis and collimated. If any of these measurements are incorrect, the camera will be out of focus. In fact, an f/1.5 Schmidt camera is sensitive to focus variations of only .001 inch.

Go to the previous page ---- Pros and Cons of a Schmidt Camera
Go to the next page -------- Initial Inspection
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