There are a number of simple mistakes that can occur after late night fatigue interferes with the operation of a Schmidt camera. The Celestron Schmidt uses two different film holders that position the emulsion at the exact focus of the camera, one for white light photography, and the other for photography with filters. The filter material slightly shifts the focus of the camera, thus the filter film holder is machined slightly different to take the focus shift into account. The only way to tell the two film holders apart is a flat washer under the film holder pressure plate on the filter film holder. In the dark, especially when it is cold in the dead of winter, it is easy to overlook or simply not be able to feel the identifying flat washer on the Schmidt's filter film holder. Unless the two different devices are separated, you might use the wrong film holder. Using the filter film holder without a filter, of the nonfilter film holder with a filter, will result in the discovery of the Donut Cluster. This amazing star cluster seems to be composed entirely of ring-like planetary nebulae that are really out of focus stars.
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