Removing and Installing the Mirror

Before removing the mirror from its cell, you need to verify that it is mounted fully flush against the front lip of its cell. If it is not, you need to carefully record the amount of gap between the mirror and its cell at each of the three points where the Invar rods meet the cell. When reinstalling the mirror, duplicating this gap will greatly ease collimation. To check the gap, label each Invar rod so you know where the measurements were taken. Carefully insert thin feeler gauges between the edge of the mirror and the cell casting where it overhangs the mirror to intersect the Invar rods. Start with the smallest gauge, usually .0015-inch, and work your way up to larger sizes until you find the one that will not slip into the gap. Back up one size and this is your mirror to cell gap at that point. Once the clearances are logged, you can proceed with mirror removal.

Once the cover is off, you will see which of three mounting methods hold the mirror in place. The mirror is resting on the cast rim of its cell, perhaps with several small cork spacers in the gap between the edge of the mirror and the cell. If you are going to remove the mirror, you should mark the position of the mirror and spacers so they can be put back in the same place. Use an engraver to permanently etch an alignment mark near the edge of the mirror. This mark should correspond with a permanent landmark feature on the camera body such as the hinge on the film-loading door. This will aid in reassembling the optics in the same orientation that they came from the factory.

As previously mentioned, one scheme uses three springs to press the mirror against the front face of the mirror cell. If this scheme is in your camera it will be obvious as soon as you remove the first back cover screw. The cover will begin to rise up by itself. Force the cover back down when removing the rest of the screws to prevent damage to their threads. These springs seat in three bosses cast into the rear cover. With the springs removed, the mirror is loose and can be lifted out once the mirror collimation hardware attached inside the rear of the tube is removed. A variation of this method involves using the springs with a smooth-faced rear cover. In this case, the springs are attached to the cover with RTV silicone and the mirror may also be siliconed to the tube.

A second mirror mounting scheme involves three thick RTV silicone pads that have been squished between the rear of the mirror and three mirror collimation brackets that attach to the forward screws of the previously mentioned triangular pattern of screws on the rear of the tube. This is the system used in my own Schmidt camera manufactured by Celestron in 1977. To removed the mirror, the silicone has to be cut out with a razor blade or Exacto knife, then the three brackets removed from the inside rear of the tube to allow clearance for the mirror to be lifted out.

A third mirror mounting scheme found in Epoch built or modified cameras is a silicone bead all the way around the mirror cell between the cell and the tube assembly, with the silicone squished into the space between the cell and tube. Again, the silicone has to be cut away to free the mirror.

When it came time to remove the mirror from my camera, the worrying about taking it apart was far worse than the actual procedure of taking it apart. Actually, disassembling the 8-inch Schmidt was rather easy. After marking the tube where all the shims and corrector and mirror orientation went, I pulled the mounting screws out of the collimation adjuster brackets and cut through the silicone with an Exacto knife. I found that buried inside each silicone pad that glued the mirror in place was a square inch spot of Bakelite that acted as a pad between the mirror back and the collimation adjusting screws. Once these were out and the silicone cleaned off the back of the tube, the mirror fell right out. This emphasizes the need to perform the mirror removal steps while the camera is nose down.

Before removing the mirror, engrave the camera's serial number on the back of the mirror in line with the camera's serial number plate on the camera body. This will allow aligning the mirror to its original position when it is reinstalled.

On my camera, there was one cork shim about a half inch wide every 120 degrees around the rim of the mirror centering it in the cell. There was a small 1/4-inch square spot of cork (very flattened) under the lip of the cell where the Invar rods meet the face of the mirror. There is certainly no effort to make any floatation support for the mirror. It is simply held into its cell by being pushed into place with the three Bakelite pads under the collimation jackscrews, then being siliconed in three places.

Wow! An 8-inch f/1.5 mirror is a deep bowl! For those who wondered, the mirror is 8.75 inches in diameter and the cell masks it down to 8.5 inches in diameter.

Most of the silicone holding the mirror can be cut or shaved away with a razor or Exacto blade. But scraping away the final thin layer may also gouge the metal of the camera body. A trick for cleaning off the residual silicone from the back of the mirror and the inside of the tube is to rub it vigorously with a coarse cloth or rag. Such a rag cleans silicone of an object as surely as sandpaper cleans rust off metal.

The rim of the mirror cell is visible inside the tube. Note the cork pads at the base of each Invar rod.

Once the mirror is removed the whole cell, Invar rod cage and spider can be removed. If you remove this assembly, it is probable that when reassembled the collimation and focus process will have to be performed. Because of the inherent dificulties in aligning and focusing a Schmidt, I would advise leaving the Invar cage in place unless some circumstance dictates it must be removed.

A "handle" made from several layers of masking tape can be used to lower the mirror into its cell. There is no finger room between the mirror and tube so this system allows inserting the mirror without having to drop it into the cell. The previously engraved camera serial on the back of the mirror is aligned with the camera's serial number plate to return the mirror to its original position in the cell.

The mirror installation is completed by attaching the collimation adjusted backets. The final position of the mirror may change during the collimation and focusing tests, so the mirror is not secured in position with silicone until the camera is tested for collimation.

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