Adjusting Mirror Collimation

The mirror in the Celestron/Epoch Schmidt camera is siliconed into place inside the tube assembly. This process provides a robust method of securing the mirror. It should stay in collimation for the lifetime of the camera unless the mirror is removed for aluminizing or has been subjected to near catastrophic shock impact shock. On my camera, once the rear cover is removed, there are three tiny Allen-head screws set into three brackets attached to the inside rear of the tube behind the mirror.

These setscrews adjust the tilt of the mirror. Only one or two of the three screws are used as they simply press up on one corner of the mirror to nudge it into alignment. If your camera uses the same system, refer to the paper arrow on the corrector plate and adjust the screws as needed to center the arrow and its reflection exactly on the black dot at the center of the corrector. Once the mirror is in the right position, glue it into place with RTV silicone.

Some cameras do not have these adjusting screws. In this case, place the camera nose down on its corrector plate end. Insert the mirror into its cell. If the mirror is not collimated by simply setting it on the perimeter ridge at the front of the mirror cell, then put small cardboard spacers around the ridge perimeter until the correct tilt is achieved. In theory, you would need only one shim of the correct thickness at the right spot on the perimeter to correct the axial aim. Then you lock it in place with silicone. To safely maneuver the mirror, a "handle" can be made out of duct tape and stuck to the back of the mirror so it can be lifted up to slide in the shims.

Centering the film holder completes the collimation of the instrument. This is checked by placing an artificial star, or pinhole light source, in front of the camera along the center axis of the optical tube. Observe the position of the star on the film holder. It should be shining on the vertex of the curved film holder. Shifting the entire spider cage using three lateral setscrews next to where the Invar spacer rods intersect the spider assembly centers the film holder.

A simple way to monitor the mirror collimation while the camera tube is up side down is to place the camera on a glass top table and lay a flat mirror on the floor. This way, the collimation can be checked with the paper arrow mentioned in the above section while working with the mirror shims and viewing the results in the floor mirror. This way you can see the results of shim changes without crawling under the table.

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