Removing the Corrector Plate

In most cases, the camera can be disassembled and serviced without removing the corrector plate. However, if the interior of the tube assembly is to be repainted, corrector removal is mandatory. If at all possible, simply remove the corrector cell from the front of the tube while the corrector is still installed. The cell is held on the front of the tube by machine screws.

If it is necessary to remove the corrector from its cell, it is vital to mark it so it is reinstalled with the proper surface facing outward and the plate is clocked to the original orientation within its cell. Accidentally loosing track of which side of the corrector is which can be a time consuming mistake. The variation of thickness on the corrector plate is only a few thousandths of an inch. You can't detect this with the eye. Most old household windowpanes have greater variation in thickness. (At least you can see the ripples in them) And don't think you can figure out which side of the corrector has the depression figured into it by pouring water on it and seeing where it puddles. This does not work because surface tension is too great between the glass and water, it just beads up. Make a note of the location of cork shims around the edge of the corrector. They keep the corrector centered.

A good way to mark the corrector for proper orientation is to use a diamond glass scribe to mark the very edge of the corrector in the area normally covered by the retaining ring. This retainer comes off by removing the eight screws holding it in place. Various data related to the orientation of components can be scribed on the rim of the corrector as a permanent record for the instrument.

The corrector is marked with tape to aid alignment at reinstalation, then the eight retaining ring screws are removed.

The retaining ring is made of either metal or plastic, and lifts away to free the corrector plate. Once the retaining ring is off, insert a hand through the film-loading door and gently push up on the corrector. Often the corrector is stuck. Prying on the edge of the corrector with a screwdriver tip can chip the glass. Pry between the rim of the corrector plate and the edge of the cell with a pointed plastic tool or a sharpened wooden stick similar to a "Popsicle stick".

Several folds of paper towel protect the corrector when it is pushed out by hand.

The corrector plate in my camera has four sets of paper shims around the perimeter of the glass. Each is a stack of three to six thin strips of paper slightly heavier than typing paper. I guess Celestron chopped up whatever was handy. There was printing on the paper, but it was too cut up to understand what it said. What really surprised me was how chipped up the edge of the corrector in my camera was. It is chipped almost all the way around its rim. This must have happened at the factory because there are no chips in the holding cell. None of the chipping is visible once the retaining ring is in place.

The corrector on my camera was chiped all the way around the rim of the plate.

I have heard Celestron made the correctors out of ordinary window glass. I don't know if this is true, but the thickness of the corrector is very similar to windowpane glass.

Though it was difficult, I can tell which side of the corrector has been ground into the curved Schmidt corrector shape. When I looked at the reflection of straight lines that were at an angle to each other, such as the roof and side of my house as seen out the garage door, the corrector side showed those straight lines as mushy and bowed compared to the normal reflection from the flat side of the corrector. The corrector is 8.5 inches in diameter and is masked down to exactly 8 inches by its retainer.

Go to the previous page ---- Adjusting Corrector Plate Position
Go to the next page -------- Checking Mirror Collimation
Back to -------------------- Table of Contents