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
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
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
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