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SCT Corrector Plate Removal

Both the inside and outside of my corrector plate had accumulated enough slimy humidity over the first year and a half of my N11's existence that it required a good cleaning, inside and out. The mirrors did not collect this same nastiness so I'm inclined to think it was something in the coatings that predisposed the corrector OR simply the corrector's more extreme temperature swings that allow dew to form.  Directions for cleaning glass surfaces are posted here, this document deals only with removing and replacing the corrector plate.

This document only applies to the 8 inch and larger SCTs from Celestron.  For the 5 inch optical tube, visit this page on Robert Pudlo's web site:

After carefully removing the 8 screws in the retaining ring, I lifted the ring and found that Celestron has glued a thin strip of cork all around the ring. Oddly enough, they even took the effort to paint the cork black. Unfortunately that seems to cause some problems when it comes to lifting the corrector out of the cell as I will describe in a moment. I also quickly found the tiny serial number etched into the glass over by the right side of the corrector - in other words, the 3 o'clock position when looking at the front of the scope.  Before removing the corrector you must locate the etched numbers since the optical quality of the scope is dependent upon the primary mirror, corrector plate and secondary mirror all having a set orientation.  Using a permanent marker, I marked next to the screw hole that the serial number aligns with. UPDATE:  Current SCTs and the Edge HD manufactured by Celestron do NOT have an etched number indicating orientation and no marks on the secondary and primary mirrors.  Orientation is still important (though not as important as with older models) so use a permanent marker to make corresponding marks on the edges of the corrector, secondary and primary mirrors (if you actually remove the primary mirror - not described here) and the edge of the corrector cell so that you can reassemble the optics in the correct orientation.

The next thing I noticed is that Celestron is no longer using spacers to center the corrector in the cell. In fact, it is apparent it was simply placed in the cell and where ever it landed, so be it. There is about 1 or 2 mm of space around the edge of the corrector and in my scope, the corrector was shifted most of the way to the upper right. As good as my optics have been, I think it is safe to say that this offset has little effect when using the scope via the eyepiece end since the collimation of the secondary seems to adjust as necessary.  This is in fact expected since the primary and secondary are both spherical with no true "center". But, it will obviously cause problems for anyone imaging using Fastar.  Later when I replaced the corrector plate I used small pieces of folded paper in the gap between the edge of the corrector and the walls of the corrector cell in order to center it in the cell.  I placed four pieces of folded paper, evenly spaced at 90 degree separation.

Now about the problem with the painted cork. A thin strip of black-painted cork is glued all along the ledge the corrector rests on. The paint had a very strong hold on the corrector plate. I lifted up on the secondary housing with as much force as I dared (fearful that when it came loose I would bang the corrector against the cell and - CRACK - everything would be on it's way back to Celestron...) but it just wouldn't come loose. Some have recommended using a wooden stick to pry the corrector loose, but that was too much force for my comfort, so I came up with a different solution.  I dipped a cotton swab ("Q-Tip") in alcohol, then pressed the swab into the gap between the corrector plate and the cell walls.  The alcohol immediately spread out between the cork and the bottom of the corrector.  I worked my way all the way around the corrector, dampening the entire cork.  After a short soak, I pulled up on the secondary mirror housing and the corrector popped free with very little effort.

My NexStar 11 has a Fastar secondary, so after removing the corrector, I removed the secondary and placed it in a Tupperware bowl and sealed it to protect the secondary. While I had the corrector out, I also set about to tighten the secondary cell.  It has always been a little loose allowing the entire secondary assembly to rotate when tightening the Fastar retaining ring. To tighten the cell it is necessary to keep the threaded ring (the part that protrudes from the front of the corrector cell) stationary in the correct orientation (the slot for the secondary key pointed towards the etched serial number) while tightening the secondary baffle (the part protruding from the back of the corrector cell). Sounds easy but in practice it turns out to be difficult as they don't much like to tighten down sufficiently. It was only possible to tighten the assembly by holding the baffle stationary while turning the threaded ring. Since the threaded ring is the part that requires strict orientation, I had to rotate the secondary assembly and tighten the ring down and through trial and error got everything to line up.

In retrospect, I believe the problem was due to the material of the baffle - it is a plastic (nylon) and when grasping it to turn it, the plastic deformed and would bind against the corrector, preventing it from turning. The threaded ring was metal and didn't suffer from this, so it turned easily. It might have been possible to turn the baffle if I had grasped it closer to the corrector, but then I might also have scraped my fingers against the corrector when doing so.

It is not advisable to tighten the secondary cell too tightly, you may stress the corrector plate enough to affect the views.  Tighten just enough to prevent it all from turning.  It has been suggested that a Fastar corrector cell has some sort of cement or glue that should keep it from turning, but no such cement was evident on my scope. 

I would also mention that the secondary cell can move laterally in the hole in the middle of the corrector plate.  The next time I remove the corrector I will completely unscrew the secondary baffle and use the folded paper trick (four pieces as 90 degree separation) to force the secondary cell into a centered position.

Next was the cleaning.  Cleaning a Fastar corrector is very simple since you can set the secondary aside and not worry about the mirror. After cleaning, I turned the OTA front side up (by the way, I had the N11 setting on the floor without the tripod during this operation) and replaced the corrector, insuring the etched serial number was aligned with the marked screw hole. I then replaced the secondary and inserted the paper shims mentioned earlier to center the corrector in the cell. After replacing the retaining ring and screws, the job was done. By the way, only tighten the screws until just snug - too much and you can stress the corrector resulting in distorted images.

The last remaining detail is a clear night so that you can collimate.

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Michael Swanson
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