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SCT Corrector Plate Removal
Read this entire document before starting!

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.  Additionally, my Fastar assembly had rotated and needed attention.

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:

An important note before getting started:  Celestron SCTs have historically been tuned to work best with the three optical elements (primary mirror, secondary mirror and corrector plate) oriented to a specific rotation.  The secondary mirror had a permanent marker line on the edge. On Fastar optical tubes (the feature allowing the secondary to be removed and replaced with a lens assembly and camera for f/2 imaging), that marker line was aligned with the pin on the secondary mounting plate which fits into the slot on secondary holder mounted in the corrector. The marker line on the secondary should be aligned with the tiny etched number (sometimes just a few etched dots) out at the edge of the corrector plate. That etching then is aligned with a permanent marker line on the primary mirror. You generally cannot see that mark on the primary but, it is aligned with the focuser knob on the back of the scope.  This alignment does not need to be precise, just within a few degrees.

That said, current SCTs and the Edge HD manufactured by Celestron (since approximately 2006) do NOT have an etched number indicating orientation and no marks on the secondary and primary mirrors.  Orientation is probably 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.  Additionally, the pin and slot on Fastar secondary assemblies no longer align to any specific orientation so do not expect it to align with the focuser knob as in older models.  If your Fastar secondary has rotated (as mine had), the correct orientation may not be certain to you.  If you remember the orientation when you first got the scope, go with that.  Otherwise, on the alt-az models (NexStar GPS, SE, CPC, Evolution, etc.), the Fastar label should be level in normal reading direction when viewed from the front.   With the German EQ models (CGEM, AVX, etc.) or optical tubes purchased separately, the Fastar label is usually level when the optical tube is sitting on its dovetail bar.

Here is how I proceeded with a NexStar 11 GPS.

After carefully removing the 8 screws in the retaining ring (fewer screws in some versions), I lifted the ring and found that Celestron has glued a thin strip of cork all around the ring (not all models have this cork). 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 9 o'clock position when looking at the front of the scope.  Using a permanent marker, I marked next to the screw hole that the serial number aligns with.

The next thing I noticed is that Celestron was 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.

On newer SCTs, Celestron has introduced 3 or 4 nylon screws thread through the corrector cell into the edge of the corrector plate in order to accurately center the corrector to improve Fastar imaging.  You will need to loosen each of these screws about a half turn to remove the corrector.  Remember how much you turn and turn all screws the same amount so you can tighten each screw equally upon reinstallation and thus retain the factory centering.

Now about the 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.

As mentioned above, 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.  At the time of this writing, Starizona has gaskets available for most SCTs to help hold things in place.

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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