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Cleaning and Modifying the C11 Beast - Part 1: Removing the Corrector Plate

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Cleaning and Modifying the C11 Beast

By Joseph Guzmán

This article is part 1 in the How-To series “C11 Cleaning & Modification”:

  1. Part 1: Removing the Corrector Plate
  2. Part 2: Installing a Rail Carry Handle
  3. Part 3: Removing the Primary Mirror
  4. Part 4: Flocking the Interior
  5. Part 5: More Grab and control handles
  6. Part 6: Installing side control rails
  7. Part 7: Cleaning the optical surfaces - Primary Mirror
  8. Part 8: Cleaning the Corrector Plate
  9. Part 9: Installing the Corrector Plate and Closing the Tube

Part 1: Removing the Corrector Plate

Figure 1-1.

I obtained the Celestron 11 last October through lucky happenstance - right place at the right time and at a very low ball price… very. Close to a 10 grand scope & mount - it was a fraction … a very small fraction.

It was used only briefly I'm told - (too big and heavy), and was stored away in an outside storage locker for close to five years. When I first saw the scope, it was clear that it was not stored properly, as the chest it was in was rotted and the outside chest hardware rusted. Although the front aperture was capped, the rear visual back was not covered and left open - but I had to have it. And after goodwill and luck… it became the main telescope in the Chicago Astronomer arsenal.

But, it was not pristine, as the corrector plate had accumulated some gunk and haze… not only on the outside, but on the inside surface of the glass. The primary mirror was fine, with some minor dust particles… and even in this condition, the Chicago Astronomers have been enjoying great views through it during the past 8 months.

Sooner or later, I had to take it apart to give it a good cleaning.

That time has come and while it's apart… make some modifications to it.

(I will do this massive project in Parts - to help break up the steps and go a little insane on how far I have gone… :P)

But first… an assessment…

Figure 1-2.

I transported the OTA to my basement workshop, covering the workbench with clean white paper and using the foam packing as a base to steady and support it. The temps were warm, (high 80's), and humidity rather high, so no cleaning this evening, but a disassembly task was ahead of me.

Figure 1-3.

Figure 1-4.

Examining the tube and plate, there were no surprises or other permanent or potential problems that I did not expect, but the corrector plate is quite dirty, as shown above in the primary mirror reflecting back the grime on the plate. There was although, evidence of a moth cocoon wedged in between the retaining ring and the tube…

Figure 1-5.

And, a close examination under bright lights - shows the gunk…

Figure 1-6.

Figure 1-7.

Figure 1-8.

Figure 1-9.

Some would say that the dust and other gunk on the plate is OK and not a major factor in viewing. I disagree. The amount here is diffusing the light entering the tube - making the targets "soft"… which I have noticed. And there is also a slight hazy halo around bright objects such as the Moon and Jupiter. And the contrast not as deep as it should be - where dark space should be was a matted grey low contrast haze.

Around the secondary, is a ring of I-don't-know-what-the-hell-it-is sediment haze …

Figure 1-10.

Figure 1-11.

Figure 1-12.

Yep, this plate is coming out… tonight.

Figure 1-13.

I have had this 8 vial holder for many many years, just waiting on a shelf for this job to keep the screws and other items safe. I will use latex gloves as well when I handle the plate.

Figure 1-14.

Looking at the screws, they didn't seem scuffed up, but none… were torqued down tight - they all were rather loose…!

Oh My Lord… :'(

Not so loose as to allow the plate to rattle, but too loose for comfort.

Figure 1-15.

I alternate in a star-cross pattern in removing the screws…

Figure 1-16.

No odd sounds or unusual movements. Good.

Donning the gloves, I remove the retaining ring, slipping a thin screwdriver between the OTA and the ring…

Figure 1-17.

Figure 1-18.

Figure 1-19.

The ring comes off very easy. I have read where other SCT owners have had a hard time removing the ring and needing some solvents to ease it off.

A friend who owns a pizza business knows of my astronomical pursuits and I ask him for two large pizza boxes. I stored them in the garage for about a week now and ready to make use of them. I place the retaining ring in one and set it aside…

Figure 1-20.

I examine the mating surface under the retaining ring and find things… ok. No rot, mold or other nasties. Very cool… :)

Figure 1-21.

Figure 1-22.

Figure 1-23.

The cork spacers look good… ;D

It's time to remove the corrector plate, but first - I index the plate to the OTA - orientating it with reference marks, both on the glass and the tube…

Figure 1-24.

I make four marks around using hash marks…

Figure 1-25.

I would like to see if this way it's coming out, is the A-1 orientation for primo sharp viewing. Perhaps one evening, I'll have a Chicago Astronomer rotate the plate while I observe and see what happens.

Taking a break, I survey the scene and ready to remove the plate now and my second pizza box at the ready…

Figure 1-26.

I grab the secondary holder and slightly attempt to move it… and it does move - easily and without a fight…

Figure 1-27.

Figure 1-28.

Figure 1-29.

Figure 1-30.

I heard horror stories where the corrector plate remains stuck solid and alcohol solvents are needed to break the bond and the use of a plastic/wooded pry tool to move it away from the seat. I needed neither.

It's great when a plan works… :-[

I examine the secondary mirror…

Figure 1-31.

It's slightly dusty, but nothing major. Some air will probably be all that is needed to clean it…

Figure 1-32.

Figure 1-33.

Figure 1-34.

Tube minus Corrector Plate.

The backing of a couple of the spacers has come loose and one has slipped off it's home. No big…

Figure 1-35.

I gather up the four cork spacer pads and place each one in a separate vial and will return them to their original locations…

Figure 1-36.

Figure 1-37.

Figure 1-38.

I examine the plate and find no scratches or other nasties, but there is a lot of dust, fibers and haze…

Figure 1-39.

I saw a control number earlier when I removed the retaining ring, but here is a closer look…

Figure 1-40.

"5967" - I would like to do a search and see what turns up.


Since I am going to clean both sides of the corrector plate, I will lose the reference marks I placed earlier. So I carried them over to the side of the plate. Should be good…

Figure 1-41.

I place the plate in my new clean pizza box and set it aside for cleaning later…

Figure 1-42.

But first… Some modifications to the OTA - in Part 2: Installing a Rail Carry Handle

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