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[RobbW]

My newly acquired Celestron C8 SCT saw first light at our star party outside the Adler on Saturday, Sept. 4. During the session, Joe shined his flashlight inside my OTA and made some observations about streaks and other nasties on my primary mirror. Of course, me being anal and OCD, I have been bothered by this ever since and have decided to disassemble my OTA and clean it off. I am following Joe's detailed write-up from when he cleaned the C11 beast. I am taking photos along the way and will post my own write-up eventually.

In the meantime, I have a few questions. Joe, feel free to jump in here with any advice or answers you may have.

1). After removing the corrector plate, I noticed there is only one cork spacer remaining. The others are MIA. There are not stuck to the edge of the corrector place or swimming around in the OTA. I have no idea where they are. I assume they were lost when the previous owner flocked the interior of the OTA. Where can I get cork to replace the missing spacers? Do the spacers have to be cork or can I use some other material? How thick should the material be? How many spacers should there be? Where should they be placed along the rim of the OTA?

2). After removing the primary mirror from the OTA, I determined that the delicate methods Joe used to clean his mirror were not going to be sufficient. Cleaning my mirror was going to be a major operation. I decided to completely disassemble the mirror from the baffle tube/spindle. Removing the mirror from the spindle required a little more effort that I expected due to this rubbery caulk-type gunk that was holding the back of the mirror to the spindle. I carefully pried it apart and cleaned off this rubbery stuff. There is also a 1/4" thick rubber O-ring that sits between the mirror and the spindle as a buffer. What I need to know is what was this rubbery caulk-type stuff? It was the color of red clay. Is this something that is necessary when I go to reassemble my mirror? Or is the rubber ring sufficient?

3). I used Joe's packing tape method of cleaning off the flocking inside the OTA. However, in a couple locations, the adhesion of the tape was a little much for the flocking material and caused the flock to separate from the OTA. The flock stayed put when I pressed it back against the OTA. However, I'm wondering if I would be better off pulling out the flocking material in order to re-glue it to the OTA? Or should I just leave well-enough alone if the flock appears to stay put when I press it back against the OTA?

4). What kind of grease should I use on the central tube and focus assembly? Joe, I know you used bearing grease because that's what you had on hand. I will need to pick something up at the hardware store. What would be recommended?

Thanks for any help and advice. I will post more at a later time.

[Chicago Astronomer - Astro Joe]

Robb,

Your scope provided sharp crisp views at our session, but I can understand the impulse to clean it of gunk.

Some say leave it be, but I'm of the school to clean it off and if you are meticulousness, slow, careful and easy, no damage to the mirror will occur.

Depending on what was left on the mirror, (flocking adhesive?), it may not be able to come off. I'll wait on your task for more insight.


1 - Cork Spacers...

Yea...I think they were lost too when he opened up the OTA. I think you should replace them. They serve to buffer the corrector plate from the OTA against hard jars and ease the expansion & contraction of temperatures. And it helps to "center" the corrector plate to the primary mirror.

I see more collimation efforts in your future.

If you can't find cork, regular cardboard will serve the same.


2 - Rubber stuff....

The baffle tube should not be "rubbery", but instead a smooth flowing covering of lubricant. The Rubber O-ring is there to prevent the mirror from traveling too far out and falling off and will be re-installed.

Now, I used regular wheel bearing grease when I re-assembled the C11. But some felt that, although the grease I used works fine, it may "outgas", and recommending a special expensive lubricant that is used in vacuums environments. "Dow Corning High-Vacuum Grease", but there are mixed reviews - with some totally hating it so much that they removed it. I would get a high quality - high temperature thick lubricant for the baffle. In the summer it gets warm in the OTA and the grease should not thin out and drip...a reason I used the wheel bearing grease I had...and wheel bearings get pretty hot!

A search on this topic will bring up much info.

Now, perhaps the lubricant the guy used "dried" out. Happens, and it became more viscous. Or, there is a large amount of mirror flop - where the main mirror axis wanders around when traveling up and down the baffle. This is a common bane of all SCT users and have experienced it in mine as well. You will notice that the image of your target will wander out of the field of view when focusing and you need to readjust your aim. The extra viscous lubricant on the tube might be there to hold the mirror in place more.

I hadn't noticed, but how was the travel of the mirror when focusing? Smooth, jerky, rough spots?


3 - Flocking...

I wouldn't remove it.

I would use a toothpick with adhesive, (rubber cement), and reset the lifted area of flocking.

The flocking looked pretty good, but unless you are seeing some bald spots, loose fibers falling off or other negative observation - I would just fix up the areas that need touching up.

I am pleased you decided to open up the tube and clean it out, as there were some particles noticed on the inside of the corrector plate.

Once it's back together..(and no accidents occur... ), you will rest assured that you have brought it to the best possible condition.

[RobbW]

Thanks, Joe. I knew I could count on your advice. Here's some follow-up:

1). I bought 1/16" cork sheet at Ace Hardware. So I have that to use. However, during my research, I also found a few people who have used and recommend using the soft side of self-adhesive Velcro as a spacer instead. One gentleman even lined the entire inner circumference with the Velcro instead of small pieces spaced around the tube. He claims it does a better job of cushioning and centering the corrector plate plus it stays in place better than the cork. What would you think about that? I have both cork and Velcro so I can use either one.

2). Sorry, Joe, I think I may have confused you. Yes, there is a thin rubber O-ring at the end of the baffle tube to prevent the mirror from falling off. However, that's not what I was referring to. I completely disassembled the primary mirror and removed the mirror from the spindle/hub/baffle assembly. After unscrewing the metal collar ring that holds the mirror down onto the hub and removing the cork gasket that's between the collar ring and mirror, there was this dried (but still flexible) caulk or sealant type substance on the back side of the mirror. My guess is that it was some type of adhesive that stuck the mirror to the bottom of the hub assembly and prevent it from moving at all. Without this sealant/adhesive, there is just a rubber o-ring/gasket between the back of the mirror cell and the metal hub on which it rests. I think the rubber ring then the mirror cell then the cork gasket then the metal collar tightened down would be enough to keep the mirror from jiggling about at all. But I think someone may have used the adhesive/sealant as an extra precaution.

3). Okay, I'll leave the current flocking in place. I didn't know if the flocking being pulled off the OTA by the tape would cause more troubles down the road. But I'll just dab some rubber cement and call it a day.

I already have the mirror and corrector plate cleaned. It took a while, but wasn't too bad. The mirror is almost spotless now. There is one tiny little spot that still has some of that haze/streak, but I'm not going to risk washing it anymore just for that. I hope to be able to just live with it!

While I still have everything apart, do you think it would be useful to flock the housing for the secondary mirror, either the inside or the outside of it? I read a couple blogs where others did it, and I couldn't decide if it would really be worth the extra effort and risk.

BTW, there is no finderscope for my scope. What I received is all there was. So, now I have to figure what I'm going to do. I have red dot finder I can use, plus I have my 75mw laser pointer. I just need to find ways to fabricate mounts for them. Do you think an actual finderscope would be necessary? If so, would you have any recommendations on what I should use for one, assuming I can actually find one?

Thanks!

[RobbW]

Sept 7, 2010 12:59:18 GMT -6 Chicago Astronomer - Astro Joe said:

Now, I used regular wheel bearing grease when I re-assembled the C11. But some felt that, although the grease I used works fine, it may "outgas", and recommending a special expensive lubricant that is used in vacuums environments. "Dow Corning High-Vacuum Grease", but there are mixed reviews - with some totally hating it so much that they removed it. I would get a high quality - high temperature thick lubricant for the baffle. In the summer it gets warm in the OTA and the grease should not thin out and drip...a reason I used the wheel bearing grease I had...and wheel bearings get pretty hot!

Have you personally noticed any detriment to using the wheel bearing grease? I picked up a multipurpose heavy duty lubricating grease by Lubrimatic from Ace Hardware.

[Chicago Astronomer - Astro Joe]

Robb,

You already have a dovetail base mount on your OTA, ready for a 9x30 (or whatever), finderscope.

Look in Amazon, Ebay or Telescopes dot com for a finderscope...and yes...you will need one.

Pics on what you are encountering and the procedure would be of great help to see what you are referring to. But I would leave the flexible stuff alone on the back of the primary. It ain't bothering nobody.

The holder for the secondary mirror is cone shaped and the cut of the flocking is of a complicated one. Any deadening of reflective surfaces is good, but this is just too close to the secondary for me and I might have gotten some adhesive on the mirror surface...so I left it be. but if you think you can accomplish this safely, I would do it....as well as the inside of the baffle.


A detailed pictorial would be of great value to others who have similar equipment.

[Chicago Astronomer - Astro Joe]

Robb,

No...nothing negative from the wheel bearing grease I used.

I have looked for drippings or other signs of failure, but so far it's all cool.

[RobbW]

Yep, I understand that pics will be good for all. I've been posting from work today, so no access to pics. I plan to post a full pictorial when I'm done. Just trying to get some questions out of the way so I can get things done when I'm finally back home working on my scope.

As for the flexible stuff under the mirror, I already removed it. I had to in order to separate the mirror from the hub. I cleaned off any remaining gunk that stuck to the under side of the mirror since I figured it was going to fall off when I cleaned the mirror anyway. I'm just trying to ascertain its actual purpose in case it is necessary to reapply an adhesive when I reassemble the mirror assembly. I got this stuff at Ace called "Make a Gasket" or something like that. It's a tube of sealant that forms a rubber gasket. It's the closest thing I could find to what I think was originally on my mirror assembly.

As for flocking the secondary, posts I had read elsewhere showed the cone from the secondary simply being unscrewed from the correct plate and then flocked. I wasn't aware that was possible until I saw that post. I don't know about flocking the baffle tube though. That seems like too tight of a fit to do an acceptable job. Also, don't you have to worry about vignetting or blocking the light path it you start flocking things like the secondary housing and the baffle tube?

[Chicago Astronomer - Astro Joe]

Robb,

Only you can weigh the benefits of darkening the surface trade-off to slight vignetting. A 2% vignette to better contrast...I would, but your call.

I don't understand why the removal of the mirror from the hub. Was it loose and came off easily? The mirror can flex, warp or deform if not uniform with the adhesive barrier.

That's a heavy glass-like base behind the mirror, that will hold on to temps, (hot and cold), that might migrate to the mirror - prolonging equalization.. (Maks have a similar problem with that massive glass corrector plate in front.)

Waiting for the pics.