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Cleaning corrector plate and primary on C6-SCT

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#1 Sean13

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Posted 25 February 2013 - 08:06 PM

I am going to be taking the corrector plate off, well the cell holding the entire corrector plate and secondary, not actually taking the glass out of its holder, to do some work to the baffle tube in the middle of the primary.

While the scope is apart I would like to properly clean the primary and corrector plate, as I see some greasy humidity spots on the primary, and some weird water spots on the inside of the corrector plate (maybe from condensation). How do I go about properly cleaning the optical surfaces? Is just a good wipe with a microfiber sufficient? Should I use any water or a cleaning solution? Do I have to be extremely careful when touching the primary or are the coating pretty durable on these?

#2 KerryR

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Posted 25 February 2013 - 09:27 PM

I've had my LX90 8" and CPC11 completely apart while I added active venting.

I cleaned the primaries on both using the same methods used for Newts, using Dawn, Iso. Alcohol and distilled water. Rinsing, however, proved very difficult on both: The last trickles of water that ran from the holder/carrier always left a white residue as they ran away from the center and off the edge. On the LX90, the primary was easily removed from the carrier, and rinsing it in the normals way worked fine. Celestron glues the primary onto the holder, so this isn't possible. Therefore, I had to wash and rinse 1/2 of the mirror at a time, allowing the rinse water to approach but not touch the carrier. This avoided the residue.

Based on what I experienced, and on what I've read, modern SCT mirror coatings are very similar if not identical to those found on any modern first surface mirror-- Al overcoated with SiO-- and are quite durable. I encountered no visible sleeking.

I can't remember how my c6 corrector cell is attached, but, if memory serves, it's the same as the 8 and 11" sct's-- screws and nuts. So, I don't think you'll be able to get the front cell off without removing the corrector. It's very easy to remove and replace the corrector if you place index marks (under the retaining ring, out of sight) for rotational alignment, as well as mark/index the shims and their locations.

#3 Sean13

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Posted 25 February 2013 - 10:29 PM

The c6 front cell looks to only be held on with 4 screws plus 2 on the dovetail and no nuts, so I'm thinking it comes off without removing the corrector but we'll see. Would you advise removing the rear cell in order to clean the primary better/easier? I don't quite understand about cleaning 1/2 at a time, but I'm sure i'll figure it out once I get started.

I'm not familiar with how to mix the dawn, alch, and water for a cleaning solution, is there some more detailed instructions on making the solution?

#4 mgwhittle

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Posted 25 February 2013 - 10:39 PM

Unless you just have to and the spots are small, I would leave the primary alone.

Having said that warning, I have had good luck using just acetone to clean small spots on a primary mirror. You wont have to worry about rinsing or using lots of water. You have to use a very very small amount of acetone on a Pecpad or Optiwipes type pad. I will put a small amount, just to dampen but not wet a small section of the pad with acetone and gently wipe with no pressure across the spot. You might need to make a couple of passes with new pads dampened with acetone. Again, just dampen the pad with acetone, if you are wiping the spot and the acetone doesn't instantly dry, you are using too much. The good part about this is if you use too much and it leaves a bit of film, then just use less on your next and subsequent passes until you find the least amount that removes the stain and dries instantly. This works well with very small spots which is what it is sounding like you are describing.

And to say it again....if the spots are small, my advice is to leave the primary alone. You are only doing cosmetic cleaning at this point, you will never see those spots effect the image.

#5 Sean13

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Posted 25 February 2013 - 10:50 PM

Its hard to describe what I'm seeing. Its a haze, fairly large in size, and when aiming a flashlight it really stands out. Other then the haze I just have small dust pieces that would likely blow away with light compressed air. The corrector on the other hand has some spots that look like water spots, and a couple weird looking rings tword the edges.

#6 mgwhittle

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Posted 25 February 2013 - 11:21 PM

Don't clean any mirror based on the flashlight test. Do you see the haze under normal room lighting? If you don't, don't mess with it.

Here is my advice, if you are unfamiliar with the cleaning process for mirrors, an SCT mirror that can't easily be removed from its cell, unlike a Newtonian, is the hardest mirror to learn the proper technique on. That baffle tube has grease on it and you can really get into a mess if you get even the smallest amount of grease transferred on to the mirror from a bad technique. Really think about if removing that haze is worth it in the end before proceeding. Cleaning the corrector will be easier.

#7 Sean13

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Posted 25 February 2013 - 11:48 PM

The scope normally sits in an unlite garage, so its hard to say if I could see if under "normal" lighting conditions. If I were to take it out during daylight, and look at the mirror, yes I would still see the haze.

I'm having some optical problems with some tear shaped glares ruining my pictures with bright stars in them. I'm trying to eliminate any possibilities so I was wanting to get a good clean mirror surface to rule that haze out as a cause of the problem, as well as the grease at the end of the baffle tube that may be causing reflections.

#8 LauraMS

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Posted 26 February 2013 - 02:28 AM

As has been pointed out by others I would strongly suggest to not remove the primary. You mention that you don't have much hands-on experience with maintenance of your SCT. Although I have ground my own mirror when I was much younger, I would say the same is true for me today. Last year I had to clean the corrector plate of my 8SE because of quite severe haze on the inside. For me it was rather frightening to just remove the corrector, clean it and return it (in the appropriate position) to the front of the telescope. You have to do it yourself to know why. I succeded but have also a lot of respect doing something similar on the primary. Other's have written why - it's significantly more difficult and chances of doing something seriously wrong are high, in particular if someone is unexperienced with maintenance of optical instruments.

And it is obviously the best rule to better not touch optical surface because if not absolutely necessary. Almost always some scratches remain. And the imaging process allows for surprisingly large amount of dust etc before image quality degrades.

Did you check collimation? In my experience this may also cause a lot of trouble. I think I also wouldn't remove the corrector plate just because of a few remnants from water drops on the inside - they hardly will influence image quality.

If you are seriously worried about haze on primary - can you imagine to show it someone from your local astronomy club, or a telescope dealer nearby (who might also be able to clean it if absolutely necessary).

Good luck - hope you soon have your scope back in good condition :-)

#9 Sean13

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Posted 26 February 2013 - 02:57 AM

I have to take the scope apart anyway to determine the cause of the issues, so I figured it would be nice to do while apart. I'm not taking it apart just to clean things, but it is a little dusty in there so I might as well.

Collimation as far as I can tell is good. I've been thinking about playing with it to see if my reflections/glares move around in the picture.

#10 rmollise

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Posted 26 February 2013 - 07:36 AM

Don't clean any mirror based on the flashlight test.


Yep. _Leave your mirror alone_...

#11 Eddgie

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Posted 26 February 2013 - 09:30 AM

First, I do not know if what you are contemplating is even possible. The reason you read about people pulling the corrector all the time is becasue in the past, it has been the only way to get into the scope.

In all of the SCTs I have owned, the front cell is held on by screws that go into nuts on the inside of the tube.

In the past (and this may have changed), these nuts were not welded in place.

When you unscrew the screws, one of two things happens. Once you get pressure off of it, the nut just spins and won't come off, or you get the screw out and then the nut falls into the tube (and on to the mirror if you are really unlucky.

But that is only half of the problem.. Now you have to get the nut back on.

I suppose that you could glue them in place before putting the cell back on but otherwise, there is no way to get them back on to the screws.

You should be able to eyeball this. The nuts sit very tight up under the correctory cell rim.

Using a flashlight, look at the reflection of the back of the corrector cell using the primary. If you see nuts holding the cell in place, then I think this is still the case, that the nuts simply are held in place by the tension of the screw.

You could try losening one to see if it spins, but then you have a loose nut with no way to tighten it, and dude, nobody likes a loose nut.

But maybe they have changed the design so that the cell threads on the way it does with the smaller MCTS????

Sometimes you feel like a nut, sometimes you don't. But if you've got nuts, then you may be better of doing it the old fashioned way.

#12 Sean13

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Posted 26 February 2013 - 10:21 AM

If the advise is not to touch my own primary is there anywhere I can ship it off to to be cleaned? I'm just not happy or satisfied putting something dirty back together, I just see no reason for it. Granted I have no experience taking an OTA apart, I'm not a moron. I modded my Canon, managed to get the filter smuged and cleaned it again. I've also completely stripped down the CG-5GT mount and polished the internals, along with re-grease and reassembly. The OTA is the last thing that I haven't torn down, but it is next. Sorry if I seem snappy but it just seems like I'm getting the response that I'm too dumb to touch the mirror because I haven't done it before. I plan on being in this hobby for life, so I have to learn at one point or another, now is as good a time as any. That being said, I'm an automotive mechanic, and I know that sometimes there are certain things you shouldn't touch/do unless absolutely necessary even if you know how to do it properly because of the room for error. If people still feel I should not touch the primary myself, I will heed your advice, however I want to make it clear that I'm not a run of the mill amateur when it comes to performing complex and precision tasks.

The front cell on my scope had no nuts, only 4 machine screws thru the cell into the tube, and 2 screws on the dovetail into the cell. I've already taken the front cell off, this is the case. In the event the corrector itself needs to come off this doesn't look like a problem. Looks like about 6 or so screws around the hold down ring and likely some shims and another ring below that one. No biggie if I have to pull it also.

#13 Sean13

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Posted 26 February 2013 - 10:47 AM

Is there an effective way to photograph the inside of the scope in a way such to show off the glare so I can post it here to see if it warrants a cleaning? Every picture I've taken is either too dark to show it or too washed out to show anything.

#14 KerryR

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Posted 26 February 2013 - 11:10 AM

You'll have to figure out if you want to take the risk and learn how to take the thing apart. It's not particularly hard, but there's a lot of potential for mistakes. For me, doing this stuff on occasion is something I enjoy; it's part of the hobby for me, and I'm willing to take risks. Other folks justifiably avoid such things, and focus on other aspects. Proceed as you see fit for your interests and abilities.

The general process is:
Leave the front and rear castings attached to the ota.
1) Remove the corrector from the cell.. Mark the rotational location of the corrector so that it can be returned to the same rotational location. Mark and label the shims that go between the edge of the corrector and cell so that each shim can be returned to it's original position, which ensures the corrector is returned to it's original lateral location.

2) Pull the rubber coat off the focus knob. Inside this is a screw. Remove this. There may be a set screw on the side as well. Loosen that. Point the scope slightly up and unthread the focus knob from the threaded shaft. You point the scope slightly up to prevent the mirror sliding forward on the baffle once the focus knob is removed.

3) Go to the front of the scope. On the front of the baffle is a retaining ring fitting into a slot. Remove the ring.

4) You can now grab the carrier, the part the primary is attached to, and gently pull it up and off the baffle. Turn the mirror slightly sideways once it's off the baffle so that the edges of the mirror can pass through the notches in periphery of the corrector cell.

5) As others have mentioned above, you may want to try spot cleaning with acetone and optical wipes. This would avoid having rinsing issues. If you wash with soap and water, you'll need to keep the water away from the carrier portion, so rather than wash with the mirror horizontal, wash with it on it's side. Wash the lower half, using gravity to keep the fluid away from the carrier. Once that half is clean and dry, rotate the mirror 180*, and wash the 'new' bottom half. Or do it in thirds instead.

The grease on the baffle is integral in reducing focus shift. So, if you had to wipe the baffle clear of grease, be liberal with replacement grease. Super Lube works well and is temperature stable, unlike some other greases. The trick here is to apply plenty of grease, but keep it off the mirror. My process was: apply 'too much', replace the mirror and use the carrier to distribute the grease on the baffle, all the way to the full back position, remove the mirror, and clean away the excess grease that accumulated near the rear of the baffle as well excess grease from the rear of the carrier. Clean out any excess that entered the interior of the baffle at the top.

In general, do most of the work with the ota at 20-45*. This will help ensure that anything you drop into the ota will hit the side of the ota and not the primary.

#15 Sean13

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Posted 26 February 2013 - 11:39 AM

Excellent, that was a great help. I understand what it meant by cleaning the mirror 1/2 at a time now. I plan on attempting some spot cleaning first, hopefully that will be all I need.

The grease I'm talking about isn't on the outside of the baffle, its on the inside, right around the tip (tword the front of the scope) and on the inside lip. I can clearly see a color change between the flat black paint and the part smuged with grease. I also see some shiny silver looking areas in the same place, perhaps where 2 different paint jobs came together, but not very well. I suspect one of these issues is causing my internal glare/reflections.

#16 rmollise

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Posted 26 February 2013 - 12:15 PM

If the advise is not to touch my own primary is there anywhere I can ship it off to to be cleaned?


It doesn't need cleaning. What you see will not affect images in anyway. If you insist, Celestron will do a cleaning for you, and will be happy to take your money, I expect. ;) Or...you can do it yourself and risk doing far more harm than good.

#17 orion61

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Posted 26 February 2013 - 12:27 PM

Do Not take the whole fron cell off!
You will most likely induce Astigmatism to the system if you do.
if you pull the corrector put a drop of pain on the corrector and cell so you know where to put it back,
I don't believe they mark them any longer. Most folks say the performance suffers after messing around with the corrector on the c6. Have it Professionally done. That is if you really need it done. Chances are it isn't affecting performance. It is hard to say without pictures.

#18 KerryR

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Posted 26 February 2013 - 01:46 PM


Can you post an astro image or sketch that shows the aberration in the image that's causing you to want to pursue this avenue? It's possible some folks here will be able to recognize the issue, including possibly recognizing that it's normal...

I agree with the others that you're unlikely to see performance improve after messing with or cleaning the ota. Flashlights ALWAYS create anxiety.

If you index everything and work carefully, it's unlikely it'll be worse, unless you have an accident.

As long as you can live with any mistakes you might make, including total loss due to, say, shattering the corrector or chipping the primary as you remove it from the tube, then proceed. You'll learn a lot, and might enjoy the process. I did, even though I had some minor mishaps. I no longer fear my SCT's guts. I find this liberating.

If the idea of tinkering doesn't appeal to you (I enjoy it), and you're sure you want/need the thing cleaned, Optic Wave Labs offers a cleaning service. Celestron does as well, but stories of dealing with their service department are not inspiring. Dr. Clay Sherrod (sp?) has offered, I believe, SCT servicing in the past that might have included cleaning.

#19 Sean13

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Posted 26 February 2013 - 03:05 PM

Here is a picture with a mild version of my issue visible (it does get a lot worse then this photo). It is an internal reflection, and not related to the camera. This image has been run thru PI, so the glare is minimized and smoothed. The real glares are almost rigid, they look like what an old metal watch band would look like laying on table.

I've thought very carefully about how the reflections work, how they move, what they look like, and what effects them. They look like the pattern of the baffle tube being exaggerated to one side. I have posted this image to other c6 owners and it appears its a manufacturing issue with the c6, and the vast majority of other owners experience the same thing. One person cured the problem by flocking the baffle tube, however I don't want to flock but rather do a correct paint job on the baffle if this is indeed the cause of my problems.

Posted Image

The collimation is good. However in reading about proper collimation it appears I've only done what is called "rough collimation" and not a "critical collimation" Could it really be so far off still that its causing these glares?

I'm definatly a tinker. I love to take stuff apart even if it isn't broken. I'm usually very careful and precise, otherwise I don't think I would have been successful in modding my camera. I'm up for the challenge and I'd much rather my learning process happens when I'm in this price range of equipment as opposed to years down the road when a lot more money is on the table.

#20 KerryR

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Posted 26 February 2013 - 04:34 PM

Is this visible visually, or only in imaging?

Seems like I've read something about a screw poking into the light path on some C6's, but you'd be able to see that when looking up the tube from the rear.

If the culprit is not obvious when looking up the tube, a worthwhile experiment might be to roll up a piece of paper very slightly longer than the baffle, stick it in there and let it unroll-- it'll cover up whatever the offender is. Then take or look or take a pic and see if the issue vanishes. If it does, you've isolated the issue to the inside of the baffle.

#21 Sean13

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Posted 26 February 2013 - 04:54 PM

Thats exactly what I'm planning on trying with a piece of black construction paper. I don't believe I see it visually, however I only used the scope for one or two nights visually before I began imaging with it and haven't had my eyepiece and diagonal on there since.

#22 Geo.

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Posted 27 February 2013 - 09:44 AM

Add Kodak Photo Flo or a similar wetting agent to your rinse. http://www.freestylephoto.biz/ sells Kodak and seven other brands. Wetting agents decrease water-surface tension and minimize water marks and streaks on film to promote faster and more uniform drying. Work the same on optics. They are concentrates, I'm still using some I got in college in the '60s.






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