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Replacing a Celestron C9 1/4 corrector plate & co

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#1 Ajohn

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Posted 10 January 2013 - 11:49 AM

I had a nasty Christmas present in the from of a C9 1/4 with the corrector plate broken during shipping and wondered if any one has any experience of replacing one. More mechanically and how they are aligned if they are. I've read about rotating them for best correction but that makes no sense at all. They need to be square to the mirror and it's hard to see how rotating them would achieve anything. I assume they ensure squareness via manufacturing the tube accurately or adjusting the cell in some way.

I've just found out that there is another problem as well. It's a USA made scope not one of the more recent Chinese ones. The official way of repairing those is to send them back to the US - out of the question from the UK. I can get a Chinese plate but no one in the UK knows if the design is the same. Any ideas?

One other minor problem. The lifting handle on the mirror cell is loose so ideally I need to remove the cell and tighten the fixings. There are an amazing number of screws. Hard to know which ones really need undoing.

I had a quick look around to see if there is any sort of servicing manual but so far have failed to find anything. Might be on a dealer site etc.

:bawling: Hope some one can help. The weight of the 2ndry and throwing the double boxed scope about seems to have broken it and as usual optics are not insured so it wasn't worth paying the extra.

John
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#2 pstarr  Happy Birthday!

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Posted 10 January 2013 - 01:04 PM

There are magic maker lines on the back of the secondary and primary mirror. These lines represent the best correction that was achieved when the optics were rotated in relation to the corrector. These lines and the serial# etched on the corrector all need to line up for best performance. Place the secondary in the corrector with the line facing the serial number on the corrector and then just line it up with same line on the back of the primary. Because your will be using a corrector that has not been tested with your scope, I would get an artificial star or one of these very small Christmas tree lights. Place it a hundred yards or so away and look at the diffraction pattern inside and outside of focus. Rotate the corrector until you get the best results. That being said, I did the rotation of the corrector to see if I could improve on the factory setting and found it made minimal difference. Hope this helps.

#3 Ajohn

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Posted 10 January 2013 - 01:30 PM

Thanks. I'm wondering if Celestron UK have identified the scope correctly. It's a C9.25 XLT starbright with I think a carbon fibre look tube rather than carbon fibre. It taps like metal. They introduced the XLT in 2003 according to the web and got took over by Syntra in China in 2005. No idea when the carbon fibre thing happened.

It's been fitted with Bob's knobs at both ends. My biggest worry is spending a fair amount of money on a Chinese plate and finding I have a *BLEEP* scope. Mind you some would say they are *BLEEP* anyway.

John
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#4 Ajohn

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Posted 10 January 2013 - 01:38 PM

Sorry about the *bleep* I used a short word that begins with c and ends with p which just means rubbish and one or two other things but wouldn't cause offence in this context.

John
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#5 DAVIDG

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Posted 10 January 2013 - 03:03 PM

A couple of clues to the optical design difference if there are any between your C9.25 and the ones made in China would the spacing between the corrector and the primary mirror. The diameter of the secondary, the exact focal lengths and of course if one scope uses any additional optics.

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#6 MKV

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Posted 10 January 2013 - 04:07 PM

John, why don't you just get in touch with the Chinese and and ask them if their plates are compatible, or if they can replace it for you?

People on this forum don't have access to their data or their facilities.

Mladen

#7 Ajohn

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Posted 10 January 2013 - 04:59 PM

Sorry about the name spelling Mladin - using a small keyboard that has slightly smaller key spacing and not used to it yet.

I feel it is worth asking on here as there might even be some one who worked for Celestron about. Celestron do sell the plates from time to time and there is also the chance that some one has fitted one. The XLT is still in production and only the Chinese plates are available.

I've taken the plate out. As it has the modified collimation screws on the secondary mirror some on has been in before. No signs of any serial number on the plate at all but there are felt tip alignment marks. All rather loosely assembled in my view complete with a missing cork pad. There should be 3 around the edges. I suspect that explains the breakage.

John
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#8 MKV

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Posted 10 January 2013 - 07:46 PM

No problem John, but it's not Mladin either; it's Mladen. Maybe you should just use my initials or copy-and-paste or just skip my name altogether! I hate to revisit this issue, but, hey, that's my name, it has a meaning and a sentimental value, so if I seem insistent I hope you'll understand. Thank you.

It sounds like you bought this scope as a "pre-woned" item. Therein lies the rub. The old adage says - buy cheap, pay twice. I have learned the hard way so now I make sure I pay once! And, I if i can't afford something, I don't buy it. It actually saves me money.

I think the breakage can definitely be traced to the missing cork pads. Someone was "fixing" the scope for a sale and forgot to put the corks back or simply misplaced them and sent the the scope anyway.

I don't know of too many successful stories where a broken Celestron corrector plate has been replaced cheaply. If, on the other hand, this scope was bought from a legitimate Celestron dealer, then you should be reimbursed for shipping it back, and being repaired.

You may still try to contact the Chines and work out something with them.

Good luck!

Mladen

#9 Ajohn

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Posted 11 January 2013 - 07:54 AM

Sorry again Miaden. I will get it right. 1st time must have been left fingers going quicker than right. 2nd, maybe I should wear glasses.

The scope has been fitted with what I think are called Bob's knobs. Why do that on the 2ndry collimation screws pass. That must be when the cork pad was "lost" as it involves removing the corrector plate and removing the 2ndry holder from that and fitting the new one.

I have sold several used scopes and there has been nothing wrong with them at all. Must admit if I had realised that the 2ndry holder wasn't original I probably wouldn't have bought it.

Seems a new plate is £200 in the UK. A lot too spend if things don't work out. I'm just about to see if I can contact Syntra. I am also going to contact the seller concerning how a new corrector plate is going to be paid for especially as the outcome is risky.

John
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#10 Jon Isaacs

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Posted 11 January 2013 - 08:05 AM

This would seem to be a situation for the CATs and CASSes forum, in particular, Rod Mollise. If there is anyone on the outside of Celestron who would know, it would be Uncle Rod...

Jon

#11 ed_turco

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Posted 11 January 2013 - 11:24 AM

I am afraid to say it, but anyone who loses the corrector plate on a Schmidt-Cass is really out of luck. These plates are not interchangeable and even if you found one, it would take a master optician, and I'm not!), to make things right. Even if the optician tries to be reasonable in his fees, the money it would cost would exceed the cost of the same telescope new.

#12 MKV

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Posted 11 January 2013 - 12:41 PM

Sorry again Miaden.[sic] I will get it right.

This is the THIRD time John that you butcher my name! Maybe you should just not use my name, since you either can't or don't want to get it right. Okay?

Mladen

#13 Driven1

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Posted 11 January 2013 - 03:18 PM

I don't know how bad your situation is with your corrector, but I had a similar situation on my scope with a pretty bad crack emanating from the secondary holder out to the outer edge of the plate. Being in the exact situation you're in of not being able to find a replacement, I fixed mine with Permatex Bullseye Windshield Repair resin. You can get instructions here of how I went about it. http://tech.groups.y...s/message/46392

#14 kfrederick

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Posted 11 January 2013 - 05:47 PM

If part of the corrector is there save it There is a book on replacing one I would say it would be very hard to make for almost all glass pushers the book is by Robert Piekiel Making Schmidt-Cassegratn Telescope Optics . I think any glass pusher should own this book .

#15 Ajohn

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Posted 11 January 2013 - 05:47 PM

Sorry once again. I was in something of a rush.

I will try the other forum excellent idea as was china.

Not concerned about alignment and have a fair idea that I have made in USA with China glass scope. All I do know is that it has been done by some but getting factual information concerning the outcome isn't easy.

John

#16 Ajohn

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Posted 11 January 2013 - 05:55 PM

If part of the corrector is there save it There is a book on replacing one I would say it would be very hard to make for almost all glass pushers the book is by Robert Piekiel Making Schmidt-Cassegratn Telescope Optics . I think any glass pusher should own this book .


Actually I probably could make one out of window glass as the makers did until they switched to a water white version of the same. It would still be a fairly expensive option and no spec is available. I would need a vacuum pan, oil, blank for the plate and 2 reasonably rigid blanks of the same or larger size for the tool. And then it would need coating. There is a way of doing it without all of this as well but that method would be really difficult but probably more accurate.

John
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#17 kfrederick

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Posted 11 January 2013 - 06:55 PM

If you have at least half of the plate then calipers can check the profile it is a positive lens in a bigger neg one or something like that not easy to make and the secondary may need worked also Just reading from the book not worked anything like this

#18 Ajohn

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Posted 13 January 2013 - 06:43 AM

I'm beginning to think it's best to write the scope off and talking to the seller about a refund. I reckon 50/50 would be fair. It's what I would do if this happened to something I sold. Big problem with optics. Carriers wont insure them usually just for loss which is a bit pointless if the package is tracked. If I buy used of a private seller again I think I am going to insist on that sort of deal before buying. I've also told him it would be worth checking for missing cork pads around the plate if he sends another by post - I most certainly will. Looks like in my case there should have been 6 and there are only 2. An accident just waiting to happen really.

John
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#19 Geo.

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Posted 13 January 2013 - 11:32 AM

These plates are not interchangeable and even if you found one, it would take a master optician, and I'm not!), to make things right.


This may have been true in the day of hand figuring at Celestron, but that's no longer done. Celestron will now replace just the broken corrector. The orientation of the corrector is said to not be critical. Rather, the position of the secondary is primary, to coin a phrase. Rotating the corrector does little but rotate the secondary to a position where the least aberration.

Overall the Celestron optics coming out of China have been excellent. The Chinese know the military value of good optics and have spent a great effort over the last half century to develop this industry.

Carbon fiber tubes were produced for the Nexstar GPS line and were available as OTAs from 2003 for a few years. They are are unpainted and a clear CF pattern. The aluminum tubes of this period were painted black or gray.

#20 Geo.

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Posted 13 January 2013 - 11:52 AM

One other minor problem. The lifting handle on the mirror cell is loose so ideally I need to remove the cell and tighten the fixings. There are an amazing number of screws. Hard to know which ones really need undoing.


You needn't, and probably shouldn't, break the tube assembly. The primary will have to come out. This is done by removing the corrector, the focuser assembly, the baffle tube circlip/o-ring and then the primary and carrier assembly. The latter is removed through the length of the tube and out the front. There are cutouts in the front cell that permit the primary to removed once rotated longitudinally. When you've done this this is what you'll see. The arrows are the handle screws.

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#21 Ajohn

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Posted 13 January 2013 - 12:37 PM

I've had a couple of theads on this topic - pretty obvious why. :( Info is mixed. One included a link to a white paper on the edge. It's pretty detailed in terms of how they are made and as I suspected the correctors are sucked down into a former and then ground spherical - much the same as the vacuum technique really but the plate will always go to the same position. They are still hand figuring the 2ndry on these now. As controlling radii seems to have been very accurate for some time now chances are this may be finessing the results produced by the corrector.

I did find some info on when the scopes 1st went to China that commented that they too were figuring the 2ndry. Nothing recent but it seems to be a fact that when my scope was made they did figure it. I would suspect that this would be to improve on the results given from a specific corrector plate. I don't think it makes sense to use this method to correct for something else. If the a mirror was too far out for instance the simplest thing to do would be to reprocess it. One of the problems with these scopes is that all the parts hang together rather closely. I don't know if others agree but for what could be called figuring I don't think they could make much difference to serious problems with the mirror. It's likely to be a bit like figuring a parabola really that is very close but over or under done.

There are a couple of curious things about the scope though. Carbon fibre doesn't ring like I expected it too. Probably me and it doesn't sound like a Meade tube. More odd is the complete lack of a serial number on the corrector plate which makes me wonder how they managed to keep the parts together as there are no marking at all on it. This is why I have kept on asking about using a Chinese plate but I'm concluding that I could spend 200 quid plus the time to set it up and finish up with a dubious scope.

One thing I would add. Some one who took the plate out to get dust out of a scope found 3 cork pads. From where the remaining 2 on mine are it looks like there should be 6. One by each screw. If any one sends one via the mail it would be a good idea to check that these are all there. I also suspect I might have problems buying the missing ones. I fairly sure this is why the plate broke - nothing to stop it jolting around and hitting metal.

John
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#22 Geo.

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Posted 14 January 2013 - 11:23 AM

I think you're assumptions make sense. I've replaced Celestron correctors and the images seemed perfectly good. Of course you have to be up front about selling an SCT with mixed optics. There are usually only three cork spacers, less if the corrector centers without them. The last C8 I worked on had one thin card stock shim.

Correctors get broken in various ways, but I think any lateral impact is more likely to chip the edge than crack or smash one. I've seen them smashed loose primaries, impact on protruding Bob's knobs, end impact where the mass of the secondary does the job, etc. All makes Vixen's modified Cassegrains look attractive.

#23 Ajohn

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Posted 14 January 2013 - 01:05 PM

I wonder about mixed optics as there is no serial number on the corrector. I paid for an upfront scope. I don't think that is dishonesty on the part of the seller though.

Bob's knobs but clear of the cover plate. The retaining ring was rather loose though. :-) I've wondered about Vixen's version too but wanted a light weight scope.

This is what was revealed once the cover came off. I thought that Celestron had improved the 2ndry adjustments. I probably wouldn't have bought it if I had known it was a mod. They don't need tweaking very often so I don't see it as a sensible mod. They go out when something somewhere moves - the mirror remains in the centre of the corrector. There is one pad by the dovetail and another by the next screw anticlockwise.

John
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#24 mikey cee

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Posted 14 January 2013 - 06:05 PM

Cracked? Where? :confused: :tonofbricks:

#25 Geo.

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Posted 15 January 2013 - 10:50 AM

The SN is on the surface of the corrector under the retaining ring as indicated below. It is very small perhaps 2mm by 10mm and very near the edge. I t really looks like this was dropped on its end. I'm shipping one to Moldovia today. Fortunately, I'm off the hook once it gets to a reshipper in New Jersey.

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