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Is there a way to fix SCT Mirror shift

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#26 jhayes_tucson

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Posted 25 May 2014 - 02:30 PM

3..I put the rear screws just in front of the first surface of the mirror and the forward screws about 10mm behind the front edge of the slider tube. This allows you to adjust the slider tension screws from the front with the corrector plate out of the way. This way you will not have to pull the mirror assembly back out to make adjustments, should you need to.


Oh yeah…of course! But heck, that sounds even scarier--drilling and taping right next to the front surface mirror. Did you do anything special or were you just super careful??

Thanks…this is super helpful.

John

#27 PowellAstro

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Posted 25 May 2014 - 02:32 PM

My 14 has mirror lock and an electronic micro focuser but I like being able to focus with the standard knob so I did this mod I came up with. The mirror in the 14 is very heavy and I have used it for years after the mods and there is no mirror shift and no adjustments have been needed after the initial setup.

#28 PowellAstro

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Posted 25 May 2014 - 02:34 PM

Did you see point two? I removed the mirror from the slider. So the only piece I was working with was the slider tube.

#29 jhayes_tucson

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Posted 25 May 2014 - 02:39 PM

Did you see point two? I removed the mirror from the slider. So the only piece I was working with was the slider tube.


I'm sorry, I did see it but I misunderstood. My mirror is RTV'd to the backplate/slider assembly (with a LOT of RTV) and I didn't see how to get it off gracefully. Was yours glued on, or could you just unscrew it?

John

#30 PowellAstro

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Posted 25 May 2014 - 02:43 PM

Yes, look at this thread I posted on another C8 rebuild. You can see the mods and changes I made.

C8 Rebuild

#31 jhayes_tucson

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Posted 25 May 2014 - 02:50 PM

Yes, look at this thread I posted on another C8 rebuild. You can see the mods and changes I made.

C8 Rebuild


Hey, that's great…thanks! I'll just have to see if I can detach the primary from the slider tube. Obviously, that's the best option if I can make it work. I've got a new mount coming in a month and I've got to pull the scope and do some machining anyway so I'll just do the whole thing then. I really appreciate the suggestions.
John

#32 PowellAstro

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Posted 25 May 2014 - 02:57 PM

If you just go slow, a box cutter will help remove the silicon glue without to much fuss. The o-rings are a much better way to mount the mirror and easy to get the right tension.

#33 jhayes_tucson

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Posted 25 May 2014 - 03:38 PM

If you just go slow, a box cutter will help remove the silicon glue without to much fuss. The o-rings are a much better way to mount the mirror and easy to get the right tension.


I agree. It's been a while since I had it apart, but mine looked like it had a bunch of RTV up in the central hole. Maybe between razor blades and heat, I can get it off. The big problem is supporting the whole thing while working on it without damaging anything. I like the idea of a sliding it onto a stout horizontal dowel so I can rotate it and nothing can come loose. I'll have to work out a jig before giving it a try. I also like your approach with the O'rings to replace the RTV. Eliminating the focus shift and mirror flop would be a huge improvement for my scope so I'll definitely give it a try. I've got a few other mechanical improvements I need to do to the tube as well while I have it apart. It may be 5 weeks before I can get to it, but I'll post the results when I get it done.
John

#34 PowellAstro

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Posted 25 May 2014 - 03:51 PM

I really like my Meade 14 after the mods. I can use 800x and flip inside to outside focus and the star never moves. This really helps with AP as well. The mirror lock is nice but I cant always use it and reach focus with reducers, micro focuser and filters installed. Thats why I did this mod in the first place, so I could use all my attachments needed and not have the mirror shifting around.

#35 freestar8n

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Posted 25 May 2014 - 04:15 PM

Hi-

I'm not clear what you are trying to achieve. If you are looking for a smooth and backlash free system for visual work - then use an add-on crayford focuser plus the mirror locks.

If you are aiming for good imaging, then use OAG and mirror shift won't matter. And even if you glued down the mirror - you would still need OAG for overall flexure.

So - those are the two main solutions if you are aiming for particular results.

In practice - even for visual work, if you get used to removing backlash in the focuser by moving clockwise past focus, then carefully coming back to focus ccw - it will all be repeatable - and you won't have any problems focusing.

Frank

#36 jhayes_tucson

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Posted 25 May 2014 - 06:45 PM

1) OAG and ONAG are not possible with Hyperstar imaging.
2) Primary mirror focusing is really nice if you can eliminate focus shift and that should be entirely possible.
3) Adding an external focuser reduces back focus from the 6" nominal design position where correction and baffeling are optimized. It's probably not a huge deal, but i prefer to keep everything closer to the backplane.
4) Anytime you can tighten and/or stiffen the mechnics, it helps everything and hurts nothing--assuming of course that you don't damage something during the modification. But, heck that could also happen just by taking the scope off the mount! :crazy:

#37 PowellAstro

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Posted 25 May 2014 - 08:55 PM

+1

On my 14, I can't get everything I use attached plus an external focuser and still reach focus. I sometimes use two 6.3 reducers, AO-7, filter wheel and off axis guider and with an external focuser added, I can't reach focus. So, to have the unit work right with the internal focus and no image shift is a big, big plus.

Here is one example with two reducers:

Attached Files



#38 jhayes_tucson

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Posted 25 May 2014 - 09:50 PM

Sweet! Cool object and nice photo.

#39 freestar8n

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Posted 26 May 2014 - 02:12 AM

I was asking the OP since he is the one requesting help. My impression is that he is doing visual, in which case for smooth focus back and forth a crayford should solve his problems.

For hyperstar and any other system with automatic focusing using the primary, as I have said - all that matters is repeatability. The focusing software won't care or notice any image shift as long as backlash is removed.

Also for hyperstar, OAG is possible, though not commonly used or needed, with the qsi-wsg cameras. And common path guiding is possible with any dual chip sbig camera. The qsi's are small enough to work with 11 and 14" hyperstar.

No - I don't think primary mirror focusing shift can be entirely removed - nor does it need to be with automatic focusing.

Some people have added home-made mirror clamps and locks that attempt to fix the mirror in place - and anything like that could alter the figure of the mirror in a way that could be detrimental to high power work.

So for the OP - if you are after a better focusing feel for visual work - I would add a low profile crayford focuser. If you are imaging, I would add robofocus on the primary focuser and use automatic focus software. That will allow precise and automatic focusing both for hyperstar and normal sct imaging.

Frank

#40 Ali Khan

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Posted 26 May 2014 - 05:03 AM

Is the mirror locked or unlocked during transportation?

#41 Alph

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Posted 26 May 2014 - 10:56 AM

John,
The nylon screws might not be able to support the C14 primary mirror. I am waiting for someone try the grease suggested by a former Meade employee Regreasing the baffle tube

#42 PowellAstro

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Posted 26 May 2014 - 11:20 AM

The nylon screws support the Meade 14 mirror and I am sure it's as heavy as the C14 mirror. The screw tips only take up a few thousandths in space between the baffle and slider tube. You still have the grease and the same support as before. There is very little pressure on these screw tips and this does work. Many large dobs have nylon pads for bearings with a lot more weight on them and they work fine as well.

#43 Ed Holland

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Posted 26 May 2014 - 11:36 AM

Out of interest, what are the approximate mirror weights in these larger scopes?

PowellAstro, I seem to remember in one of your posts that you had added shims between the sliding tubes?

#44 bremms

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Posted 26 May 2014 - 11:45 AM

It would not have been difficult for the manufacturer to machine a wide groove for a split hard plastic cylindrical slider (think motorcycle fork) on each end of the mirror baffle. They could be sized to fit and it would be easy to fit new ones at any time. Even the bearings are not the best idea in my book.

#45 ColoHank

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Posted 26 May 2014 - 11:54 AM

Would it be possible to somehow shim the gap between the tubes with Teflon tape, the kind plumbers use to seal pipe joints, and do away with the grease altogether?

#46 Alph

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Posted 26 May 2014 - 12:05 PM

It would not have been difficult for the manufacturer to machine a wide groove for a split hard plastic cylindrical slider (think motorcycle fork) on each end of the mirror baffle. They could be sized to fit and it would be easy to fit new ones at any time.


Well, the the mirror slider could be sized to fit the baffle tightly. Why don't they do it? I think they leave a gap for thermal expansion and contraction.

Even the bearings are not the best idea in my book.


The bearings work great on Meade ACF f/8 OTA's. I don't see any mirror shift in my 12" ACF f/8 when focusing.

#47 jhayes_tucson

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Posted 26 May 2014 - 01:32 PM

1) I think that the nylon screw solution should work well (as has been demonstrated.) I agree that there just isn't sufficient load to cause it to bind--particularly with a little bit of lube. Nylon on anodized aluminum works pretty well even without any lube. The whole idea is to simply zero clearance the sliding tubes.

2) I didn't weigh my mirror when I had it apart, but my guess is that it weighs maybe 20-25 pounds. It's big but it isn't hard to pick up with one hand. Unless the angle is just right, the nylon screws would only take a fraction of that total load (most of the time.) Pointed straight up the screws would take zero load (obviously).

3) I totally agree that there a lot of ways to design this thing so that it comes out of the factory with zero clearance. Appropriate clearances, sleeves, or a kinematic, Crayford style mechanism could all work. In my view, Celestron should invest a bit of engineering time to fix this problem. I don't think that an effective solution has to be complicated or expensive.

4) Shimming could work as a fix, but I think that in practice, its going to be hard to beat an adjustable screw that gets locked in place. Teflon tape is a possibility but it only comes in a fixed thickness and the adhesive backing might not be as rigid as you would like. I believe that getting everything to operate smoothly with shims could be a challenge--unless you get lucky.

John

#48 PowellAstro

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Posted 26 May 2014 - 01:46 PM

My mirror was right at 20lbs in the Meade 14. I did grind the tips of my screws flat so there would be the most contact area.

#49 tomcody

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Posted 26 May 2014 - 02:00 PM

If the shift is caused by loosness and backlash in the focuser? you could install a Starlight instruments micro focuser, an easy 15 minute project.
Rex


The shift is due to a small amount of tilt in the primary due to the fact that there is A) Too much mechanical slop in the baffle tube mount, and B) The focuser drives the mirror from the side. The other problem for those of us doing photography is that gravity can cause the mirror to shift in tilt as the telescope tracks during an exposure. When you are trying to achieve arc-second level image stability, virtually any mirror motion really screws things up.
John

Before taking a scope apart to do modifications to it, I would first install a premium micro focuser to reduce any play there, can't hurt and may be all that is needed!
Rex

#50 Alph

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Posted 26 May 2014 - 02:36 PM

I think that the nylon screw solution should work well (as has been demonstrated.)


This has not been demonstrated to work on a GE mount. The M14 mentioned above rides on a fork mount. For a GE mount, you might need to use two set screws at 120 degrees apart to provide a stable 3 point support.






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