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Is there a way to fix SCT Mirror shift

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#151 GeneralT001

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Posted 01 July 2014 - 07:21 PM




A quick clarification please.

If the primary mirror is moving IOT achieve focus, how can you change the focus once you lock it? How is adding an aftermarket focuses (Crayford style)able to move the locked primary mirror? I know it works (from reading your response) but I just don't know how. :question:

Thanks


The add-on SCT Crayford adds a draw tube to the back of the scope (were you would normally screw on a visual back, prime focus adapter, or diagonal. This is very similar, if not identical to a Crayford on the side of a Newt or the back of a Refractor, and it works the same way... Rather than move the mirror, it moves the eyepiece.

Your diagonal (or what have you) then fits into that.

I got mine for the ability to fine tune the focus, I've never had that much of an issue with image shift during focus.


OK. The light is on. :rainbow:

Many.t thanks!

#152 BarryBrown

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Posted 02 July 2014 - 07:04 PM

In lieu of using roller bearings, it seems like building the mirror shaft to tight tolerances could pose problems in temperature extremes. On a cold night, the whole thing might bind up because the gap closes; when it's warmer you might get image shift as the gap opens.

#153 PowellAstro

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Posted 03 July 2014 - 12:14 AM

This should not happen as the baffle and the slider are both made of the same metal and should expand and contract at the same rate.

#154 bratislav

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Posted 08 August 2014 - 10:33 PM

Just to give some perspective here. A 30 arc second tilt primary in a C11 (this moves the edge of the primary barely 20 microns!)  introduces a third of a wave of coma, and very visible disruption in diffraction pattern, as demonstrated by false color FFT plot here. I doubt that tolerances on sliding primary baffle tubes in a commercial SCT are anywhere near that good. My own C11 will shift several arc minutes at focuser direction change, and will flop easily way more than 1 arc minute after a meridian flip. You don't want to see Zemax plots of that!

Attached Thumbnails

  • C11_30arcSecTilt.jpg


#155 jhayes_tucson

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Posted 09 August 2014 - 09:45 AM

That is a very good demonstration of why it would be a good thing to fix the problem!  I personally believe that with the right mechanics, it should be possible to hold the tilt of the primary to with in +/- 15" (and maybe a bit better.)  You won't do it with a bushing or sleeves or nylon screws.  The basic idea of the sliding the mirror on the baffle tube is a good one but at that level it will take a stout kinematic design.  I think that it would be possible to incorporate a good mirror lock into that design as well.  Those sliding rods just aggravate the tilt problem.  In my view, a better approach would be to firmly lock the mirror slider to the baffle tube using a precision collet.

John



#156 Ed Holland

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Posted 13 August 2014 - 05:52 AM

I think it was CN member Don Clement who also suggested that the mirror (carrier) could be suspended on stout flexures, thereby constraining motion to one axis. This could eliminate mirror shift and thanks to their elastic behaviour, could be preloaded in the direction of focus motion to eliminate backlash. It might be cheaper to implement than a precision bearing. The constraint for precision machining to match the mirror carrier's bore to that of the baffle tube would be removed. Setup might require a means to ensure concentricity of the mirror with the optical axis of the remaining components in the system

 

Just idle thoughts....

 

Ed



#157 jhayes_tucson

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Posted 14 August 2014 - 12:39 AM

I think it was CN member Don Clement who also suggested that the mirror (carrier) could be suspended on stout flexures, thereby constraining motion to one axis. This could eliminate mirror shift and thanks to their elastic behaviour, could be preloaded in the direction of focus motion to eliminate backlash. It might be cheaper to implement than a precision bearing. The constraint for precision machining to match the mirror carrier's bore to that of the baffle tube would be removed. Setup might require a means to ensure concentricity of the mirror with the optical axis of the remaining components in the system

 

Just idle thoughts....

 

Ed

 

 

I recall seeing a picture that Don posted showing what he built, but I've never heard anything about how well it worked.  Flexures are in principal a very good idea but they do have some potential limitations--mainly that they are typically very good at constraining movement along a desired axis, but only within a limited range.  Stacking flexure elements to increase range can increase the overall size and complexity of the device.  I personally think that a kinematic solution to the baffle slider would be the most simple and cost effective and could be made to work extremely well.  But, the proof is in the pudding so it's hard to say without actually building something to see how well it works.

John



#158 Ed Holland

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Posted 14 August 2014 - 11:18 AM

I agree about range, but the required travel for an 8" SCT or a small Mak isn't so large (I have no experience with larger telescopes) I'm almost tempted to mock up the idea and see where it might fall down, and how or even if it could fit inside a rear cell assembly.

 

On the other hand I certainly agree with your comments about kinematic design being a very desirable and effective approach.

 

It is simply interesting to consider the possibilities.



#159 FlyBD5

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Posted 16 August 2014 - 10:51 PM

Did you try taking out and adjusting the stock focuser to remove backlash?



#160 jhayes_tucson

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Posted 16 August 2014 - 11:02 PM

Did you try taking out and adjusting the stock focuser to remove backlash?

 

 

I'm sorry but I'm not quite sure what you are asking.  We are talking about the mechanical stability of the stock focusing mechanism in the Celestron SCT and the problem is not related to backlash in the drive screw.



#161 FlyBD5

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Posted 17 August 2014 - 08:12 PM

 

Did you try taking out and adjusting the stock focuser to remove backlash?

 

 

I'm sorry but I'm not quite sure what you are asking.  We are talking about the mechanical stability of the stock focusing mechanism in the Celestron SCT and the problem is not related to backlash in the drive screw.

 

 

Okdok. :)



#162 Ed Holland

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Posted 17 August 2014 - 09:24 PM

I did briefly mention backlash in the discussion of flexures, but not as a major point ;). If the time honoured advice alway to approach final focus with anti-clockwise rotation of the focuser is observed, backlash wont trouble you. My. C8 seems to have minimal backlash any way. 

 

Ed



#163 charlesgeiger

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Posted 17 August 2014 - 11:43 PM

jhayes,

 

Didn't you rebuild your system with the 5/32" screws forward of the mirror at 120 degrees (4 screws) and said it didn't work for you?  I can't seem to find this part of the thread...I believe you stated it might work if you could drill behind the mirror on the carrier....at any rate, please update us..thank you in advance...



#164 jhayes_tucson

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Posted 18 August 2014 - 04:15 PM

Yep...here it is:

 

http://www.cloudynig...t/#entry6124061

 

John



#165 jhayes_tucson

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Posted 18 August 2014 - 04:15 PM

I'm struggling to post with my iPad while on the road and it is sending duplicate posts.  Is there a way to delete a duplicate?  I don't see any delete button...

 

 


Edited by jhayes_tucson, 18 August 2014 - 04:19 PM.


#166 Herr Ointment

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Posted 18 August 2014 - 04:32 PM

If this thread keeps going my C8 is in real jeopardy

 

.



#167 jhayes_tucson

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Posted 18 August 2014 - 11:12 PM

If this thread keeps going my C8 is in real jeopardy

 

.

 

Hahaha...is it going to get bigger or smaller or just disappear all together?



#168 Lord Beowulf

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Posted 19 August 2014 - 12:24 AM

Hi all, 

 

I scanned this thread with interest as I was about to start my own thread on a very similar topic.  I have several Celestron SCTs, that I've had varying success doing imaging with due to mirror flop.  My latest is an 11" Edge HD CPC, which while advertised as an ideal imaging system, clearly illustrates (on multiple fronts) that Celestron has not given much thought to fully automated imaging setups, such as the one I'm trying to bring online in my newly finished observatory.  While the Edge HD does have the mirror clutches that should allow you to lock the mirror into position once focused, that doesn't do me any good for my remote control focuser, since I can't leave the locks clamped down (although I think I'm probably going to try, given I shouldn't need a large amount of motion for tweaking the focus).  At any rate, I'd have to agree that there's not really much excuse for the sloppy design of the SCT focusers these days.  I understand how the SCT started with a simple slip-tube design, but high quality linear ball bearings aren't exactly expensive these days.  There's no reason that the mirror flop issue couldn't be addressed especially for something like the larger Edge HD and ACF scopes.  The other possibility I've thought of, which would potentially be retrofit-able to my Edge would be to use a fine chain drive to tie three lead screws together rather than just using the single push/pull point of the current focuser.  Essentially you'd replace the two clutches with identical drive screws and tie all three focuser screws together.  That would force the mirror to track true and should eliminate the flop.  This would be pretty clunky as an after-market mod, but if built into the scope design, the connecting linkages (e.g. chain) could be inside the housing.  You could also use a worm gear drive shaft mechanism to tie the drives together.  

 

I can think of a few other ways to accomplish a central adjustment scheme, including using a threaded tube for driving the mirror.  That would give a center drive point and require tighter tolerances to begin with.  The requires some keying to ensure the mirror doesn't rotate, so we're back to some sort of linear bearing anyway.  Another similar concept would be a twist plate behind the mirror that would move a set of lever arms or even scissor lift type mechanisms around the perimeter.  The more pivot points, however, the more chance of something being loose.  The neat thing about this would be that it would only require a single push point to drive the plate, although it would have to be from the side rather than back.  None of these are perfect, but just trying to think outside the box (or tube in this case)!

 

Beo



#169 BWAZ

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Posted 19 August 2014 - 12:38 AM

The new Meade 10" f/8 OTA doesn't exhibit a tiny trace of mirror flop AFAICT. The design is not that complicated, though.



#170 jhayes_tucson

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Posted 19 August 2014 - 11:27 PM

Hi all, 

 

I scanned this thread with interest as I was about to start my own thread on a very similar topic.  I have several Celestron SCTs, that I've had varying success doing imaging with due to mirror flop.  My latest is an 11" Edge HD CPC, which while advertised as an ideal imaging system, clearly illustrates (on multiple fronts) that Celestron has not given much thought to fully automated imaging setups, such as the one I'm trying to bring online in my newly finished observatory.  While the Edge HD does have the mirror clutches that should allow you to lock the mirror into position once focused, that doesn't do me any good for my remote control focuser, since I can't leave the locks clamped down (although I think I'm probably going to try, given I shouldn't need a large amount of motion for tweaking the focus).  At any rate, I'd have to agree that there's not really much excuse for the sloppy design of the SCT focusers these days.  I understand how the SCT started with a simple slip-tube design, but high quality linear ball bearings aren't exactly expensive these days.  There's no reason that the mirror flop issue couldn't be addressed especially for something like the larger Edge HD and ACF scopes.  The other possibility I've thought of, which would potentially be retrofit-able to my Edge would be to use a fine chain drive to tie three lead screws together rather than just using the single push/pull point of the current focuser.  Essentially you'd replace the two clutches with identical drive screws and tie all three focuser screws together.  That would force the mirror to track true and should eliminate the flop.  This would be pretty clunky as an after-market mod, but if built into the scope design, the connecting linkages (e.g. chain) could be inside the housing.  You could also use a worm gear drive shaft mechanism to tie the drives together.  

 

I can think of a few other ways to accomplish a central adjustment scheme, including using a threaded tube for driving the mirror.  That would give a center drive point and require tighter tolerances to begin with.  The requires some keying to ensure the mirror doesn't rotate, so we're back to some sort of linear bearing anyway.  Another similar concept would be a twist plate behind the mirror that would move a set of lever arms or even scissor lift type mechanisms around the perimeter.  The more pivot points, however, the more chance of something being loose.  The neat thing about this would be that it would only require a single push point to drive the plate, although it would have to be from the side rather than back.  None of these are perfect, but just trying to think outside the box (or tube in this case)!

 

Beo

 

Been there, done that.  Years ago I built a Cassegrain with primary focusing driven by three lead screws driven by a toothed PIC chain.  The problem is that it is fundamentally over constrained.  Slack in the chain drive causes the screws to turn out of phase.  That leads to binding and tilt.  Even if you tighten the screws with an idler, the tension on the screws gets so high that the whole thing locks up.  I can tell you from experience that the three screw idea is VERY hard to implement.  I machined the thing very carefully and still couldn't get it to work well...like at all.

John



#171 TG

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Posted 21 August 2014 - 04:52 PM

 

Hi all, 

 

I The other possibility I've thought of, which would potentially be retrofit-able to my Edge would be to use a fine chain drive to tie three lead screws together rather than just using the single push/pull point of the current focuser.  Essentially you'd replace the two clutches with identical drive screws and tie all three focuser screws together.  That would force the mirror to track true and should eliminate the flop.  This would be pretty clunky as an after-market mod, but if built into the scope design, the connecting linkages (e.g. chain) could be inside the housing.  You could also use a worm gear drive shaft mechanism to tie the drives together.  

 

 

Beo

 

Been there, done that.  Years ago I built a Cassegrain with primary focusing driven by three lead screws driven by a toothed PIC chain.  The problem is that it is fundamentally over constrained.  Slack in the chain drive causes the screws to turn out of phase.  That leads to binding and tilt.  Even if you tighten the screws with an idler, the tension on the screws gets so high that the whole thing locks up.  I can tell you from experience that the three screw idea is VERY hard to implement.  I machined the thing very carefully and still couldn't get it to work well...like at all.

John

 

 

That's interesting. If I correctly understand what you gentlemen are describing, Celestron's already implemented and shipped this feature in their blue/white tube f/12 Celestron-Pacific scopes. The A-P Mak reportedly also has this mechanism. The difference though is that in both cases a belt was used, not a chain.

 

Tanveer.



#172 Herr Ointment

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Posted 21 August 2014 - 06:01 PM

I've asked before and I will ask again. Can someone point me toward an image or drawing of the "blue tube" focusing mechanism? Thanks!



#173 jhayes_tucson

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Posted 21 August 2014 - 06:16 PM

 

 

Hi all, 

 

I The other possibility I've thought of, which would potentially be retrofit-able to my Edge would be to use a fine chain drive to tie three lead screws together rather than just using the single push/pull point of the current focuser.  Essentially you'd replace the two clutches with identical drive screws and tie all three focuser screws together.  That would force the mirror to track true and should eliminate the flop.  This would be pretty clunky as an after-market mod, but if built into the scope design, the connecting linkages (e.g. chain) could be inside the housing.  You could also use a worm gear drive shaft mechanism to tie the drives together.  

 

 

Beo

 

Been there, done that.  Years ago I built a Cassegrain with primary focusing driven by three lead screws driven by a toothed PIC chain.  The problem is that it is fundamentally over constrained.  Slack in the chain drive causes the screws to turn out of phase.  That leads to binding and tilt.  Even if you tighten the screws with an idler, the tension on the screws gets so high that the whole thing locks up.  I can tell you from experience that the three screw idea is VERY hard to implement.  I machined the thing very carefully and still couldn't get it to work well...like at all.

John

 

 

That's interesting. If I correctly understand what you gentlemen are describing, Celestron's already implemented and shipped this feature in their blue/white tube f/12 Celestron-Pacific scopes. The A-P Mak reportedly also has this mechanism. The difference though is that in both cases a belt was used, not a chain.

 

Tanveer.

 

 

 

I don't think that it matters what you use--a belt will have exactly the same problem as a chain.  The design that I came up with was way over constrained and that's  the fundamental problem that makes any three screw approach difficult (note:  I did not say impossible.)  I used a PIC timing belt and getting things synced and tensioned properly is very difficult. In my view, this is a case where a kinematic solution is required.  Otherwise the tolerances are just too tight.  Maybe Celestron got it working, but I'd like to know how well it worked.   My guess is that they gave up on it for a good reason.  I believe that the baffle tube/slider solution can be made to work extremely well, but it has to be implemented properly.  Simply relying on tight mechanical tolerances is the wrong way to go about it.

John



#174 PowellAstro

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Posted 22 August 2014 - 12:27 AM

Dont use three drive points, use one just like it has now but move it to the center as close as you can. Then use a belt to drive the  focus screw. This way the focus knob can stay where its at. The closer to the center the focus drive screw is, the less tilt the mirror will have from focus action. 



#175 TG

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Posted 22 August 2014 - 05:00 PM

 

I don't think that it matters what you use--a belt will have exactly the same problem as a chain.  The design that I came up with was way over constrained and that's  the fundamental problem that makes any three screw approach difficult (note:  I did not say impossible.)  I used a PIC timing belt and getting things synced and tensioned properly is very difficult. In my view, this is a case where a kinematic solution is required.  Otherwise the tolerances are just too tight.  Maybe Celestron got it working, but I'd like to know how well it worked.   My guess is that they gave up on it for a good reason.  I believe that the baffle tube/slider solution can be made to work extremely well, but it has to be implemented properly.  Simply relying on tight mechanical tolerances is the wrong way to go about it.

John

 

 

Bob Piekiel's your best bet on info about how well it worked and the details of implementation. He advertises his e-books on Astromart often and you can send him a message there if you're a member.

From what I understand, Celestron dropped it in the quest to make cheaper, affordable SCTs, a mission where they succeeded by any measure.

 

Tanveer.




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