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C14: How much mirror flop is too much?

catadioptric Celestron optics SCT collimation
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#26 gnowellsct

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Posted 08 August 2018 - 11:20 PM

chip, They have these packaged foam thingies you put them under around and on top in the box and pull the tab or whatever and they inflate around the package locking it into place and making for a very very solid shock resistant package.  

 

Here's a pic, I greatly fear it does not adequately show how nicely the foam curves around the c8.   But you see how there's a cradle at the bottom.  I think this stuff is better than the cut packing foam that was around my original c14. 

 

You'll have to ask them about insurance and if they say no ask if you can pay them to insure it.    I honestly didn't think about it for my c8.  I don't know what the policy is.

 

GN

 

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#27 ccwemyss

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Posted 09 August 2018 - 10:29 PM

Thus far, I've heard from three people. It seems that Greg N. and John have no mirror flop with their C14s and Tom Glenn has enough in his 9.25 to require re-collimation about every hour for planetary imaging. That's not enough of a sample yet to answer my question, but does indicate that some scopes have only focus shift. Hopefully some more people will add to the data. 

 

Regards,

 

Chip



#28 WadeH237

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Posted 10 August 2018 - 11:06 AM

My C14 has mirror shift.  I've not measured it, but for visual use, it is not objectionable at all.  Collimation mostly holds fine between sessions.  The exception is on nights of truly exceptional seeing and observing Jupiter or Saturn.  In that case, I touch up the collimation in the area and check it again after the object transits.  Sometimes, I'll do another touch up after the transit.

 

For imaging, the impact is greater.  Image shift at the microscopic level is continuous, so an OAG or ONAG is absolutely required.  Automated focus requires setting up backlash compensation so that it always approaches final focus in the CCW direction with sufficient turns to overcome backlash.  Also, it can take the mirror up to 2 minutes to settle after a slew.  I see this mostly when doing an automated sky model (which points the telescope at hundreds of targets to plate solve them).  In that case, about 5% of images show trailed stars.

 

Finally, I would say that it's not possible to have focus shift without *some* impact on collimation.  I would guess that it takes quite a bit of it to impact collimation such that you could notice it over all but the very best seeing conditions.



#29 ccwemyss

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Posted 17 August 2018 - 04:21 PM

Just a bit of follow-up. I finally got somewhat better conditions, and after working on the collimation decided to back off the retaining ring, in case there was any pinching of the mirror. I didn't see much change in the diffraction rings or images as a result, but the flop increased to about 7'. Then I installed some spring-loaded flop stoppers and a Crayford style focuser on the back.

 

Interestingly, with the tension on the other two points of the mirror, focus shift dramatically increased to 7'. A star centered in the field of the 10mm Delos would jump entirely out of view with a change in focus direction. I finally settled on a focus position with the knob turning clockwise to provide tension on the third point.

 

Collimation is fairly good across the sky, but I need to lock everything and fine tune it so that I can tell whether it is a little bit off in different directions or if it is just off in the same direction everywhere. It's nice to be able to focus without even the 1' shift I was seeing before the flop stoppers. The jury is still out on whether there is residual flop -- I need another good night for testing. 

 

Regards,

 

Chip W. 



#30 jhayes_tucson

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Posted 17 August 2018 - 06:23 PM

Chip,

I'll be interested to hear how those things work.  It's not very surprising that they don't help with focus shift and that they make it a bit worse. For imaging, I think that it's better to just lock the primary in place and use the secondary to hold focus.

 

John



#31 ccwemyss

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Posted 20 August 2018 - 09:49 PM

Well, the flop-stoppers didn't give any improvement. If anything it is worse, at about 15' of shift from one side to the other. When it's on its right side, collimation is good, but on the other it is clearly off. This is with both of the flop-stopper shafts locked down and the focus knob turned clockwise until snug and then held in place with a wide rubber band. 

 

Since the flop is still happening when the triple tree is stabilized, and given that tightening the retaining ring helped, it seems that the problem may be that the mirror has come loose from the mount, or at least that the RTV has considerable flex. Although it is interesting that when the mirror mount is locked on two sides, the focuser can still cause about 7' of shift by itself. That could mean that the slider and baffle tube allow a lot of play too.

 

Funny thing was that I began testing tonight using a small, distinctive crater on the moon, near the terminator, as the target. When I flipped it the first time, it was almost dead center and I was ready to cheer. But since I always repeat the test, I tried it again on a different crater, and this time the moon wasn't even in the field. Then I realized that the first test had, just by chance, landed on the one other crater with a similar appearance that was about half way across the moon. 

 

Regards,

 

Chip W. 



#32 555aaa

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Posted 23 August 2018 - 02:30 PM

If I understand what you are doing it is a bad test because every time you flip the mount from counterweight east to counterweight west you add in an unknown amount of free play in the RA worm. How do you know if you are loaded against the same side of the tooth profile? You can't be unless you move the counterweights to always be east or west heavy. If you have several arc min of flop, turn off the mount model and look at the dec error between a star at zenith and a star at say 45 dec elevation. There is a small error from refraction (< 1 arc min from zenith to 45 degrees)  but the dec and ra worms won't have a reversing load so it should tell you if something else is moving. The shift when you focus is of course not that effect.


Edited by 555aaa, 23 August 2018 - 03:05 PM.


#33 ccwemyss

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Posted 23 August 2018 - 10:56 PM

If I understand what you are doing it is a bad test because every time you flip the mount from counterweight east to counterweight west you add in an unknown amount of free play in the RA worm. How do you know if you are loaded against the same side of the tooth profile? You can't be unless you move the counterweights to always be east or west heavy. If you have several arc min of flop, turn off the mount model and look at the dec error between a star at zenith and a star at say 45 dec elevation. There is a small error from refraction (< 1 arc min from zenith to 45 degrees)  but the dec and ra worms won't have a reversing load so it should tell you if something else is moving. The shift when you focus is of course not that effect.

I'm moving the mount manually (loosen the clutches, flip, get object in finder, tighten clutches), then I use the centering controls to get the object exactly on the finder crosshair, then check in the eyepiece. Go back and forth three times to ensure repeatability. On one side, an object on the crosshair is centered in a high power eyepiece and collimation is good. On the other side, the object is far off center in a wide field eyepiece, and the collimation is poor. Flop stoppers securing the mirror mount didn't help. Other sources of shift (secondary, corrector, diagonal, tailpiece, etc.) were all checked. It appears that the mirror is moving independent of its mount. Tightening the retaining ring helped, but it can't be tightened enough to get rid of flop and not pinch the mirror.

 

Regards,

Chip



#34 555aaa

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Posted 24 August 2018 - 12:15 PM

Thanks for the explanation, Chip. Sorry I misunderstood.



#35 freestar8n

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Posted 24 August 2018 - 08:21 PM

I'm moving the mount manually (loosen the clutches, flip, get object in finder, tighten clutches), then I use the centering controls to get the object exactly on the finder crosshair, then check in the eyepiece. Go back and forth three times to ensure repeatability. On one side, an object on the crosshair is centered in a high power eyepiece and collimation is good. On the other side, the object is far off center in a wide field eyepiece, and the collimation is poor. Flop stoppers securing the mirror mount didn't help. Other sources of shift (secondary, corrector, diagonal, tailpiece, etc.) were all checked. It appears that the mirror is moving independent of its mount. Tightening the retaining ring helped, but it can't be tightened enough to get rid of flop and not pinch the mirror.

 

Regards,

Chip

The mirror may "flop" a bit when you do a meridian flip - but the net change should be reduced if you make sure to re-focus on each side of the meridian, by pulling the mirror back a bit with clockwise turns of the primary focus knob - and then approach focus with ccw turns.

 

This will help place the mirror in a repeatable orientation relative to the baffle tube - and also help maintain collimation and reduce the effective "flop."

 

Did you re-focus in this manner before and after the meridian flip?  For imaging this can all happen automatically - and for visual work it's a fairly natural thing to do that doesn't take long.

 

Frank



#36 mconnelley

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Posted 24 August 2018 - 11:15 PM

Hello:

   

  Your symptoms sound like something that happened to me.  I had restored an old C10, and in the process took the primary mirror off of the spindle that slid up and down the baffle tube.  This was really easy to do, so I didn't realize that the mirror had been glued (or at least stuck) to the cork pad on the top of the spindle.  When I put the scope back together, the collimation and pointing were very unstable, especially when moving to the other side of the sky.  I could hear a slight 'thunk' when I moved the scope to the other side of the sky.  

 

  Upon disassembling the scope, I could feel the primary mirror sliding around.  I glued the mirror back to the spindle (probably with silicone), and the problem went away.  

 

  As I recall from recently rebuilding a C11, the primary mirror pretty stoutly bonded to the assembly that slides up/down the baffle tube and has the arm with the focusing screw.  My guess is that your mirror somehow came loose.  Pull the mirror out of your C14 and see if it's still bonded to the focuser/slider/tube thingie.  

 

 I don't think tightening that nut on the front of the mirror is really the solution you want.  It sounds like you're squeezing the mirror to try to keep it in place.  Applying stress to a mirror in a way like that usually isn't a good thing.  

Cheers

Mike



#37 ccwemyss

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Posted 26 August 2018 - 10:45 PM

The mirror may "flop" a bit when you do a meridian flip - but the net change should be reduced if you make sure to re-focus on each side of the meridian, by pulling the mirror back a bit with clockwise turns of the primary focus knob - and then approach focus with ccw turns.

 

This will help place the mirror in a repeatable orientation relative to the baffle tube - and also help maintain collimation and reduce the effective "flop."

 

Did you re-focus in this manner before and after the meridian flip?  For imaging this can all happen automatically - and for visual work it's a fairly natural thing to do that doesn't take long.

 

Frank

Originally, the shift was nearly half a degree (out of the field of view of the widest eyepiece available for the C14). Counterintuitively, there was almost no shift in response to changing focus direction. 

 

Tightening the retaining ring on the mirror (which was definitely loose), reduced flop to as little as 4' of shift, but focus shift increased to about 1'. However, that seemed to be pinching the mirror, so I backed it off until the ring was stopping with very light pressure. The flop shift increased to about 7'.

 

I then tried a set of flop stoppers that applied spring tension to two points on the rear of the mirror cell. That didn't seem to reduce the flop shift. So I locked them in place, and then found that focus shift had increased to about 3'. The flop shift was still about 7', even when the focus knob was turned CW until it was snug (balancing the tension on the other two points).

 

It seems that that mirror is able to shift even when its cell is held statically. The original small amount of focus shift could just be that the slider moves so easily that it avoids tilting. But when locked at two points, pushing or pulling on the third point reveals some slop between the slider and the baffle tube. Or it could be that the focuser arm of the cell applies a force to the back of the mirror that takes out or increases some of its looseness. 

 

Regards,

 

Chip W. 



#38 ccwemyss

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Posted 26 August 2018 - 10:54 PM

Hello:

   

  Your symptoms sound like something that happened to me.  I had restored an old C10, and in the process took the primary mirror off of the spindle that slid up and down the baffle tube.  This was really easy to do, so I didn't realize that the mirror had been glued (or at least stuck) to the cork pad on the top of the spindle.  When I put the scope back together, the collimation and pointing were very unstable, especially when moving to the other side of the sky.  I could hear a slight 'thunk' when I moved the scope to the other side of the sky.  

 

  Upon disassembling the scope, I could feel the primary mirror sliding around.  I glued the mirror back to the spindle (probably with silicone), and the problem went away.  

 

  As I recall from recently rebuilding a C11, the primary mirror pretty stoutly bonded to the assembly that slides up/down the baffle tube and has the arm with the focusing screw.  My guess is that your mirror somehow came loose.  Pull the mirror out of your C14 and see if it's still bonded to the focuser/slider/tube thingie.  

 

 I don't think tightening that nut on the front of the mirror is really the solution you want.  It sounds like you're squeezing the mirror to try to keep it in place.  Applying stress to a mirror in a way like that usually isn't a good thing.  

Cheers

Mike

This does sound similar to the symptoms I'm seeing. In some other threads, however, I've seen cautions about regluing the mirror because it can end up with a tilt that introduces astigmatism. If it turns out that the RTV has separated on one or more of the arms, I wonder if a thin layer of epoxy would bond without a significant change in geometry. Silicone seems like it would end up thicker. 

 

Regards,

 

Chip W. 



#39 mconnelley

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Posted 27 August 2018 - 03:24 AM

Hello:
 

   If the bond has failed between the mirror and the focusing assembly, then it's best to pull the mirror off of that assembly, clean out the remaining glue, and glue it up again.  I don't know if epoxy is a bad idea.  If you suspect that the glue has failed in a few places, but not others, then you still need to pull the mirror off to fix the problem.  You want to glue glass to metal, not old glue to old glue, which is what would likely happen if you can't clean out the old adhesive.  The thickness of silicone isn't a problem, as long as it's even.  

 

   Gluing the mirror is such a way that it's straight is important.  I don't recall worrying about this much when I did mine, probably because you can collimate the primary mirror of a C10.  Here's an idea to try.  Put a laser collimator into the focuser tube, and hold a card at the radius of curvature of the mirror.  Hopefully you'll just see one spot, from the laser itself.  However, if the mirror is tilted then you'll see a faint second spot that is made by the mirror reimaging the spot on the card.  If you take the mirror off and glue it back on, you can tweak the tilt of the mirror by making sure its dot is behind the spot from the laser. This is how I set the collimation for the primary mirror of my C10.  

Cheers

Mike



#40 freestar8n

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Posted 27 August 2018 - 05:10 AM

Originally, the shift was nearly half a degree (out of the field of view of the widest eyepiece available for the C14). Counterintuitively, there was almost no shift in response to changing focus direction. 

 

Tightening the retaining ring on the mirror (which was definitely loose), reduced flop to as little as 4' of shift, but focus shift increased to about 1'. However, that seemed to be pinching the mirror, so I backed it off until the ring was stopping with very light pressure. The flop shift increased to about 7'.

 

I then tried a set of flop stoppers that applied spring tension to two points on the rear of the mirror cell. That didn't seem to reduce the flop shift. So I locked them in place, and then found that focus shift had increased to about 3'. The flop shift was still about 7', even when the focus knob was turned CW until it was snug (balancing the tension on the other two points).

 

It seems that that mirror is able to shift even when its cell is held statically. The original small amount of focus shift could just be that the slider moves so easily that it avoids tilting. But when locked at two points, pushing or pulling on the third point reveals some slop between the slider and the baffle tube. Or it could be that the focuser arm of the cell applies a force to the back of the mirror that takes out or increases some of its looseness. 

 

Regards,

 

Chip W. 

Hi-

 

My question was about whether you re-focused after you did the meridian flip - and if you pulled the mirror back - then pushed it forward.  Unless the mirror is mechanically loose in the way it is attached, the focusing system should do a good job of loading it against gravity with nearly the same geometry with respect to the baffle tube.

 

If you simply locked it somehow - then you are relying on how well those locks can hold the mirror in place.  But you can achieve the same effect reliably without locks at all - if you simply pull the mirror back - then push it forward against gravity.  It has no opportunity to float - and no need for additional support - if it is being pushed against gravity by the same mechanical system - as it is pointed up to the sky.

 

Frank



#41 ccwemyss

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Posted 27 August 2018 - 10:22 AM

Hi-

 

My question was about whether you re-focused after you did the meridian flip - and if you pulled the mirror back - then pushed it forward.  Unless the mirror is mechanically loose in the way it is attached, the focusing system should do a good job of loading it against gravity with nearly the same geometry with respect to the baffle tube.

 

If you simply locked it somehow - then you are relying on how well those locks can hold the mirror in place.  But you can achieve the same effect reliably without locks at all - if you simply pull the mirror back - then push it forward against gravity.  It has no opportunity to float - and no need for additional support - if it is being pushed against gravity by the same mechanical system - as it is pointed up to the sky.

 

Frank

Yes, I ran the focus in and out several times, since I was checking collimation on both sides of focus each time. So I was also checking the resulting shift both when pushing and pulling on the mirror.

 

With the flop stoppers installed, changing focus direction had a noticeable effect, but still less than half of the total shift. Before they were installed, it had almost no effect. With the stopper shafts locked, it was still possible to produce image shift by turning the focus knob (it would still move about half a turn). 

 

Note that the OTA was nearly horizontal on each side, and collimated when on its right side. When pointed vertically, the flop shift was less and collimation was not as far off. On its left side, flop shift was maximized, and collimation was unusable. The flop shift also made it impossible to build a good model with the mount. 

 

Regards,

 

Chip



#42 bobhen

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Posted 27 August 2018 - 10:42 AM

The mirror may "flop" a bit when you do a meridian flip - but the net change should be reduced if you make sure to re-focus on each side of the meridian, by pulling the mirror back a bit with clockwise turns of the primary focus knob - and then approach focus with ccw turns.

 

This will help place the mirror in a repeatable orientation relative to the baffle tube - and also help maintain collimation and reduce the effective "flop."

 

Did you re-focus in this manner before and after the meridian flip?  For imaging this can all happen automatically - and for visual work it's a fairly natural thing to do that doesn't take long.

 

Frank

I had a C11 with mirror flop and no amount of systemactic refocusing solved the issue.

 

The scope had to go back to Celestron. They fixed it for free and it worked fine from then on.

 

Bob 



#43 freestar8n

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Posted 27 August 2018 - 04:59 PM

I had a C11 with mirror flop and no amount of systemactic refocusing solved the issue.

 

The scope had to go back to Celestron. They fixed it for free and it worked fine from then on.

 

Bob 

Yes - it sounds like there are some with genuinely loose mirrors that need to be fixed.  But I think in other cases people may not be re-focusing in a manner to load against gravity.  And the OP is apparently collimating with the scope horizontal - which reduces the loading provided by gravity.

 

I'm just not sure how common this issue of loose mirrors is - because there are many of us who don't have the issue - and with multiple sct's.  And a key point is that even if it is the case, it isn't a flaw in the design - but an error in assembly that apparently can be fixed.

 

Frank



#44 freestar8n

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Posted 28 August 2018 - 07:47 AM

I just measured my "mirror flop" with EdgeHD11 and a Baader Vario Finder-Guider by centering a star with the EdgeHD and measuring the location in the guidescope - then flipping to the other side of the meridian and centering a different star on the other side - each about 30 degrees away from zenith.

 

The shift in the guidescope star location for me is about 2' - even without refocus.

 

This has nothing to do with the accuracy of the mount - because I am exactly centering the star in the EdgeHD - and then looking at the shift of the star position in the guidescope.  And it's about 2' - with a non-modified EdgeHD11 purchased from Astronomics.

 

I have EdgeHD11,  C11 and EdgeHD8 - and none of them have shown a dysfunctional flop on meridian flip.

 

So it may happen - if something is wrong in the scope.  But it is not inherent in a limitation of the focusing system.  

 

People seeing a large shift on meridian flip may be improperly measuring a scope when it is horizontal - or they may be affected by a scope that is improperly focused and not loaded against gravity.  Or they may be measuring a flop in the guidescope and its camera by itself.

 

In some cases they may actually be measuring a real problem in their mirror attachment.  But it isn't an inherent problem in the design that affects all sct's with primary mirror focus.  It works great for me - and others.

 

Frank



#45 ccwemyss

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Posted 28 August 2018 - 09:19 AM

Yes - it sounds like there are some with genuinely loose mirrors that need to be fixed.  But I think in other cases people may not be re-focusing in a manner to load against gravity.  And the OP is apparently collimating with the scope horizontal - which reduces the loading provided by gravity.

 

I'm just not sure how common this issue of loose mirrors is - because there are many of us who don't have the issue - and with multiple sct's.  And a key point is that even if it is the case, it isn't a flaw in the design - but an error in assembly that apparently can be fixed.

 

Frank

Just to clarify, I didn't intentionally collimate it on one side. Before I realized that the mirror was flopping, I collimated on a star where it was upright. But once the retaining ring was snugged up, collimation shifted to being best on one side. Since I was interested in establishing the magnitude of the effects of the flop, I left it there. Before I tightened the ring, I found that the flop was causing noticeable mis-collimation at other angles, but not as severe as with the side-to-side flip after tightening. In-focus bright stars were clearly not round, but not horrible. 

 

The other significant problem was that the flop shift was making it impossible to build a good model with the mount. Pointing was off by more than half a degree in some cases. When a goto mount can't even hit a target the size of the moon, it's pretty much useless. With a 6" f9 refractor on the same mount, pointing is within 3' and often within 1'. 

 

This OTA is 18 years old. If it has been this way since it was made, then one has to wonder about how it was used. The previous owner (who works on telescopes professionally) bought it from the original owner a couple of years ago, de-forked it, replaced the tube with carbon fiber (chipping the corrector in the process), and then sold it because he felt it was too big, and because his Celestron GEM mount had turned out to be defective, and he was tired of dealing with it. He assured me that there was no issue with mirror flop when he had it, and that the mirror seemed properly attached to the cell when he had it apart. But the fact that the mount wasn't working raises questions about whether he mis-diagnosed the problem.

 

I suspect there are a fair number of scopes that are gathering dust because their owners don't know how to diagnose the source of poor images in them. Those eventually get sold, with claims of being "like new" because they have "only been used a few times." In some cases, it may just be bad collimation, but others may have manufacturing defects that then get passed around until they are disposed of or someone takes the time to do a careful diagnosis, and then can't resell it in good faith.

 

One would think that Celestron would use better quality control on their flagship optics, but there are other threads (John H. had one in particular) where it seems that even with the C14 Edge-HD, the RTV application can sometimes be faulty.

 

I'm not set up to pull the mirror from its mount and properly remount it, so the only solution may be to send it to Celestron. But they won't give any kind of estimate (other than for cleaning and collimation) or assurances that it is repairable, until they inspect it, so it's not clear whether it's just throwing good money after bad to send it in, or if it would be better to part it out and try to recoup some of the loss without pawning the problem off on someone else. 

 

Regards,

 

Chip W. 



#46 ccwemyss

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Posted 28 August 2018 - 09:37 AM

I just measured my "mirror flop" with EdgeHD11 and a Baader Vario Finder-Guider by centering a star with the EdgeHD and measuring the location in the guidescope - then flipping to the other side of the meridian and centering a different star on the other side - each about 30 degrees away from zenith.

 

The shift in the guidescope star location for me is about 2' - even without refocus.

 

This has nothing to do with the accuracy of the mount - because I am exactly centering the star in the EdgeHD - and then looking at the shift of the star position in the guidescope.  And it's about 2' - with a non-modified EdgeHD11 purchased from Astronomics.

 

I have EdgeHD11,  C11 and EdgeHD8 - and none of them have shown a dysfunctional flop on meridian flip.

 

So it may happen - if something is wrong in the scope.  But it is not inherent in a limitation of the focusing system.  

 

People seeing a large shift on meridian flip may be improperly measuring a scope when it is horizontal - or they may be affected by a scope that is improperly focused and not loaded against gravity.  Or they may be measuring a flop in the guidescope and its camera by itself.

 

In some cases they may actually be measuring a real problem in their mirror attachment.  But it isn't an inherent problem in the design that affects all sct's with primary mirror focus.  It works great for me - and others.

 

Frank

Excellent! This is really useful data! Just what I was hoping to get when I started the thread.

 

I was seeing over 20' initially, got it down to about 4' with tightening the retaining ring, but then backed it off because it seemed to be pinching a bit, and now have about 7'. 

 

I decided to pull the scope and mount from the pier and put my Nexstar 11GPS in its place (which has been sitting indoors for about 18 months). Continuing my string of luck, the main board smoked the second night. So it looks like I will have to de-fork it, put a rail on it, and mount it on the G11. I'll do a similar set of measurements for comparison. It had no apparent image shift before, but an alt-az fork only tips 90 degrees on one axis, so it's not as stressful on the mirror cell.

 

Regards,

Chip W. 



#47 bobhen

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Posted 28 August 2018 - 12:41 PM

Yes - it sounds like there are some with genuinely loose mirrors that need to be fixed.  But I think in other cases people may not be re-focusing in a manner to load against gravity.  And the OP is apparently collimating with the scope horizontal - which reduces the loading provided by gravity.

 

I'm just not sure how common this issue of loose mirrors is - because there are many of us who don't have the issue - and with multiple sct's.  And a key point is that even if it is the case, it isn't a flaw in the design - but an error in assembly that apparently can be fixed.

 

Frank

Frank,

 

One way to tell the difference is that mirror shift occurs when focusing and does not require re-collimating while mirror flop occurs when the scope is on a GEM and moved from say low in the east to low in the west and will defiantly require re-collimating.

 

It is usually the larger SCTs that have this issue because the mirrors are heavier.

 

When S & T did a review of a C14 back in the day they also mentioned that for high power study that their sample required a collimation tweak.

 

It might not be a design flaw but a better design would probably eliminate the issue altogether, albeit at a cost increase.

 

It might not be common but it’s common enough that reports like this surface every once in a while and who knows how many just live with it or don’t diagnose the problem.

 

This is just a guess on my part but it could be that mirror flop was never an issue when all SCTs were fork-mounted. But, when people started using the larger SCTs on GEMs, which “in certain positions” might cause more stress on the mirror attachment, it became more of an issue.

 

Bob



#48 ccwemyss

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Posted 28 August 2018 - 07:04 PM

Frank,

 

.

.

.

This is just a guess on my part but it could be that mirror flop was never an issue when all SCTs were fork-mounted. But, when people started using the larger SCTs on GEMs, which “in certain positions” might cause more stress on the mirror attachment, it became more of an issue.

 

Bob

A wedge-mounted fork will put the mirror through the same range of orientations as a GEM. It's just an alt-az fork that will keep one axis consistent. Celestron didn't make an alt-az 14, like Meade, so the fork mounted C-14s would normally have been wedge mounted.

 

Manufacturing errors just happen. Especially in small quantity production situations, if somebody gets distracted and skips a step, it can get out the door.

 

My original request was for people to report measurements on their scopes, to see if we could get some data establishing a norm. It sounds like Frank's C11, at 2' is only slightly more than what other people have reported for focus shift, so I would expect it to retain usable collimation across the sky (except, perhaps, for high power studies, as you mention). 

 

Regards,

 

Chip W.



#49 freestar8n

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Posted 28 August 2018 - 09:35 PM

Hi. My focus shift with edghd11 is about 30-45” when you turn the knob clockwise and then forward. And I think that is a typical value.

The 2’ I report is the shift relative to a guidescope after doing merid flip with different stars about 40 degrees up.

Part of that 2’ could be in the tube or secondary and even some in the guidescope. Hard to tell.

Frank

#50 ccwemyss

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Posted 26 October 2018 - 09:52 AM

Just a bit of follow-up. I ended up sending the OTA to Celestron. They confirmed that the mirror was not attached to the supports, and removed the old RTV, re-attached the mirror with new RTV, rotated the optical components to get the best alignment, and collimated it. With the mirror removed, they cleaned and re-greased the slider, and cleaned all the optics. The cost was the same as the basic service. The shipping was actually more expensive than the service. The scope is on its way back, and I'll post new shift numbers once I have time and weather that allows me to check it. 

 

The repair experience was a bit mixed. On the one hand, I got to speak to the repair supervisor who was extremely helpful and knowledgable. But the email communication was a bit confused. I emailed a description of the problem and a list of other things I wanted checked. When it arrived, they sent me email saying that they would clean and collimate it and return it. After a few emails, I finally got to speak to the supervisor who said he would ensure that all of the issues would be addressed. After a couple of weeks, I got another email saying that my OTA had been cleaned, collimated, and was being shipped. So once again I called to be sure that they actually did what I asked. The receptionist couldn't reach the repair supervisor, so she scrambled to get my message to the repair department and called the shipment back from Fedex. The supervisor called me back a couple of hours later to assure me that it had been serviced as requested, and that they had confused it with another OTA in the email. Now they are shipping it again. Basically, when I was able to speak directly to real people, they were incredibly helpful and responsive. There just seems to be an issue with connecting email for service requests under ticket numbers with email related to receiving and shipping. 

 

But, to partly answer the original question I posted, the amount of shift I was seeing was indicative of a serious problem in the optics, and was outside of the norm. I have to wonder how long it has been this way. The previous owner was complaining that his mount was not working and needed to go in for service, but that could have been due to the impossibility of building a model with inconsistent star positions. It also makes me wonder how many of the people who have "fixed" their shift problems by tightening the retaining ring are actually dealing with separation of the RTV on one or more arms.

 

Chip W.




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