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Mirror flop with C11HD, how bad

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#1 BWAZ

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Posted 06 September 2012 - 01:19 PM

I'm playing with the idea of acquiring a C11 HD, but have some concerns about the possible mirror flop issue. I owned a C11 last year but had to give up due to the excessive mirror flop exhibited in that sample, from which I kept losing the collimation once there was a meridian flip.

I wonder with the mirror lock how the new C11HD handles the mirror flop issue.

#2 Footbag

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Posted 06 September 2012 - 03:52 PM

I've never experienced mirror flop with my CPC-800. My new Edge HD 800 doesn't exhibit it either, but that is with the locks. I do believe that with the locks, you won't have a problem with it.

#3 BWAZ

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Posted 06 September 2012 - 06:31 PM

I owned two C8's and both didn't have much mirror shift/flop either. But a C11 has much heavier mirror and from my experience I suspect the mirror support system may not be up to the task.

#4 Dan Finnerty

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Posted 06 September 2012 - 07:00 PM

I get significant mirror flop on my C11 Edge, up to several arc minutes perhaps, though I've never measured it carefully. It was annoying at first (coming from a refractor), but I've learned to cope. I don't see any significant collimation shift, even at high powers, judging by the out of focus diffraction patterns.

One thing I have learned to look out for is to lock the mirror when doing mount alignments, as there can be enough mirror shift on opposite sides of the meridian to affect modeling precision. After completing modeling (I have a Losmandy C11/G2) I then unlock the mirror and focus/center for whatever target I'm viewing and if I need precise goto, I can sync on a star near the object to correct for mirror shift effects.

#5 EFT

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Posted 07 September 2012 - 10:44 AM

Having the mirror locks allows you to completely avoid mirror flop when you add a crayford focuser to the back of the scope. Simply lock the primary down and use the crayford from there on. It's tricky for AP work since you have to think about back focus issues, but for visual it's a piece of cake. In any circumstance, there should not be enough flop to mess up collimation. If it is that bad, then I would suspect that something else is wrong (e.g., the primary is loose in its holder).

#6 BWAZ

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Posted 07 September 2012 - 01:44 PM

Thanks for your responses!

Dan, a couple of arc minutes don't sound too bad. My C11 exhibited much more than that when used with a binoviewer, and in this case, the primary mirror moved farther towards the sky, possibly resulting in a larger gap between the main baffle and the primary holder and thus excessive lateral movement. Not only did I lose the collimation of the scope, but I couldn't build the pointing model when I carried out the star alignment for the Goto function of my mount. The flop was less severe when I use an EP w/ a 1.25" diagonal though. In the end, I lost my faith in the C11 and sold it at a great loss to someone who could cope with the issue.

Ed, that's my thought too. I would mainly use the C11HD for visual since I've got other scopes for AP. A Crayford focuser sounds like a solution, but I wonder if I could use my binoviewer with the focuser in place though.

#7 EFT

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Posted 07 September 2012 - 02:02 PM

Thanks for your responses!

Dan, a couple of arc minutes don't sound too bad. My C11 exhibited much more than that when used with a binoviewer, and in this case, the primary mirror moved farther towards the sky, possibly resulting in a larger gap between the main baffle and the primary holder and thus excessive lateral movement. Not only did I lose the collimation of the scope, but I couldn't build the pointing model when I carried out the star alignment for the Goto function of my mount. The flop was less severe when I use an EP w/ a 1.25" diagonal though. In the end, I lost my faith in the C11 and sold it at a great loss to someone who could cope with the issue.

Ed, that's my thought too. I would mainly use the C11HD for visual since I've got other scopes for AP. A Crayford focuser sounds like a solution, but I wonder if I could use my binoviewer with the focuser in place though.


I'm not sure on the binoviewer since I don't use one myself. You would need to check on the back focus distances.

In any case, it sounds like the C11 you had was particularly problematic. So far, I haven't heard of anyone with the C11HD that has a severe mirror flop problem.

#8 lightyear44

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Posted 08 September 2012 - 06:47 AM

I'll be posting an add for my C11 on a local site here in Ontario today. Tired of the flop. Having to stop imaging to re-centre my target every hour or two. I expect to get enough $ for it to purchase a Edge-8.
I'll miss the super long focal length for those small targets, but should be happy enough with an 8-inch. -David.

#9 Greg Boynton

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Posted 08 September 2012 - 04:55 PM

Are you sure it's mirror "flop" and not just a loose primary lock nut? The effect is similar but more severe and you loose collimation.

#10 Dan Finnerty

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Posted 08 September 2012 - 07:31 PM

This sounds like something I should know about for future reference. How do you check?

#11 Greg Boynton

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Posted 08 September 2012 - 09:36 PM

Check your collimation against a low altitude star and then flip the scope so that the opposite side of the tube is towards ground. Recheck collimation on the same star. If the primary lock nut is loose, the mirror will shift on it's carrier and collimation will be out.

#12 Dan Finnerty

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Posted 09 September 2012 - 10:35 AM

How do you tighten the loose lock nut? Do you have to disassemble the rear cell? Remove the corrector plate and go in through the front?

#13 BWAZ

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Posted 09 September 2012 - 11:40 AM

Are you sure it's mirror "flop" and not just a loose primary lock nut? The effect is similar but more severe and you loose collimation.


Though not the C11 I'm concerning about, I suspect my older C11 suffered from some loose part in the mirror support mechanism.

#14 Greg Boynton

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Posted 09 September 2012 - 12:01 PM

I forgot to mention, is there a huge amount of shift when you focus? If that's the case, then the problem is incorrectly machined parts in the baffle tube assembly. If focus shift is just a normal amount, then I would proceed with checking the primary lock nut.

I've posted on this subject before and given the magnitude of the damage to the views, I think it should be a sticky.

On a C11 you have to remove the corrector sorry to say. You will need a spanner tool to tighten the nut. The yahoo C14 group has a file that shows dimensions for making a spanner out of 3 inch pvc pipe and a couple 4-40 screws. You have to cut and drill a bit to make one, but it's not a big deal. I made one and it works on all the C14's and C11's I've tried. I just checked the dimensions of the pins on my homemade tool and they are approx 3.450 inches on center.

To use it all correctors must be removed with the exception of C14 fastar which has a large enough opening for the tool to work through. The pvc pipe fits over the central baffle and the cut off screws stick out as pins from the end. The pins fit into the corresponding holes in the lock nut and allow you to tighten it. The tool is safe and self guiding. No chance of hurting the primary.

I prefer to put the tool in with the ota horizontal and then turn it facing up. Gravity will then help you. I've found that it's useful to work the nut back and forth a bit. The goal is to get the primary to settle back into the factory position and properly compress the cork and fibre washer. You can make it quite snug (hand tight) without pinching the primary. It needs to be tight enough to hold the relatively heavy mirror precisely when orientation changes.

In theory, the cassegrain primary should be supported on it's center of gravity (by hardened silicone caulk in this case) and the tension of the nut shouldn't matter much. I think that maybe at F2 it's front heavy or maybe they didn't put in quite enough silicone.

For what it's worth since I've been aware of this problem, I've discovered it in several C11's and C14's.

#15 Dan Finnerty

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Posted 09 September 2012 - 10:45 PM

Greg,

that is really useful information. I'm filing it away for future reference in case I ever have problems.

#16 BKBrown

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Posted 10 September 2012 - 05:28 PM

I have never had a lick of mirror flop in my C11 Edge to the best of my recollection, and even the focus shift is minimal...I highly recommend an external focuser and using the mirror locks :grin:

Clear Skies,
Brian

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