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Is there a way to fix SCT Mirror shift

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#101 pjensen

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Posted 02 June 2014 - 12:44 PM

Had the C8 out last night and couldn't help but notice at least "a Saturn's worth" of mirror shift, possibly more...


Dumb question here. When does this mirror shift occur - when you reverse direction on the focus knob? Or anytime you focus?

I see only a small shift when reversing - about the width of a star.

Or am I missing something here?

#102 Ed Holland

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Posted 02 June 2014 - 02:00 PM

In the case of my scope, it is during focus reversal - not really a problem if one follows the rule of approaching best focus with only anti-clockwise rotation of the control.

Being picky, and obsessive compulsive, and obsessive compulsive, it would be nice to iron out this entirely trivial wrinkle.

Ed

#103 Herr Ointment

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Posted 02 June 2014 - 02:06 PM

The focus on my CPC 1100 HD changes after goto slews much to my amazement.

#104 Live_Steam_Mad

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Posted 02 June 2014 - 02:23 PM

The focus on my CPC 1100 HD changes after goto slews much to my amazement.


As far as I know that is usually due to mirror flop of the primary mirror on the primary's outer baffle tube carrier? It sounds like the primary mirror is slightly loose, like when e.g. on Meade SCT's / Mak's, the section to the rear of the mirror that the focusser arm screws into which screws on, is not screwed on quite tight enough, and there is a slight space between it and the large O ring and then the primary mirror and then the cork ring around the front of the primary and the metal ring at the front holding it onto the outer baffle tube. So the mirror can flop from left to right, as it were, in the tube as the tube goes to different orientations, and collimation / focus can shift as a result.

I can show you the idea of the mechanism in these pictures from when I took my Meade 7" Mak apart to remove the internal 3.5 Kg weight ;-

https://plus.google....976030517144...

Fortunately when I checked my 7" Meade's Mak's 8 1/4" primary it was firmly held and would not rotate or flop even slightly.

I think Celestron RTV glues their primaries onto the carrier though, where Meade does not, which may present you with problems?

If your primary flopping around slightly is the cause, you will need to have it repaired by Celestron if under warranty, or take the corrector off (noting the corrector position exactly), pull the metal C clip off the inner baffle tube, pull the primary out by the outer baffle tube, and tighten the assembly. Be careful not to rotate the primary or to tighten too much which could cause Astigmatism. The part at the back of the primary which the focusser arm screws into is held on to the end of the outer baffle tube by a screw thread and a couple of grub screws it seems. Make sure not to tighten them too much or you will crush the threads. Needs only just sufficient to hold it firmly with no movement.

I've seen this happen before on the forum, and that was the solution to the problem. If the mirror flops a lot, collimation will be affected and your planetary views at high power will be affected badly I would assume.

Regards,

Alistair G.

#105 pjensen

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Posted 02 June 2014 - 04:14 PM

The focus on my CPC 1100 HD changes after goto slews much to my amazement.


Are your mirror locks tightened down (just slightly snug)?

I will adjust the focus once in the evening, snug down the locks - and it stays in focus all evening through a dozen slews (all over the place).

#106 Herr Ointment

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Posted 02 June 2014 - 04:21 PM

Well thanks for that, Alistair! Someday I'll have to get after it as I will not ship the thing to California.

You're right, it does effect things unless I collimate at the target....hardly ideal. I can check the mirror for poor mounting through the vents and I'll do that tonight.

#107 Herr Ointment

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Posted 02 June 2014 - 04:25 PM

I don't use the locks, actually removed the sliding lock pins so I don't stress things in a moment of forgetfulness.

I'm a focus knob twiddler.

#108 T1R2

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Posted 02 June 2014 - 04:29 PM

also the higher magnification you use, the more shift will be seen, at low powers mine is hardly there.

#109 Ed Holland

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Posted 03 June 2014 - 09:46 AM

also the higher magnification you use, the more shift will be seen, at low powers mine is hardly there.


Thats right. The shift is constant at the image scale, but more noticable as you blow it up with shorter fl eyepieces.

I'm tempted to assess my scope's shift, then dismantle and measure the tolerances to see how much clearance between the tubes is responsible for what I see. From there I can then attempt a modification.

Quick Q for PowellAstro: with tube to tube play removed, I take it you have noticed no binding in the focuser action?That is to enquire, clearance is not required for things to operate properly?

#110 PowellAstro

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Posted 03 June 2014 - 03:11 PM

@ Ed

No problem at all. If I was using metal screws that may have been an issue. With the nylon, this is not a problem and I have had no binding. However, when I adjusted the screws, all I did was to remove the slop.

It only takes a few thousandths of an inch to produce image shift.

#111 Ed Holland

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Posted 03 June 2014 - 03:54 PM

Thank you PowellAstro :)

I've built/repaired/tinkered a fair share of mechanical items, and consider myself to have a good feel for this sort precision, so might give it a go.
I can also see that it only takes a miniscule off axis movement of the mirror to cause significant shift of an object a few arc-seconds in size. Not surprising that this shows up in mass produced parts - I doubt (but don't know) whether the tubes are machine finished to control ID & OD in production?

I'll investigate the screws :)

Ed

#112 PowellAstro

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Posted 03 June 2014 - 08:23 PM

I had units apart with .020 in them which is bad shift and I have had them with .006 in the front of the slider and .035 in the rear. They do not seem to be consistent. A split ring slider with two collars would correct this issue and would be easy for the manufacturer to implement.

#113 Ed Holland

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Posted 03 June 2014 - 09:38 PM

Thanks for sharing the numbers. I agree there are potentially effctive ways to combat the variations without incurring the expense of super precision components. Your split/collar idea is a good one.

#114 Howie Glatter

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Posted 04 June 2014 - 06:22 AM

"I agree there are potentially effctive ways to combat the variations without incurring the expense of super precision components."

I can not understand why the manufacturers do not simply implement closer tolerances on the cylindrical bearing surfaces of the baffle tube and the mirror bushing. If the surfaces are made cylindrical within 0.0004", and with a diameter difference no greater than 0.002, there would be no visible image shift when reversing focus direction. I think that CNC machines in good condition, watched over by good mechanics, can do it. It might not be quite as fast as what they're doing now, but it would be good for them in the long run. The proof is that at least two smaller volume manufacturers in the U.S. have done it.

#115 Starhawk

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Posted 04 June 2014 - 07:29 AM

It would be a one-way trip. If you improved the tolerances,you would have a point in manufacturing where new OTAs really were a new model and incompatible with old OTA parts because the new tolerance parts would be incompatible with the old parts. Of course, the time to do this would be when introducing the EDGE HD. It seems they still wanted to be able to service old tubes with new mirrors, though. Otherwise, there could be a reserved set of serial numbers for the new parts (for example, C8s with a letter "N" prefix have the narrow lolerance baffle tube and mirror carrier.

With that said. There is one SCT where every example I have used had no mirror flop: C5s.

-Rich

#116 Ed Holland

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Posted 04 June 2014 - 03:37 PM

It would be a one-way trip. If you improved the tolerances,you would have a point in manufacturing where new OTAs really were a new model and incompatible with old OTA parts because the new tolerance parts would be incompatible with the old parts. Of course, the time to do this would be when introducing the EDGE HD. It seems they still wanted to be able to service old tubes with new mirrors, though. Otherwise, there could be a reserved set of serial numbers for the new parts (for example, C8s with a letter "N" prefix have the narrow lolerance baffle tube and mirror carrier.

With that said. There is one SCT where every example I have used had no mirror flop: C5s.

-Rich



I don't see why compatibility with older parts would be a problem. Are there not enough differences between Edge scopes and regular SCTs? Even the C8 has had different rear cell castings over the years, amongst other things.

The issue, in this plain and simple "way of the world" situation is that the present design doesn't cause the manufacturer enough customer complaint issues to stimulate a change. That status quo remains, however much we debate the simplicity and necessity of a fix here on CN.

Cheers

Ed

#117 Ed Holland

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Posted 04 June 2014 - 03:51 PM


Oh and I'm now seriously considering mods to my C8, and possibly 127mm Mak, if the space allows. I think 6-32 set screws should do the trick, and be minimally invasive. Was thinking to use 6 of these: 3 point contacts spaced 120° around the tube at the top and also bottom ends. Probably over-engineered though.

Ed

#118 ColoHank

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Posted 04 June 2014 - 03:55 PM

The issue, in this plain and simple "way of the world" situation is that the present design doesn't cause the manufacturer enough customer complaint issues to stimulate a change. That status quo remains, however much we debate the simplicity and necessity of a fix here on CN.



But when dissatisfied customers spend their dollars elsewhere, telescope companies struggle and then end up on the auction block. It's the same sad story for some domestic automobile manufacturers (time and again, it seems). The question is, will they ever learn from their mistakes?

#119 jhayes_tucson

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Posted 04 June 2014 - 04:37 PM

Oh and I'm now seriously considering mods to my C8, and possibly 127mm Mak, if the space allows. I think 6-32 set screws should do the trick, and be minimally invasive. Was thinking to use 6 of these: 3 point contacts spaced 120° around the tube at the top and also bottom ends. Probably over-engineered though.

Ed


Ed,
Six screws will probably work, but I think that you only need four to make it work well. Four points are all that are needed to define the position and the side of the baffle tube can do some of the work. You don't want it to bind and less might be better in this case.
John

#120 PowellAstro

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Posted 04 June 2014 - 05:39 PM

@ Ed

Six screws will work fine, especially on the smaller scopes. It will also allow a small amount of adjustment to make sure the main mirror is aligned on the OTA optical axis. Just put the two sets on the top of the slider and the single set at the bottom. It also gives more contact area if you can find flat tip screws or file them and make the tips flat.

#121 TCW

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Posted 04 June 2014 - 06:04 PM

There have to be some amateurs out there with access to CNC machines that could build a precision set of mirror tubes. Certainly some of the after market vendors have the capability.

#122 Ed Holland

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Posted 04 June 2014 - 06:42 PM

Thanks both :)

McMaster has flat-tipped all nylon set screws, #6-32 x 1/8" long that should do the trick without undue intrusion into the OTA.

Ed

#123 jhayes_tucson

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Posted 04 June 2014 - 08:00 PM

Thanks both :)

McMaster has flat-tipped all nylon set screws, #6-32 x 1/8" long that should do the trick without undue intrusion into the OTA.

Ed


I will probably go with 10-32 on my C14 since it will engage the same number of threads and have more surface area (both around the threads and bearing on the tube) to spread the load.

John

#124 Ed Holland

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Posted 04 June 2014 - 08:19 PM


That makes sense, the C14 is a much bigger beastie!

#125 A6Q6

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Posted 05 June 2014 - 02:39 AM

I have a Celestron 11" Edge HD. The mirror shift is horrible even for small changes. Because of this I installed a moonlite focuser. Nonetheless, it bothers me and wondering is there a method to fix this for a DIY?

I would feel the same way if I bought a new Celestron Edge HD of any size. I have seen mirror shift in 3.5" Questars, which I feel is Inexcusable for the Questar Company. I don't remember if I noticed it in a Questar 7. My Quantum 6 has none even after 35+yrs and much to my surprise my new to me pawn shop Classic orange tube C5 has none. I still would like to get a C11 or C14 but I don't think I could live with Mirror Shift.


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