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Mirror moving when focusing

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#26 stanislas-jean

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Posted 30 November 2012 - 01:48 AM

All the compact scopes has focusing problems.
2µm of thermal expansion on the metal tube involves 40µm at focus point. The degradation is sensitive.
Regarding these views the steel tube is better than aluminium, the resin sensibly better and carbon well selected also better. This contribute to anneal this movement.
The problem is that the optical glass also are not so stable and contributes to a focus point move, sensitive on some nights of great temperature gradient. After the menusque too with a temperature difference between outersurface colder than the inner.
If you push the analysis deep you can find source of troubles, one can be the air currents inside the tube that cause aberration and focus point movements.
If you quantify each and combine them in some scenario, you will conclude to avoid the SCT use and you will built something completely different than a sct.
After you will find 2 schools, open tube the klevtov, the closed tube a newtonian with a flat closure glass. The cost will be quite different with stable glasses, 0 expansion, material neutral as carbon or resin, perfect alignment and simple to do, almost no focus point parasitic movements.
Returning back on sct the great improvement will be a resin or carbon tube, thermally expansion un-existant, better thermal exchanges with outer air. For the rest all other troubles will exist but a step gain. The fixation of the main mirror is a must, so no shifting in any case, also.
Stanislas-Jean.

#27 Sarkikos

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Posted 30 November 2012 - 08:43 AM

Norme et al,

Yea, Rick, that's true. Over the years we've all seen various levels of mirror shift. And it's not surprising, really, when you move the mirror something is gonna shift.

I consider myself fortunate my current Mak can focus on stars at upwards of 100x per inch and remain in the FOV. Easily. I doubt the star shifts more than the diameter of it's first ring, about 2.4" arc. That may change as it get's used, but for now no worrying about approaching focus from either direction.

Though, slewing across the meridian does require a slight retouch. Like you, I can easily live with that. Would probably have refocused, anyway.


I want to emphasize a distinction here between "mirror shift" - an apparent sideways movement of the image when focusing - and what I call "focus slip." "Focus slip" occurs when you have finished focusing the telescope and then the image slips out of focus. A sideways "mirror shift" of the image may or may not occur at the same time as the "focus slip." These are two different problems.

My Bosma 150mm Mak has never shown either mirror shift or focus slip. It comes to a focus with no lateral shifting of the image while I'm turning the focus knob and with no defocusing after I've finished turning the focus knob. Or at least I've never noticed either of these conditions. So if they are present, it is very minor.

On the other hand, my 90mm Mak always shows some mirror shift when I'm focusing. But I've only noticed focus slip recently when I've taken the little Mak out during near freezing temps.

The mirror shift really doesn't bother me. It's the focus slip that's a PITA. When I'm trying to catch various lunar features, I don't want to have to keep fiddling with the focus. If there's some way to correct this problem by taking the 90mm apart and maybe regreasing it or doing something else, I'd be interested. Otherwise, I might only take it out when the temps are well above freezing.

Mike

#28 Asbytec

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Posted 30 November 2012 - 09:06 AM

Mike, yea, the difference is noted. Mine has a tiny bit of either. But, if your 90 Mak is "slipping" losing focus simply slewing around the moon, that's not normal. I believe this focus slip is caused by the primary feeling the force of gravity and flopping a bit. It's probably tilting a tiny bit against the primary baffle as the scope changes it's orientation. Mirror shift is the primary being forced out of true by the focuser knob itself - during focus. Ideally, the primary should probably rest on the focuser knob and "lean" against the baffle.

For either, has anyone ever tried a very thin shim between the primary and the baffle? Just tighten up that play by a few hundredths(?) of a inch, just enough slide easily and not to bind or become tight. Not sure what you could use, but surely something very thin could be slipped into the gap and reduce the slop between the primary mirror and the baffle. Re-grease it.

#29 Sarkikos

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Posted 30 November 2012 - 11:15 AM

Norme,

Mike, yea, the difference is noted. Mine has a tiny bit of either. But, if your 90 Mak is "slipping" losing focus simply slewing around the moon, that's not normal. I believe this focus slip is caused by the primary feeling the force of gravity and flopping a bit. It's probably tilting a tiny bit against the primary baffle as the scope changes it's orientation. Mirror shift is the primary being forced out of true by the focuser knob itself - during focus. Ideally, the primary should probably rest on the focuser knob and "lean" against the baffle.


Another finer qualification of what I mean by "focus slip"... In the case of my 90mm Mak, it will slip out of focus when I am not slewing or moving the OTA at all. All I have to do is point at the ecliptic, keep the OTA stationary, and focus the scope. It will spontaneously begin to defocus. As I said, I have only seen this happen when the temps are around freezing.

Maybe I should put antifreeze in the grease?

:grin:
Mike

#30 Asbytec

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Posted 30 November 2012 - 12:39 PM

Okay, well, heck. Try it? (Na! Don't...) Not sure what would cause that. Maybe the grease is very thick and when you draw back the focuser knob it kind of holds onto the primary? The mirror is slow to settle back onto the focuser because the grease is thicker and the mirror sticks to it? Just guessing.

#31 Sarkikos

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Posted 30 November 2012 - 01:03 PM

Yes, it does seem as if something like that is happening. Eddgie mentioned that Celestron had problems with focus slip due to contraction in metal OTAs at low temps. Is that the problem or the grease or both or ? It makes sense to me that a change in viscosity of the grease due to low temps could very well cause the focusing mechanism or the mirror to slip when held toward vertical.

I've never taken apart an SCT or Mak, so they are pretty much a black box to me, besides a basic understanding of the optics involved. I break down my Newts all the time to clean them or do some upgrades. But open up my Maks? I'm very reluctant to do that.

Mike

#32 Asbytec

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Posted 30 November 2012 - 10:04 PM

Well, certainly a contracting tube would reduce the mirror spacing and change focus. But, it seems like it would do so very slowly at first, then stabilize as the temps changed more slowly. I just can't see it happening each time you pointed the scope toward the Zenith. That sounds like a gravity problem.

I wonder if the focuser actually pulls the mirror back, at least on one side. I had my Mak torn apart and forgot to study at the focusing mechanism. It seems it should push and pull the mirror, it probably does. In fact, thinking about it...it has to as the focuser itself does not move in or out. So, maybe the off-focuser side is sticking a bit.

#33 Sarkikos

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Posted 01 December 2012 - 09:58 AM

The OP was talking about the exact same problem with his C8 as I have with my 90mm Mak: focus slip after setting the focus. Eddgie also mentioned that some Celestron Cats were having this problem: focus slip after setting the focus. It seems that it's not all that unusual in Cat World.

Eddgie said it would be "easier to remove the focuser and just pull the drive screw in and out a dozen times." I don't know about that. I'm adverse to removing anything involved with the internals of a Mak.

Does anyone know some solution besides exercising the focus knob or only setting focus CCW? They seem to help somewhat in my case, but didn't seem to completely solve the problem. Maybe I will need to take the Mak apart. But then do what?

Mike

#34 Asbytec

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Posted 01 December 2012 - 10:28 AM

Well, actually he described both problems, the image jumping all over the field during focus and drifting out of focus.

I dunno, if you cannot redistribute the grease, well nothing will make the gap between the primary and the baffle get any tighter. Maybe a shim for the worst cases, if one can be inserted without binding the primary during focus.

Maybe take it apart and look at it. Figure out what's going on. Mark your mirror and corrector orientations. If the Mak meniscus comes off while still in it's cell, that's cool. Mine did and I never had to worry about loosing or misplacing shims.

#35 Sarkikos

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Posted 01 December 2012 - 11:56 AM

I don't like the idea of sticking a shim anywhere inside the OTA. I'd be concerned that the shim would come loose and foul the optics.

Maybe a different type of grease is what's needed? Many Chinese manufacturers apparently get it wrong when it comes to picking the best lubricant for many types of telescopes. I've used Diff Oil Pure Silicon #3000 for refractor focusers to improve the motion. Perhaps that would work for Cats?

Mike

#36 Sarkikos

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Posted 01 December 2012 - 11:59 AM

I think I'll follow Orion61's suggestion to PM him for more options.

Mike

#37 gcfboulder

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Posted 02 December 2012 - 01:29 PM

Anyone out there wanting the ability to do manual fine- focus with a Crayford-style focuser on their SCT scope, I have a JMI event focuser for sale. See my CN ad under focusers.






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