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Equipment Discussions >> Cats & Casses

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rigel123
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Mirror moving when focusing
      #5533542 - 11/22/12 08:47 AM Attachment (36 downloads)

I was with my brother getting him back out under the stars with his C8 and his Questar to get some views of Jupiter and some of our favorite DSO's. still amazed at the views that little Questar can give!

When using the C8 it was really frustrating trying to focus as it appeared as if the mirror was shifting (yes, I know the primary moves to focus this Cat) so that the object would just jump all over the FOV, and when you did get it focused it had a tendency to quickly drift out of focus. I assume something in the focuser is either loose or binding somewhere and wondered if this is something that is a DIY or should he send it somewhere for repair?

Any thoughts? Here we are ready to go!

Edit: stupid IPad keeps flipping the image!

Edited by rigel123 (11/22/12 09:56 AM)


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Eddgie
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Re: Mirror moving when focusing new [Re: rigel123]
      #5533582 - 11/22/12 09:23 AM

The mirror carrier is not tight on the baffle. It slides on a thin film of grease. Over time this film can thin.

The movement you see after you stop focusing is the mirror xettling back against the baffle.

There are two things you can do. The first is to attempt to restore the grease film by running the focuser through its full travel several times. This will redistribute the grease and may improve the situatiion. It is easier to remove the focuser and just pull the drive screw in and out a dozen times, but for now you may not want to do thia and just turn the focuser a couple of hundred times.

Next, the best way to focus an sct is to approach final focus using only counter clockwise movements of the focuser knob (and it is the best way to focus anyway to reduce chasing best focus by using visual accomodation). Go clockwise smoothly jost past best focus, then use tiny ccw turns pausing each timee to let tbe image settle. If you turn the knob and don't see an improvement, do not back up. Your eye will accomodate even if you go slightly past focus if you approach using ccw motion.

But a ccw approach also "lifts" the mirrur into the final position. If you approach using cw turns, you are pushing the mirror down but when you stop, it still has som room to settle against the grease film on the baffle

I never have problems gettingperfect focus onone pass using this method an mirror movement is never an issue.

Edited by Eddgie (11/22/12 09:24 AM)


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cavefrog
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Re: Mirror moving when focusing new [Re: rigel123]
      #5533593 - 11/22/12 09:30 AM

I think maybe you just need a little bit of technique. Something I think Eddgie always says is to make your final focus with a counterclockwise turn? this puts the mirror in a position that is agreeable with gravity. One may try the opposite direction too. it may depend on which side of the meridian they are on. another tip is to even out the grease on the baffle by removing the screws holding the focuser knob assembly, AFTER running the mirror all the way forward,(closest to the corrector) and pull the mirror all the way to the back of the OTA, and the forward again. run it back and forth like this to even out the grease. do this about a dozen times and then secure the assembly back on the OTA. but don't tighten it a lot. I know that with a Meade, tightening these screws sets the stiffness that the knob will work at.

Theo


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cavefrog
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Re: Mirror moving when focusing new [Re: cavefrog]
      #5533597 - 11/22/12 09:31 AM

Ha!, Eddgie! ya beat me to it!!!

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rigel123
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Re: Mirror moving when focusing new [Re: Eddgie]
      #5533632 - 11/22/12 09:55 AM

Thanks guys, that makes sense as this baby has sat unused for years. I typically use the ccw focusing on my LS6, I just had never seen so much mirror shift in my life! I'll go crank that focuser back and forth a number of times and see how that works!

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David PavlichAdministrator
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Re: Mirror moving when focusing new [Re: rigel123]
      #5533682 - 11/22/12 10:21 AM

On the "spend more money" side of the equation, I've had several SCs that I've installed a Feathertouch Microfocuser in place of the stock unit. That also tightened things up. But the best is what the guys already suggested.

Or, you can add a Crayford style focuser. You achieve focus in the way prescribed, then leave the stock focuser alone and use the Crayford the rest of the night.

David


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mayidunk
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Re: Mirror moving when focusing new [Re: David Pavlich]
      #5533736 - 11/22/12 10:55 AM

Forget spending money, use technique.

Clockwise until you've passed through the focus point, then counter clockwise back into focus. Doing it like this first pulls the mirror down past the focus point, and then pushes it back up into the focus point against the pull of gravity. Since there's no room for gravity to pull the mirror, it stays put with no more drifting out of focus.

Easy, peasy!


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Sarkikos
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Re: Mirror moving when focusing new [Re: mayidunk]
      #5533837 - 11/22/12 12:07 PM

I experienced similar focus slip with my 90mm Mak last night. This morning I set the Mak up inside and focused on the louvers in a roof exhaust. I did not see the focus slip, maybe because the Mak was pointed more-or-less horizontally?

So then I focused on the louvers and immediately pointed the scope to zenith for a few minutes. When I brought it down to sight on the louvers again, they were still in focus. No focus slip.

Maybe the near freezing temps last night had some effect on my scope's ability to retain focus? Perhaps the low temps affected the grease in a way to produce focus slip? I've never experienced focus slip in the 90mm Mak during the warm months.

This is the first time I've seen focus slip at all in this scope. Up until now, the focus has been smooth and holds tight. The only focus problems I had before was trying to keep up with bad seeing.

At any rate, I've exercised the focus through the full range a number of times this morning. Hopefully that will distribute the grease more evenly and help during low temps also.

By the way, I tried different focusing techniques last night, and they all resulted in focus slip.

I have read that the final focus in SCTs should be CCW, but for Maks it should be CW.

Mike


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Sarkikos
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Re: Mirror moving when focusing new [Re: Sarkikos]
      #5533855 - 11/22/12 12:19 PM

FWIW, the focus knob on my 90mm Mak goes through a maximum of 20 3/4 turns.

Mike


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Eddgie
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Re: Mirror moving when focusing new [Re: Eddgie]
      #5534027 - 11/22/12 02:18 PM

And my note was typed on a freaking tiny miserable little tablet keyboard. Really, tablets are going to replace keyboards? Yikes!!!

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Sarkikos
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Re: Mirror moving when focusing new [Re: Eddgie]
      #5534178 - 11/22/12 04:23 PM

Eddgie,

Any thoughts on CW for Maks vs CCW for SCTs? Or the possible effect the cold might have on focus slip in Cats?

Mike


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orion61

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Re: Mirror moving when focusing new [Re: Sarkikos]
      #5534411 - 11/22/12 08:24 PM

You can try focusing the mirror up and back a few times if the grease has shifted.
You can get rid of most of the image shift but requires taking the scope apart. I have found Meades have a bigger problem with this than Celestrons, perhaps because the mirror is a bit heavier.
PM me for more options..


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Sarkikos
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Re: Mirror moving when focusing new [Re: orion61]
      #5534705 - 11/23/12 01:11 AM

Any thoughts on whether or not cold conditions can produce focus slip? (That's what I'm calling it. I don't like the term "image shift" in this context. I take "image shift" to mean the common sideways shift of the image while focusing, even if final focus is rock steady.)

In contrast to my 90mm Mak, my 150mm Mak exhibits no image shift while focusing and no focus slip.

Mike


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rigel123
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Re: Mirror moving when focusing new [Re: Sarkikos]
      #5534968 - 11/23/12 07:53 AM

I was surprised since I don't experience this "shift" at all with my LS6. Could be newer, lighter scope though.

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Mirzam
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Re: Mirror moving when focusing new [Re: rigel123]
      #5534992 - 11/23/12 08:18 AM

Of course if the mirror is shifting that much it also raises the question whether you are holding collimation.

JimC


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rigel123
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Re: Mirror moving when focusing new [Re: Mirzam]
      #5535199 - 11/23/12 10:26 AM

Quote:

Of course if the mirror is shifting that much it also raises the question whether you are holding collimation.

JimC



Good question, when I did get it stable the star test was OK, but could use some work.


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Sarkikos
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Re: Mirror moving when focusing new [Re: rigel123]
      #5535265 - 11/23/12 11:06 AM

Quote:

I was surprised since I don't experience this "shift" at all with my LS6. Could be newer, lighter scope though.




Which "shift" are you talking about? The sideways shift while you're focusing or the focus slipping after you've focused? Let's don't be ambiguous.


Mike


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rigel123
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Re: Mirror moving when focusing new [Re: Sarkikos]
      #5536427 - 11/23/12 11:06 PM

Quote:

Quote:

I was surprised since I don't experience this "shift" at all with my LS6. Could be newer, lighter scope though.




Which "shift" are you talking about? The sideways shift while you're focusing or the focus slipping after you've focused? Let's don't be ambiguous.


Mike




I don't see either, no sideways motion when focusing and no drift after getting the object in focus.


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Eddgie
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Re: Mirror moving when focusing new [Re: Sarkikos]
      #5536454 - 11/23/12 11:34 PM

In MCTs with moving mirror, approach using CCW too.

Refractors should approach using inward focuser tube travel.

In cold weather, if you approach using clockwise which is not what I recommend, settling is longer because the grease film has more viscosity so gravity takes longer to squish out the grease.

Again, CCW is not only the best way to prevent mirror settling, It is the best direction to spproach focus period. And for refractor users, it usually best to approach finsl focus using inward movement of the focuser tube. Approching this way keeps you from chasing best focus.

Edited by Eddgie (11/25/12 02:42 PM)


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Sarkikos
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Re: Mirror moving when focusing new [Re: Eddgie]
      #5539971 - 11/26/12 09:15 AM

I took my 90mm Mak and Baader Zoom out Saturday night for about an hour to look at the Moon, and to check on the focus slip. Earlier in the day, I had turned the focuser knob all the way CW and CCW several times, finally setting it about in the middle of the range after turning CCW. First thing that night, when focusing on the Moon, I needed to turn the knob CCW, maybe about halfway between mid-range and maximum CCW. The focus held steady.

Well, it held steady for about the first half-hour. I was using the Baader Zoom, which requires a little tweak on the focus between settings. As I started to dial up and down the focal-length settings, and so had to tweak the focus a little, the focus started to slip again. I still think the cold has something to do with this - it was around freezing - because I've never experienced focus slip in the warm months.

I found, though, that if I move the knob several turns CW and then turn CCW to focus, the focus will hold steady. But that is a PITA to have to prime the focus by turning CW everytime I want to focus the Mak. As I said, I don't recall ever experiencing this during the warm months.

Would it be worthwhile to take the Mak apart and do something - regrease the mechanism? - to prevent this focus slip from happening again?

I'm not talking about image shift. I'm talking about focus slip. Those terms make more sense to me and are much less ambiguous than others I've heard.

Mike


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Eddgie
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Re: Mirror moving when focusing new [Re: Sarkikos]
      #5540001 - 11/26/12 09:52 AM

Is it possible that focus is changing due to thermal contraction of the OTA?

And approaching CCW is not just the way to deal with the problem, it is actually the best way to focus regardless of you are having this problem or not. Approaching from CCW allows you to most easily get the best focus possible because it allows your eye to accomdate the view if you overshoot slightly.

If your focus is changing though, and the image is not moving, perhaps you are having some other problem that has nothing to do with the mirror movement.

SCTs and MCTs use a very powerful negative mirror for the secondary. Even a very tiny change in spacing causes the focus to shift.

Try putting the scope out for a few hours before observing but if the temp is falling fast, it could be that if this is the problem (tube contracting) you may not resolve it.

This was the reason Celestron shipped Carbon fiber tubes for a while. As I recall, they said that the Carbon Fiber would not expand or contract the way the metal OTAs did so that for long period images, the focus would not change.

Could be your problem??


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Sarkikos
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Re: Mirror moving when focusing new [Re: Eddgie]
      #5540081 - 11/26/12 10:55 AM

Quote:

Is it possible that focus is changing due to thermal contraction of the OTA?




I'm talking about the focus not holding steady immediately after I set the focus. The focus slips as I watch. That's exactly what it does. And when it starts slipping, it tends to repeat that performance whenever I refocus. Could this be due to thermal contraction of the OTA? It's not a gradual change in focus, but a sudden slip out of focus that is repeatable.

Quote:

If your focus is changing though, and the image is not moving, perhaps you are having some other problem that has nothing to do with the mirror movement.




Why couldn't it be due to mirror movement? I do see a lateral shift in the image when I go from far-out-of-focus to a focused image. That's the nature of this little beast (in contrast to my 150mm Mak, which shows no lateral image shift at all when I turn the focus knob).

But that is not what I'm talking about. What I'm talking about is a spontaneous defocusing of the image. If I watch this long enough, there may be some lateral shifting of the image, also. But I don't let it progress that far before I try to refocus the image to sharpness again.

Quote:

Try putting the scope out for a few hours before observing but if the temp is falling fast, it could be that if this is the problem (tube contracting) you may not resolve it.




I let the Mak acclimate on my porch for about an hour the first night I noticed this focus slip. That ought to be good enough for a 90mm Mak. This last Saturday night, I took the scope directly outside from my warm house. As I said, Saturday night I did not see any focus slip until I had been outside for about a half-hour. Again, that indicates this problem may be at least partly temperature related.

Quote:

This was the reason Celestron shipped Carbon fiber tubes for a while. As I recall, they said that the Carbon Fiber would not expand or contract the way the metal OTAs did so that for long period images, the focus would not change.

Could be your problem??




You got me. I have no idea. This is definitely a metal tube. No carbon fiber anything in my house. But to make myself clearly understood again, this does not involve "long period images." Here is what I see: I focus the image, and almost immediately the image slips out of focus (there is no obvious lateral shift of the image), and I need to turn the focus knob again to refocus the image. Sounds like the problem the OP had, in that the image in my scope slips out of focus, also.

Mike


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Sarkikos
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Re: Mirror moving when focusing new [Re: Sarkikos]
      #5546374 - 11/29/12 10:28 PM

A friend of mine told me today that his SCT also shows focus slip sometimes. He has a CPC 1100.

So focus slip is not an uncommon problem among Maks and SCTs? This is something I have never experienced with Newts and refractors.

Mike


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Rick Woods
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Re: Mirror moving when focusing new [Re: rigel123]
      #5546535 - 11/30/12 12:50 AM

Quote:

Quote:

Quote:

I was surprised since I don't experience this "shift" at all with my LS6. Could be newer, lighter scope though.




Which "shift" are you talking about? The sideways shift while you're focusing or the focus slipping after you've focused? Let's don't be ambiguous.


Mike




I don't see either, no sideways motion when focusing and no drift after getting the object in focus.




Some scopes are just better mechanically than the rest. My M14 has almost zero image shift when focusing (under 10 arcsec), and no shift after focusing.
It does need a little refocusing when I slew, though. I'll settle for that without complaint.


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Asbytec
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Re: Mirror moving when focusing new [Re: Rick Woods]
      #5546559 - 11/30/12 01:22 AM

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.


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stanislas-jean
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Re: Mirror moving when focusing new [Re: Asbytec]
      #5546579 - 11/30/12 01:48 AM

All the compact scopes has focusing problems.
2m of thermal expansion on the metal tube involves 40m 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.


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Sarkikos
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Re: Mirror moving when focusing new [Re: Asbytec]
      #5546801 - 11/30/12 08:43 AM

Norme et al,

Quote:

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


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Asbytec
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Re: Mirror moving when focusing new [Re: Sarkikos]
      #5546829 - 11/30/12 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.


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Sarkikos
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Re: Mirror moving when focusing new [Re: Asbytec]
      #5546989 - 11/30/12 11:15 AM

Norme,

Quote:

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?


Mike


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Asbytec
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Re: Mirror moving when focusing new [Re: Sarkikos]
      #5547130 - 11/30/12 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.

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Sarkikos
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Re: Mirror moving when focusing new [Re: Asbytec]
      #5547184 - 11/30/12 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


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Asbytec
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Re: Mirror moving when focusing new [Re: Sarkikos]
      #5547953 - 11/30/12 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.


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Sarkikos
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Re: Mirror moving when focusing new [Re: Asbytec]
      #5548503 - 12/01/12 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


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Asbytec
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Reged: 08/08/07

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Re: Mirror moving when focusing new [Re: Sarkikos]
      #5548543 - 12/01/12 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.


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Sarkikos
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Re: Mirror moving when focusing new [Re: Asbytec]
      #5548657 - 12/01/12 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


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Sarkikos
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Re: Mirror moving when focusing new [Re: Sarkikos]
      #5548660 - 12/01/12 11:59 AM

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

Mike


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gcfboulder
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Reged: 09/27/07

Loc: CO
Re: Mirror moving when focusing new [Re: rigel123]
      #5550346 - 12/02/12 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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