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Equipment Discussions >> Cats & Casses

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rigel123
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Mirror moving when focusing new
      #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 [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 [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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