Jump to content


Photo

Mirror moving when focusing

  • Please log in to reply
36 replies to this topic

#1 rigel123

rigel123

    Voyager 1

  • *****
  • Posts: 10891
  • Joined: 29 Jun 2009
  • Loc: SW Ohio

Posted 22 November 2012 - 08:47 AM

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!

Attached Files



#2 Eddgie

Eddgie

    Voyager 1

  • *****
  • Posts: 12675
  • Joined: 01 Feb 2006

Posted 22 November 2012 - 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.

#3 cavefrog

cavefrog

    Surveyor 1

  • *****
  • Posts: 1959
  • Joined: 11 Nov 2008
  • Loc: loozyanna

Posted 22 November 2012 - 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

#4 cavefrog

cavefrog

    Surveyor 1

  • *****
  • Posts: 1959
  • Joined: 11 Nov 2008
  • Loc: loozyanna

Posted 22 November 2012 - 09:31 AM

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

#5 rigel123

rigel123

    Voyager 1

  • *****
  • Posts: 10891
  • Joined: 29 Jun 2009
  • Loc: SW Ohio

Posted 22 November 2012 - 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!

#6 David Pavlich

David Pavlich

    Transmographied

  • *****
  • Administrators
  • Posts: 27509
  • Joined: 18 May 2005
  • Loc: Mandeville, LA USA

Posted 22 November 2012 - 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

#7 mayidunk

mayidunk

    Don't Ask...

  • *****
  • Moderators
  • Posts: 3990
  • Joined: 17 Feb 2010
  • Loc: Betwixt & Between...

Posted 22 November 2012 - 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!

#8 Sarkikos

Sarkikos

    Hubble

  • *****
  • Posts: 16708
  • Joined: 18 Dec 2007
  • Loc: Per sylvam ad astra

Posted 22 November 2012 - 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. :grin:

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. :shrug:

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

#9 Sarkikos

Sarkikos

    Hubble

  • *****
  • Posts: 16708
  • Joined: 18 Dec 2007
  • Loc: Per sylvam ad astra

Posted 22 November 2012 - 12:19 PM

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

Mike

#10 Eddgie

Eddgie

    Voyager 1

  • *****
  • Posts: 12675
  • Joined: 01 Feb 2006

Posted 22 November 2012 - 02:18 PM

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

#11 Sarkikos

Sarkikos

    Hubble

  • *****
  • Posts: 16708
  • Joined: 18 Dec 2007
  • Loc: Per sylvam ad astra

Posted 22 November 2012 - 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

#12 orion61

orion61

    Aurora

  • *****
  • Posts: 4597
  • Joined: 20 Oct 2007
  • Loc: Birthplace James T Kirk

Posted 22 November 2012 - 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..

#13 Sarkikos

Sarkikos

    Hubble

  • *****
  • Posts: 16708
  • Joined: 18 Dec 2007
  • Loc: Per sylvam ad astra

Posted 23 November 2012 - 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

#14 rigel123

rigel123

    Voyager 1

  • *****
  • Posts: 10891
  • Joined: 29 Jun 2009
  • Loc: SW Ohio

Posted 23 November 2012 - 07:53 AM

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

#15 Mirzam

Mirzam

    Skylab

  • *****
  • Posts: 4447
  • Joined: 01 Apr 2008
  • Loc: Lovettsville, VA

Posted 23 November 2012 - 08:18 AM

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

JimC

#16 rigel123

rigel123

    Voyager 1

  • *****
  • Posts: 10891
  • Joined: 29 Jun 2009
  • Loc: SW Ohio

Posted 23 November 2012 - 10:26 AM

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.

#17 Sarkikos

Sarkikos

    Hubble

  • *****
  • Posts: 16708
  • Joined: 18 Dec 2007
  • Loc: Per sylvam ad astra

Posted 23 November 2012 - 11:06 AM

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.

:grin:
Mike

#18 rigel123

rigel123

    Voyager 1

  • *****
  • Posts: 10891
  • Joined: 29 Jun 2009
  • Loc: SW Ohio

Posted 23 November 2012 - 11:06 PM

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.

:grin:
Mike


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

#19 Eddgie

Eddgie

    Voyager 1

  • *****
  • Posts: 12675
  • Joined: 01 Feb 2006

Posted 23 November 2012 - 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.

#20 Sarkikos

Sarkikos

    Hubble

  • *****
  • Posts: 16708
  • Joined: 18 Dec 2007
  • Loc: Per sylvam ad astra

Posted 26 November 2012 - 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

#21 Eddgie

Eddgie

    Voyager 1

  • *****
  • Posts: 12675
  • Joined: 01 Feb 2006

Posted 26 November 2012 - 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??

#22 Sarkikos

Sarkikos

    Hubble

  • *****
  • Posts: 16708
  • Joined: 18 Dec 2007
  • Loc: Per sylvam ad astra

Posted 26 November 2012 - 10:55 AM

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.

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.

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.

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. :shrug: 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

#23 Sarkikos

Sarkikos

    Hubble

  • *****
  • Posts: 16708
  • Joined: 18 Dec 2007
  • Loc: Per sylvam ad astra

Posted 29 November 2012 - 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

#24 Rick Woods

Rick Woods

    Voyager 1

  • *****
  • Posts: 14590
  • Joined: 27 Jan 2005
  • Loc: Inner Solar System

Posted 30 November 2012 - 12:50 AM

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.

:grin:
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.

#25 Asbytec

Asbytec

    Guy in a furry hat

  • *****
  • Posts: 8203
  • Joined: 08 Aug 2007
  • Loc: La Union, PI

Posted 30 November 2012 - 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.






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics