Image slightly shifts while rotating focus

Hi everyone. I have really been enjoying my new CGEM 925 HD. I am still amazed. Tonight I think that I went thru all the doubles in the menu that were visible for tonight. Truly amazing.

The fourth night (tonight) posed a new problem (at least for me). I noticed for the very first time, that when I rock the focus back and forth that the image is shifting. It actually seemed to have started about 15 min into my observing. When I got to M13 is when I noticed it. As the night went on about 2 hours later, then it became less or intermittent. When I started the Double viewing, it seems not as bad or barely noticable. Loosening the Mirror lock knobs about 7 turns out seemed to have made "some" better, but not like the first 3 nights out. The image was rock steady in the middle while I adjusted focus in and out. Nothing in the EP moved while I was doing this. Now, tonight !
The focuser is what seemed to be at the root of it. I could take my finger and apply a very slight pressure either toward the scope back or side to side and I could see the image moving in the FOV.
This all started when I could not get my Diagonal to move so that I could adjust the position of the EP. As a matter of fact, I had a small panic in the fact that I noticed that while I was turning the threaded assembly (sorry can't remember the name) , that I was also turning the huge ring that is right up against the OTA. Somehow my smaller ring was stuck and I had to go ahead and remove the larger ring and take the assemblies into the house quickly to remove the EP. I really do not like the smaller ring for the diagonal, it is very sticky and it does not take much to get it stuck. So, it was at this point, I guess is when I started to notice things were not going well with the focusing. I do not see how in the world removing both visual backs (larger and smaller rings) would cause a focus problem. I think it was a matter of coincidence.
It could be that some of the "newness" of the mechanical parts is loosening up and might be causing this. I see that there are some screws that hold a plate onto the back OTA which holds the focuser assembly on. Could it be that these screws have become lose which is causing the focus knob to be very sensitive to the touch. It just seems now that just touching the focuser knob causes the images to shift.
I appreciate all the feedback from those that might have suffered this and I hope that it is nothing to worry about.

Thanks
Donnie

Donnie,

That is called "mirror shift" and it's a normal thing for an SCT, or any other scope that moves the primary mirror for focus. The primary slides back and forth on the baffle tube and there is always a bit of tolerance in there to allow for that movement. The focus rod pushes directly on the mirror cell of your C925, but as you can see it's slightly off center. So the pushing and pulling of the focus rod will also rock the primary mirror ever so slightly as you change direction.

I'm sure you will learn the trick to handling it soon enough, but the simple rule is to turn the focus knob very slowly the closer you get to focus. If you do that, you are less likely to pass the focus point and have to come back.

Cheers,

Ron

One more thing:

You have an Edge SCT. Double check the mirror locks to make sure they are not dragging. They will make it worse if they are.

Ron

Yes, it is the Edge. All I know is that this started happening all of a sudden (well, within about 15 min of observing)
Would going from a 74° house to about 90° make a diff? If I had to total the amount of time from OTA inside to outside when I started to notice this would have been about an hour. The other observation time, I had the OTA outside for about 1 1/2 hours before I even touched the OTA - focuser.
Maybe the grease or whatever had a thicker vicosity since it had been inside with much cooler temps.

Thanks
Donnie

One thing you might want to do and do every so often is to run the mirror back and forth lock to lock a couple of times to even out the lubricant on the baffle tube which is what the mirror is riding on. It's not a cure for the image shift, but it makes it a little less obvious.

David

I went to the Celestron site and actually found an article just now about it. It might be just what you are talking about. what I see is that the image will "move" laterally as the object passes thru focus. I assume that this is what is called image shifting?
Yes, I will try that.

My other question that I would like to post is:
Does the mirror lock knobs have a "stop" to them or they just reach a point of turning and turning and nothing happens (applies to loosening)
Also the same with Focus knob. Does it have a "stop" to it.

thanks to all.
Donnie

There is an accessory you can add to your telescope which will competely nullify mirror shift (what you are experiencing). Get a focusser which mounts on to the end of your focal train before the diagonal. You do your "final" focusing from it rather than moving your mirror and therefore cut out any possibility of shift.

The very first time I ever did any imaging, I ran into mirror shift with a camera on my telescope. I got a super image of Saturn as shown on my computer screen, but it needed a hair of focal adjustment. As soon as I touched the telescope focus knob to adjust, Saturn disappeared and all I could see was black sky. This happened a few times before I realized what was happening. A JMI focusser solved the problem and I still use it today even for casual viewing.

My other question that I would like to post is:
Does the mirror lock knobs have a "stop" to them or they just reach a point of turning and turning and nothing happens (applies to loosening)
Also the same with Focus knob. Does it have a "stop" to it.

thanks to all.
Donnie

The mirror locks on the Celestron are progressive, so there will be a point they become very hard to turn when you lock the mirror. Loosening - they just become easier. It's a bit harder to tell unless the focuser is stiff.

The focuser does have stops on both ends.

Ron

Donnie,

I learned about image shift when I started AP with my CPC 1100. Imagine, you set up an item that you are going to shoot a picture of, and then you realize the focus needs a bit of tweaking and then when you move the focus knob the whole image shifts position. That's how I learned.

The solution for me was an upgade to a focuser that adds on to the rear of the OTA. I went with the FeatherTouch ( .8" short travel because of clearance issues with my fork-mounted scope). Additionally this focuser has two focus knob speeds, regular and 10 to 1, and you can lock it down once it's focused. I'd recommend something similar if you plan to do any AP with the scope.

Enjoy your Edge!

Hope this helps,

CTSS

I was thinking about starizona has that focuser that installs in place of the existing.

Donnie

I was thinking about starizona has that focuser that installs in place of the existing.

Donnie

That would be the Feathertouch Micro focuser. I have installed these on all of my SCTs and they help minimize the shift a lot. But they cannot eliminate it; only an external focuser can do that. The external focuser adds weight, so that's another issue to consider.

Ron

James,

What you do is just add that focuser on. I have found that the range of my FeatherTouch is not as great as the stock Celestron focuser, but, that was never a problem. I just use the stock focuser to get focusing close, and then I use the FT for fine-tuning.

You can see how I added the focuser in the picture below.

I'm really happy with the way the FT focuser eliminates image shift when I'm trying to find that perfect focus (usually with the help of a Bahtinov Mask).

Hope this helps,

CTSS

Two drawbacks about the Crayford focuser at the back is it increases the effective focal length which also increases focal ratio. That's the nature of SCT scopes when adding image train length to the rear will increase effective focal length. I am an imager and I want to keep the focal ratio minimal to reduce exposure times.

Second drawback is it adds more weight to the already rear heavy scope.

I have Feather Touch microfocuser which replaces stock focuser and I see a tiny image shift when digitally zoomed at maximum with Nebulosity software and SXVR-M25C camera. I do not have much issue with mirror shift with my CPC0800.

Peter

The addition of a Crayford focuser isn't necessarily going to increase your imaging focal length. For a 9.25 EdgeHD, the recommended rear-cell to camera sensor distance to maintain a flat field is 5.75 inches. If you had a Crayford on the back of the scope, you would be making up that distance with an adjustable focuser rather than a stationary extension. Either way, your camera sensor will be the same distance back from the rear cell, and your imaging focal length will be the same with or without the Crayford.

I really appreciate all the feedback. One thing that I plan to do next time is to let the OTA reach temp more. Then after that, I will move the focuser to the extremes (which ever that amount is) - and not saying that this will work. I do find it interesting that after the third night - then all of a sudden I get this shift. Wasn't doing it before, but then again, I had the scope out much longer before I even touched the mirror lock or focuser. I am still new at this and I do not have all the answers, however, I will say that if I find anything I will certainly post it. I do like the feather touch that would replace the existing focuser and will at some point - do that anyway. I really do not want to be nit picky, I just want to enjoy this new scope.

Donnie

Taken straight from 925 Edge manual:
Critical focusing is best accomplished when the focusing knob is turned in such a manner that the mirror moves against the pull of gravity. In doing so, any mirror shift is minimized. For astronomical observing, both visually and photographically, this is done by turning the focus knob counterclockwise.
So, at least in the manual image shifting is acknowledged.
Fair enough !
Donnie

Please update us regarding your final solution, i.e. what works.

I didn't see any thing on the ring "Lock-up" on the diagional to the visual back. I'm having a terrible problem with that. I have recently converted over to 2" Nagler eyepieces and a 2" diagional and because of the added weight I have a problem keeping the diagional from rotating. I know if I swing the diagional 90 degrees in the other direction it would tighten itself but I'm left eye strong and besides the focuser is on the right side. Any suggestions on how you corrected your same problem. When the threads stick it takes alot of efforts to right the problem.

Any suggestions on how you corrected your same problem. When the threads stick it takes alot of efforts to right the problem.

Well, I am thinking that an aftermarket diagonal would have to do the job. Yes, I have found that there is hardly no play adjustment on the smaller visual back ring. Just a tiny tightening and then it is stuck and can not get it loose. I am thinking that the thread pattern is too course. They should have made the thread pattern (pitch) finer. For finer adjustment. Or have some type of thumbscrews. Does any one know of a Diagonal that would replace the stock one.

Donnie

It is the SCT rear Cell Connector that I am trying to think of. (this part is too sticky once you get it on)