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Celestron SCT Focus

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#1 carlstronomy

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Posted 06 February 2013 - 06:11 AM

I have now had my telescope for just over six months and am starting to now get a grasp of this hobby. I have done a lot work to this old Ultima 9.25, rebuilt the internals of the mount which had been assembled wrong by a previous owner at some time, lose nuts floating around inside and a few other little things along the way. It is now giving me very nice 3-5 minute subs and when I build my pier mount I know it will give good 10 minute subs at least as I managed a 9 minute sub without perfect pollar alignment last week by luck.
The only issue I now have which is not terible but a pain is when I focus the object moves over in one direction. So each time I zoom in on an object I have to recenter it and I sometimes do lose the object. I have learnt to work around this but if there is a fix I would be interested in giving it a go as it would then complete this project completely and give me perfect scope to learn with.
I am not sure what causes this maybe the primary mirror is on a light angle or the front corrector plate is on a slight angle as that would the affect the secondary. I know it is not the collimation as even when this is spot on it still happens.
If anyone has any serious idea's then please share them or a link it would be greatly appreciated.

Thanks

Carl

#2 rguasto

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Posted 06 February 2013 - 06:25 AM

Hi Carl. What your experiencing is called image shift. This is present in all SCT - although newer models are greatly improved (my C6 has almost none). Mirror shift is inherent in the design since your moving the mirror to focus. Not sure what you can do. Others may have more insight.
-Rob

#3 carlstronomy

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Posted 06 February 2013 - 06:31 AM

Thanks Rob, thats cool like I said I have learnt to work with it. I really wanted to make sure it is not something I need to add to my restoration project.

Carl

#4 Geo.

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Posted 06 February 2013 - 10:11 AM

The tolerances between the baffle tube and the primary carrier are in the 0.0001 vicinity, but that's enough to allow the carrier to rock on the baffle tube. The grease is there to lube the mechanism, but more importantly to limit the rocking. If you OTA hasn't had it's primary out for a relube it's time. Grease is basically soap and oil. The oil evaporates over time. Folks recommend Dow Corning high vacuum grease, which is a synthetic (silicone) high viscosity grease. Sawbuck an ounch on eBay.

#5 rmollise

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Posted 06 February 2013 - 10:31 AM

The tolerances between the baffle tube and the primary carrier are in the 0.0001 vicinity, but that's enough to allow the carrier to rock on the baffle tube. The grease is there to lube the mechanism, but more importantly to limit the rocking. If you OTA hasn't had it's primary out for a relube it's time. Grease is basically soap and oil. The oil evaporates over time. Folks recommend Dow Corning high vacuum grease, which is a synthetic (silicone) high viscosity grease. Sawbuck an ounch on eBay.


Lack of grease can be a problem with older OTAs, but be very cautious about regreasing. The grease on the baffle must be THIN. Too much and your problems escalate to focus drift. Too thick a layer and the mirror continues moving momentarily after you stop moving the focus control. NOT a good thing. ;)

I have yet to have to regrease the baffle of my 1994 Ultima OTA.

#6 rflinn68

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Posted 06 February 2013 - 11:34 AM

Lack of grease can be a problem with older OTAs, but be very cautious about regreasing. The grease on the baffle must be THIN. Too much and your problems escalate to focus drift. Too thick a layer and the mirror continues moving momentarily after you stop moving the focus control. NOT a good thing. ;)


I noticed that in my old Meade LX50 the last couple times out (focus drifting). It has never been opened up. Its been a struggle for me getting and keeping a good focus with that scope. I was also getting really bad focus shift so now I dont know if it needs greasing or not :question: I have since made up one of these and I've been enjoying the scope a lot more. It seems to work very well.

#7 rigel123

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Posted 06 February 2013 - 08:16 PM

I noticed this on my brother's Meade and I was told to rack the focuser all the way in and all the way out a few times to redistribute the grease. It really helped a lot to reduce the image shift

#8 carlstronomy

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Posted 07 February 2013 - 02:21 AM

Thanks everyone for your input, I have from all your comments decided that my scope is just fine and I will be doing nothing with it at this stage. It is just a slight case of image shift, my focus is always easy to achieve, nice and crisp and I get no drift. I can move my scope a good 40 degrees during a session and the focus stays good.

Again thanks everyone. :)

#9 Pete-LH

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Posted 07 February 2013 - 07:06 AM

I think you have come to the correct conclusion, There are some other threads in this section but I could only find this one from my saved items;

Greasing

One solution is to use a separate focuser between the OTA and the visual back. In fact there is one for sale here in the classified by Zhummel. You can also do this with replacing the std focuser with a combined focuser/mirror lock. I think one is offered by Hutech but my mempory might be failing me on this. Then you focus with the SCT focuer, loch it and focus with the installed SCT focuser. I have not done this myself. Vixen offers the VMC200L (8") SCT which only has the SCT focuser between the OTA and visual back. I have been tempted to get one of these (Also does not have the corrector plate)

I tried the first part of the above and never really cared for it mainly due to slippage with heavy lenses in cold weather. A better focuser would probably manage this better.
I finally went with a Feathertouch microfocuser replacement of the standard focuser. This does not eliminate mirror shift but I find being able to make small adjustments when needed makes it more bearable.

I actually sold my C9.25 at one time but the person asked to return it due to the image shift described as 1 to 2 jupiter diameters at high magnification. This is probably unacceptable for most who do AP. But I am just a visual observer(Redundant?) and with time I have optimzed being able to collimate and to be more patient for temperature equilbration. I'm glad I did not sell it after all and have enjoyed it as my larger aperture telescope for several years now. The only reason I would sell it now is to downsize to just my C8 as I get older.

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#10 carlstronomy

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Posted 07 February 2013 - 07:21 AM

Thanks Pete, the shift on this scope is sufficient to be a pain some times going up in mag on visual lenses I do have to keep moving the object back into the center. I just have to go up in stages not big jumps and I dont have a problem.
I can understand the person you sold the 9.25 to sending it back he probably had a small size ccd chip. When I first started out I used my DBK21 and I would zoom all the way up to a 9mm and could not find it once I swapped to the DBK21 I used to move backwards and forwards it used to drive me crazy.
Now I have upgraded to my 450d there is just no issue at all for ap the chip size is so large I have to miss by miles to not get it somewhere in the live view.
My main issue these days is just visual and I have learnt to work with it and really it does not bother me. If I still used my DBK more than what I do now I would probably have to do something about it.

Carl

#11 azure1961p

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Posted 07 February 2013 - 07:36 AM

Yeah the DBK field is awesomely tiny. I'm thinking of mounting my TV ranger as a guide scope or get a flip mirror attachment coupling for the cam. It can be maddening. And at the focal lengths required for imaging its a super small field of view.


Pete

#12 carlstronomy

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Posted 07 February 2013 - 08:59 AM

But I am just a visual observer(Redundant?)


Sorry could not disagree strongly enough, some nights I look at Jupiter with my 26mm eyepiece and the view is spectacular, I then put on my camera and take 1000 frames spend 20 minutes aligning, stacking, colour correcting still to find it looks worse that what I saw in my $50 eye piece and the surface detail is no where near as clear or colourful. Maybe not as good for deep space (you can still see M42 with the good old visual method) but anyone should still get the wow factor out of visual astronomy.

#13 KerryR

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Posted 07 February 2013 - 04:45 PM

I have a 2003 8" LX90 that I completely disassembled when I installed active venting.

When I reassembled, I was conservative with the grease. When I tested to ota on distant tree-tops, I found the mirror shift was enough to move the target right out of the fov with a 15mm eyepiece, and across most of the FOV of a 25mm ep, both plossl's.

I re-greased the baffle heavily, and removed the excess that was pushed out the rear of the carrier.

This dramaicaly reduced focus shifting. While shifting is still present, it's no longer enough to move the target out of the FOV with a 4mm ep.

#14 Geo.

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Posted 07 February 2013 - 04:57 PM

Well, it's true that high Xs will, err, magnify the problem. I think it's asking too much of the SCT system to think there will be NO image shift. At 260X (C9.25 w/9mm Plossl) the FOV is 11.5' and Jupiter is, say, 45" or 6.5% of the FOV. Guess that's about quater inch, which will take it off a 1/3" imager. In the day Meade sold a CCD framing eyepiece that made it possible to center an object on the imager. It had a reticule with the outlines of various sized imager FOVs.

#15 carlstronomy

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Posted 08 February 2013 - 02:20 AM

George, you are correct (impossible to have no image shift) and I started this thread to try and gain some sort of knowledge of what it was that was causing my image to move, why, and how to fix it or if I even need to. From all the great feedback here I have come to these conclusions:

1. Its called image shift
2. All SCT's have it
3. You can never get rid of it
4. You will probably make things worse trying to get rid of it if you try!
5. It is not a fault or issue unless it makes your scope unusable.

All in all simply said and summed up, "do not ask to much of a simple telescope"

Carl

#16 bilgebay

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Posted 08 February 2013 - 02:51 AM

If you come to focus by rotating the focus knob counter clockwise always it wont be necessary to recenter the object.

#17 Geo.

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Posted 08 February 2013 - 10:39 AM

I find that cats stored on their sides seem to have more shift than those stored upright and they say gravity is a weak force. The worst I've seen was a ETX125 Mak that would shift the entire FOV or about 50 acrmin! Even then running the focuser back and forth half a dozen times calmed it down to a couple of arcminutes.

#18 DHurst

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Posted 08 February 2013 - 02:14 PM

I think adding a crayford focuser on the back of the scope will solve your problem. It will ad a bit of backfocus, but you won't be focusing by moving your mirror.

#19 gonzosc1

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Posted 08 February 2013 - 03:27 PM

have you tried running the mirror out in both directions a few times. helped to redistribute the grease on the baffle tube sometimes.

#20 OLDTIMER77

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Posted 11 February 2013 - 12:11 PM

Hi Car,

To minimse the problems of 'mirror flop' on the Edge Ota's I take this step before slewing the 'scope.
Carefully screw in the two mirror knobs on the rear cell preventing the mirror from moving. Then after slewing to the required sky position gently release the two knobs before focusing on your sky objective.
Clear skies.
oldTimer

#21 kansas skies

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Posted 11 February 2013 - 07:09 PM

I had this problem on my late 70's C8. It was bad enough that refocusing while using a medium power eyepiece would cause my target to move completely out of the field. What I found that had the most positive effect for me was to clean the focusing mechanism.

My first step was to orient the OTA into a horizontal position in case the primary mirror didn't have a stop clip installed on the baffle to prevent the primary from falling down onto the corrector plate.

I'm not sure if the following is the same with your scope, but all I did was to remove the focusing knob and plate that holds the focusing shaft in place. Next, I removed a stop screw that was inside the end of the focusing mechanism and then screwed the outer sleeve of the focusing mechanism all the way out of the scope. This left me with a threaded rod sticking out the back that was attached to an arm that was attached a sleeve that was attached to the back of the primary. I then carefully moved the primary forward with the threaded rod until it contacted the stop (which was installed). At this point, it was easy to move the primary back and forth slowly a number of times to redistribute the grease in the baffle. In my case, I don't think this was the problem, as it moved very smoothly along its axis.

What I found to be the culprit in my case was the point at which the threaded focusing rod attached to the arm. It was designed to slide side to side and also pivot, but the old grease had frozen and it would not pivot as intended. What I did was to moisten, but not saturate, a cotton swab in mineral spirits (solvent) and touch this carefully to the pivot point. I then worked the shaft side to side a number of times to work in the mineral spirits until it moved freely. I had to repeat the application of the mineral spirits a few times to accomplish this. What I found to be most important here was to not rush and to apply the solvent very sparingly.

I also noticed prior to disassembly that the focuser had a gritty feel when rotated, sort of like it had detent stops. This was caused by dried grease in the bearings that were contained in the outer focusing sleeve that I removed in order to get to the inner threaded shaft. To correct this, I applied a drop of solvent (again, not too much) to both bearings and then chucked the sleeve into a drill press. I switched the drill press on and held the outer bearing races while the shaft rotated. It took no more that a couple of minutes of this and the roughness was gone.

I then put everything back together, but left the screws holding the flange somewhat loose. I rotated the focusing knob both directions from stop to stop before tightening the screws on the flange to allow the focusing shaft to center properly first.

After doing this, the focusing mechanism is now very smooth and the focus shift problem I was experiencing is almost completely gone.

Bill






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