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Is there a way to fix SCT Mirror shift

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#76 aa6ww

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Posted 30 May 2014 - 05:28 AM

you shouldnt have to accept image shift on something as new as an Edge HD scope.

I have 3 SCT's, my C14 and C11 are over 10 years old and they are rock solid up to about 350x, then just a tiny big creeps in. my 8" SE has zero image shift so far out to about 350x.
Why not call Celestron and see if they can do something about it. That's ridiculous you choose to accept image shift in something as new as an Edge HD scope.

No one should have to pay over $300 for an external focuser when the original problem should be fixed. Im sure there are dozens of C11 HD's that are rock solid, yours should be too.

Just remember to always turn the focuser clockwise until it stops before you transport any sct, so the mirror doesn't flop around when your transporting the scope.

...Ralph



I have a Celestron 11" Edge HD. The mirror shift is horrible even for small changes. Because of this I installed a moonlite focuser. Nonetheless, it bothers me and wondering is there a method to fix this for a DIY?



#77 Gil V

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Posted 30 May 2014 - 05:34 AM

I wonder why nobody has made a truly excellent SCT in this regard. It seems there are a number of fantastic premium scopes out there of other designs that command high prices, why couldn't there be a premium SCT at a higher price point (like Edge HD +)? I think I'd *easily* pay another $100 for a scope to have a truly proper zero-shift moving-mirror mechanism. $100 is almost in the noise, price-wise. Heck, I pay almost that in gas alone for a single weekend dark-sky trip!


I personally built quite a few excellent cats in this regard. It all boils down to manufacturing tolerances and product cost.

#78 Live_Steam_Mad

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Posted 30 May 2014 - 03:24 PM

Before I did the re-grease of the baffle tube on my 7" F15 Meade Mak I was experiencing huge image shift (seem to remember nearly 1/4 FOV of high power eyepiece) and when I took the OTA apart I found next to no grease left of the original Meade stuff, it had thickened to a wax and was only present behind the mirror carrier, none was present in the baffle tube anymore, no wonder the image shift was so large ;-

https://picasaweb.go...CassegrainEx...

Since I regreased the primary baffle tube, the mirror shift has become hugely less, maybe 1/2 the diameter of Jupiter. I'm very pleased with it.

Regards,

Alistair G.

#79 Live_Steam_Mad

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Posted 30 May 2014 - 03:29 PM

The nylon screws support the Meade 14 mirror and I am sure it's as heavy as the C14 mirror. The screw tips only take up a few thousandths in space between the baffle and slider tube. You still have the grease and the same support as before. There is very little pressure on these screw tips and this does work.


Got any pictures of this modification with the Nylon screws? It would be interesting to see.

If Celestron / Meade would implement the roller bearings for zero image short focussing, and also COOLING FAN on all their SCT's, then the instrument would become a lot more viable for double star / planetary work at high magnifications.

I'd be willing to pay a few tens of GBP more for 6 roller bearings and a fan, it's not a lot of extra cost but would make the instrument much more desirable and competitive. By the sounds of it, a lot of other users would like this also.

Best Regards,

Alistair G.

#80 WesC

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Posted 30 May 2014 - 04:14 PM

You know it occurs to me that there really is no mechanical need for the mirror and baffle tube to ever touch, and in fact, its probably a good idea that they do not. Tolerances can be tight enough that they are close fitting without touching.

There are two better solutions, IMO.

Something like the clement focuser, but internally. Properly supporting the mirror from the back, a stable system made to focus the mirror along multiple equidistant points instead of just one, where the focus knob is.

or

Lock the mirror down and focus externally, but build the tube with this in mind so that the external focuser doesn't add anything to the focal length as well as eat up all the back focus.

#81 TCW

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Posted 30 May 2014 - 07:20 PM

Properly supporting the mirror from the back, a stable system made to focus the mirror along multiple equidistant points instead of just one, where the focus knob is.


I believe that is what Celestron started out with and dropped when they went to the orange tube scopes.

#82 REC

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Posted 31 May 2014 - 12:25 PM

I have that problem with my ETX-125 where the star would move outside the FOV. I'm supposed to open it up and grease the barrel, but leary of opening it up. One fix was to get a diagonal that had a focusing tube on it. Now I just get the focus close and rotate the helical tube up and down.

#83 Live_Steam_Mad

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Posted 31 May 2014 - 01:06 PM

I believe that is what Celestron started out with and dropped when they went to the orange tube scopes.


Yes, the very first batch of Orange Tube 'scopes had Co-Axial focussers (I think it was up to serial number 125 if I remember right and they seemed to be a no image shift design by the looks of it) and then they quickly changed to using the conventional focusser.

Intes etc with their Maksutov Cassegrains use a pinion gear on the focusser and a large spur gear inside the rear cell and then a huge split plastic gear and metal baffle tube threads to make the mirror move backwards and forwards with zero image shift, a better design but more expensive. But has the advantage that the focusser looks and operates conventionally from the outside.

Regards,

Alistair G.

#84 TCW

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Posted 31 May 2014 - 01:10 PM

Using grease to take up mechanical slop just does not seem right! :p

I don't own a Cat but hope to so I am following this thread with great interest. :gotpopcorn:

#85 jhayes_tucson

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Posted 31 May 2014 - 02:32 PM

You know it occurs to me that there really is no mechanical need for the mirror and baffle tube to ever touch, and in fact, its probably a good idea that they do not. Tolerances can be tight enough that they are close fitting without touching.

There are two better solutions, IMO.

Something like the clement focuser, but internally. Properly supporting the mirror from the back, a stable system made to focus the mirror along multiple equidistant points instead of just one, where the focus knob is.

or

Lock the mirror down and focus externally, but build the tube with this in mind so that the external focuser doesn't add anything to the focal length as well as eat up all the back focus.


There are certainly a lot of ways to move the mirror. Doing it with flexures can certainly work (as one CN participant has shown) but it tends to me a more complicated solution. I think that the slider on the baffle tube is actually a pretty good approach, but it needs to be implemented better than the way Celestron has done it. Focusing by moving the primary requires pretty tight tolerances and/or a clever design. There are certainly some significant advantages to focusing this way. If you move the primary by x, the image moves by xm^2, where m is the optical magnification. That makes it very easy to accommodate a wide range of back focal distances with only a little bit of mirror movement. That means that for a typical SCT with m = 5, you only have to move the primary by 1" to shift the focus position by 25"! The optical magnification translates mirror tilt linearly into lateral shift at the focal plane. That means that a 1 arc-min tilt of the primary becomes a 5 arc-min tilt at the output (for m=5). So, the mechanics have to be very tightly controlled (as we have been saying.) Simply greasing the slider tube will not fix the problem--no matter how thick the grease is. There is also no way to "float" the slider tube on the baffle so that they don't touch--nor would that approach solve the focus shift problem. I think that there are basically two improvements that would completely fix the "greased pole" focusing system:

1) Zero clearance the slider tube. That could be done in a number of ways but as a retro-fix, the nylon screw idea seems quite good.

2) Rearrange the focus screw so that the slider tube is loaded symmetrically about the central axis of the baffle tube. In the Celestron design the focus screw is offset from the center and that imparts a tilt moment on the mirror. It would be relatively simple to implement a screw driven kinematic lever arm that would impart zero tilt moment on the mirror support plate. I haven't looked at the space behind the mirror to see if there is enough room to implement a retrofit of this sort--maybe, maybe not. This is something that Celestron could certainly implement with only a minor redesign.

John
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#86 TCW

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Posted 31 May 2014 - 03:29 PM

Does the mirror tilt show up as astigmatism or another optical fault?

#87 jhayes_tucson

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Posted 31 May 2014 - 03:54 PM

If the tilt is large enough, it certainly could. I haven't ray traced it, but my sense is that the center of the "zero" aberration field will shift off center for small amounts of tilt. As tilt increases, you'll start to see more significant aberration over the field. I can't give any limiting values but keeping things on-axis and centered is obviously a good idea.
John

#88 TCW

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Posted 31 May 2014 - 04:16 PM

People always talk about image shift in SCT's but never about the optical implications of a mirror that moves around.

#89 TxStars

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Posted 31 May 2014 - 04:33 PM

Yes a star image is changed as the main mirror moves, but with the image floping all over the place the changes are hard to see.

#90 WesC

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Posted 31 May 2014 - 06:57 PM

Well it definitely shows up as a mis-collimation effect.

#91 Herr Ointment

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Posted 31 May 2014 - 07:21 PM

Geez, I like the lever arm idea.

Poking around my C11 Edge and taking apart the mirror lock had me thinking about a 3 point focuser mechanism. It's like they started the job for you!

Anybody have an image or diagram link to the original Blue Tube Celestron focusing system?

#92 jhayes_tucson

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Posted 31 May 2014 - 08:04 PM

I posted photos of the assembly on page one of this thread--though the photos don't show the clearances. Unfortunately, I don't think that there is room in there for a lever arm of sufficient stiffness. It has to go between the backplate on the mirror and the inside of the rear casting so it's pretty tight. I'll look at it more closely the next time I have the scope apart. If there's room, I'll give some serious thought to how it might be modified. Pushing on both sides of a line passing through the center of the baffle tube would totally remove the tilt moment caused by the current focusing screw.

John

#93 Ed Holland

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Posted 31 May 2014 - 09:40 PM

In a answer to TCW, my C8 had a the rear cell tilted to the tube and that appeared a lot like astigmatism. However the situation was more severe I think, than could result from large clearences (slop) in the focus mechanism. Once fixed, the optics turned out to be wonderful.

#94 PowellAstro

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Posted 31 May 2014 - 10:57 PM

One easy fix for the manufacturer could be to use a split tube slider with a front and rear collar. The collar would have two set screws that applied adjustable pressure on the sides of the split ring slider removing the play. This would require no changes to the OTA, just the slider tube would get one new machining cut and the addition of the two collars.

#95 jrbarnett

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Posted 01 June 2014 - 11:07 AM

It's not a hard problem to solve. The Ruskis solved it out of the box with their MCTs. It's just (a) feeble engineering investment and (b) disrespect for the customer that causes it to persist as an issue. In other words, the Big Two in astronomy are a lot like the Big 3 in automobiles; cheap and cynical. "Why would we spend the extra 17 cents per unit when our customers are so unsophisticated that they're ignorant enough to buy it anyway with the defect?"

Perhaps Meade and Celestron will learn what the Big 3 learned. The customers you want most aren't the indiscriminate uninformed buyers, since they generally have less money to spend and replace capital items less frequently, and the prize customer demographic not only is willing to pay for quality, but often won't buy unless it is present.

Regards,

Jim

#96 jrbarnett

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Posted 01 June 2014 - 11:19 AM

https://www.astromar...auction_id=6857

Like this one?

Regards,

Jim

#97 Herr Ointment

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Posted 01 June 2014 - 12:17 PM

I'm not worthy.

#98 TCW

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Posted 01 June 2014 - 08:31 PM

Wouldn't three points be needed to totally eliminate mirror shift? Even with three points you could get a tiny bit of side to side movement but I suppose that would be insignificant.

#99 jhayes_tucson

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Posted 01 June 2014 - 10:03 PM

Wouldn't three points be needed to totally eliminate mirror shift? Even with three points you could get a tiny bit of side to side movement but I suppose that would be insignificant.


If you ignore the clock angle, a minimum of four contact points are required to kinematically define the position and angular orientation of any cylinder. You weren't specific so I'm guessing that you mean that since the cylinders contact along a line, you could get away with only three screws. The problem is that the cylinders are very close in diameter and you don't know how the cylinders actually contact (i.e. is it really along a line or at one point?). If we were talking about perfect geometric cylinders, I think that you only need a single hard point point contact to geometrically define the position, but that won't work mechanically. In general, I think that you have to look at this kinematically and go with four hard points--even though it might be slightly over-constrained. The fact that the contact points are adjustable makes it completely workable.

John

#100 Ed Holland

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Posted 02 June 2014 - 10:39 AM

Had the C8 out last night and couldn't help but notice at least "a Saturn's worth" of mirror shift, possibly more... Now wishing I hadn't read this, and the other threads, for fear of wanting to tear into and modify the 'scope :)

Cheers

Ed


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