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Is there a way to fix SCT Mirror shift

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#51 jhayes_tucson

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Posted 26 May 2014 - 02:44 PM

Before taking a scope apart to do modifications to it, I would first install a premium micro focuser to reduce any play there, can't hurt and may be all that is needed!
Rex


A) I already have a premium micro focuser.

B) No matter what kind of focuser you have, it drives the same screw so it doesn't matter what kind of focuser you have. This issue is strictly related to the way the mirror is mounted.

C) I want to take the scope apart for some other reasons in a month or so anyway and this is a nice mod to do at the same time. I spent over 25 years professionally designing, building, and aligning optical-mechanical instrumentation much more complicated than a C14 so this job doesn't scare me. Do I know what I'm doing? Yes. Could I screw it up? Absolutely. But, I'm an engineer and engineers by nature have to take stuff apart and (at least attempt) to put it back together. :p

Stay tuned. I'll let you know how it goes…
John

#52 jhayes_tucson

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Posted 26 May 2014 - 02:52 PM

I think that the nylon screw solution should work well (as has been demonstrated.)


This has not been demonstrated to work on a GE mount. The M14 mentioned above rides on a fork mount. For a GE mount, you might need to use two set screws at 120 degrees apart to provide a stable 3 point support.


Alph,

A) Forgive me, maybe I'm being dense, but what does the mount have to do with it? They both point all over the sky.

B) I'm talking about adding four screws total. Two at each position along the outer slider tube, 120 degrees apart. That should be sufficient to completely constrain the tilt of the inner and outer tubes with respect to one another.

C) I seem to be talking in lists today…

John

#53 TxStars

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Posted 26 May 2014 - 03:41 PM

Instead of nylon screws you could use roller ball set screws.
http://www.smallpart...rewsrollingb...

#54 jhayes_tucson

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Posted 26 May 2014 - 04:09 PM

Instead of nylon screws you could use roller ball set screws.
http://www.smallpart...rewsrollingb...


That's not a bad idea, but it has some potential problems. First, roller tipped screws are typically pretty big. The one you found is probably at the upper limit of what you would be able to use. The other problem is that the balls are not constrained--they float on springs. So, to get it right, you would have to use one hard point (Nylon) and one spring loaded ball at each position. The spring also has to be stout enough to hold the load (i.e. a high K factor) under the worst case of telescope orientation. The real problem is that, in my experience, ball plungers don't actually roll very well. They are great for applications that require a detent or where you need to apply force at a point, but when they have to roll, they just end up scoring the part over time. The metal will gaul and the motion will no longer be smooth. The main advantage of spring loading is that it would work better if the baffle tube is not uniformly straight or round. As long as the baffle tube is fairly uniform, I think that the Nylon screw might work better over time.
John

#55 Alph

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Posted 26 May 2014 - 05:46 PM

Forgive me, maybe I'm being dense, but what does the mount have to do with it? They both point all over the sky.



It does make a difference. You can't make any assumptions about load distribution when the OTA rides on a GEM. See message #6541779 in this thread. All I am saying is that this is not a proven solution. I would be interested though to hear how this experiment turns out in 12 months. Good luck.

#56 PowellAstro

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Posted 26 May 2014 - 06:23 PM

@ Alph

Your assumption is wrong. My 14 is fork mounted but is on a super wedge most of the time for the last few years. Three points is all that is needed to support the mirror under any orientation. Mine works just as it should mounted either way.

#57 WesC

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Posted 27 May 2014 - 07:36 PM

C) I want to take the scope apart for some other reasons in a month or so anyway and this is a nice mod to do at the same time. I spent over 25 years professionally designing, building, and aligning optical-mechanical instrumentation much more complicated than a C14 so this job doesn't scare me. Do I know what I'm doing? Yes. Could I screw it up? Absolutely. But, I'm an engineer and engineers by nature have to take stuff apart and (at least attempt) to put it back together. :p

Stay tuned. I'll let you know how it goes…
John



Careful, you may end up starting a side business... I think the first person to make a reliable, permanent solution to this issue is going to be very popular. ;)

#58 jhayes_tucson

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Posted 28 May 2014 - 09:59 AM

Well...I think PowellAstro gets all the credit for pioneering this idea. It sure sounds like he's made it work well. Whether or not I can implement it on my scope will depend a lot on de-bonding the mirror. I may order up a right angle drill bit--just in case I can't remove the mirror. I wish I could start on it right away but it will probably have to wait at least another month before I can work on it.
John

#59 rmollise

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Posted 28 May 2014 - 11:10 AM

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?


How much is horrible? If more than about 30 - 45-seconds, there is likely something wrong with your scope. The Edges, even the 11s, I've seen have usually had less focus shift than that. ;)

#60 jhayes_tucson

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Posted 28 May 2014 - 02:55 PM

Just for reference, I guesstimate that I have 10-15 arc-minutes of shift in my standard C14. Yep, that's 1/3 - 1/2 of the full field! It is a significant problem.
John

#61 WesC

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Posted 28 May 2014 - 03:29 PM

Whoa... I would guess I have about 10-20 arc seconds in my Edge 11, less if I back the mirror locks waaaaaay out.

10-15 arc minutes is just awful! I wonder if your mirror is lose on the carrier???

#62 jhayes_tucson

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Posted 28 May 2014 - 06:38 PM

Whoa... I would guess I have about 10-20 arc seconds in my Edge 11, less if I back the mirror locks waaaaaay out.

10-15 arc minutes is just awful! I wonder if your mirror is lose on the carrier???


Nope. That is one of the reasons I had it apart in the first place. I inspected everything and the mirror is firmly bonded to the backplate. At the time, I installed flop stoppers, which significantly improved the stability of the mirror for photography; however, the drag of the stabilization rods actually made the focus shift a bit worse. The core problem is the mechanical clearance between the focus tube and the baffle. You always want to focus in the same direction but in my case, the focus shift is so large that if I go too far, I have to back way up to start over because the object actually goes totally out of the field of view when I'm looking at a 10x live view image on my camera. It is very frustrating! My problem is so big that any fix is better than nothing. Frankly, after seeing the insides of my telescope, I was less than impressed by the way it was built. It looked like it was assembled with a hammer and a rat tail file by some guy (maybe in China) who normally makes furniture. The optics seem ok but the mechanics need some serious work. I've clean up a lot of the issues but there are a few remaining one's that I will fix the next time I have it apart.

To put this all in perspective: Remember that with an optical magnification of 5.78 (for the C14), a 12" long baffle tube only has to tilt about 0.010" at one end to move the image by 15 arc-min. So, to hold the image to 10 arc-seconds, the same tube has to be tilt free to within about 0.0001". That's a pretty tight tolerance.

John

#63 WesC

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Posted 28 May 2014 - 07:46 PM

Yeah, that's the downside of high mag and small FOV...

#64 TCW

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Posted 28 May 2014 - 07:55 PM

Just for reference, I guesstimate that I have 10-15 arc-minutes of shift in my standard C14. Yep, that's 1/3 - 1/2 of the full field! It is a significant problem.
John


Yikes! Is that typical? That scope would be for sale if it were mine.

Maybe I don't want a big Cat after all. :p

#65 jhayes_tucson

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Posted 28 May 2014 - 09:34 PM

Just for reference, I guesstimate that I have 10-15 arc-minutes of shift in my standard C14. Yep, that's 1/3 - 1/2 of the full field! It is a significant problem.
John


Yikes! Is that typical? That scope would be for sale if it were mine.

Maybe I don't want a big Cat after all. :p


No, I don't think it is typical at all. I bought the scope second hand and it's been just fine for how I'm using it; however, I think that it was built with some extraordinarily poor mechanical tolerances. I've already made huge improvements and I think that ultimately, I can make it better than new. Heck, that will probably be about the time I decide to trade it in on a new HD system and then I'll start all over again! ;)

John



BTW, I love the 14" aperture and highly recommend it. I may just move on to a 17 or 18" aperture after this. The problem is that I'd hate to give up the Hyperstar!

#66 PowellAstro

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Posted 28 May 2014 - 11:49 PM

Mine had about 1/10th FOV shift at 100x before I added the nylon screws to the slider. Now at 800x I have none. All I see is the image move from touching the focus knob. I see about the same amount just touching the OTA and not touching the focus knob.

#67 Ed Holland

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Posted 29 May 2014 - 11:59 AM

One might think that the manufacturers could find a number of ways to address this problem and then tout the advantage.

If the issue is manufacturing tolerance, then adjustability should be designed-in. This of course requires an extra step in assembly, and proper training of the workforce.

One idea that occurs, for manufacturing design would be to form the end of the mirror tube with slits and a thread, just as a collet chuck, and add an adjustable threaded collar. This could allow precise and stable adjustment of the fit between inner and outer tubes, and could have a low profile that would not interfere with the light path. The possibility for thermal variation would need to be tested.

Another possibility would be to groove one or other tube to accept an o-ring, or multiples thereof, to locate the tubes with respect to each other. With the right grease, this could be reliable and offer smooth motion - similar principles are used routinely in vacuum systems.

Just ideas... not reduced to telescope practice.

#68 Namlak

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Posted 29 May 2014 - 04:10 PM

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!

#69 herrointment

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Posted 29 May 2014 - 05:17 PM

I imagine studies show that Joe Blow won't pay that extra few dollars.

Really, this sounds like a simple mod and this thread and the "holes" thread has me screwing up my courage to give my 8 SE orange tube some modifications that, while not sorely needed will most likely improve the performance of the telescope.

Major stupidity aside, I can do this and that.

The CPC 1100 HD I'm not touching. But the focus shift is a problem and cold weather performance (below 10F) is beyond lousy.

So small steps ar first.

#70 TCW

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Posted 29 May 2014 - 05:45 PM

Both the C and M SCT 14" now run 10 grand. You would think they would provide more quality control where it matters. Other than optics, the focus is the single biggest issue to me. It seems like someone could machine precise sets of mirror tubes and perhaps install them as well. Just about every other conceivable option is available in the aftermarket!

If the ball tipped screws take most of the slop out, it would only ad a couple of dollars to the scopes and it would be money well spent. Listening Celestron?

#71 jhayes_tucson

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Posted 29 May 2014 - 06:16 PM

I sure hope that Celestron is listening. This is great customer feedback. The big issue is that I don't think that Celestron makes a lot of money and when you start talking about making this kind of change, they have to assign an engineering budget to it, then they have to go through an EC process so that manufacturing can implement it, and that all takes time and costs money. In my experience, it's never easy no mater how simple it seems at the outset, but really good companies commit to a program of continuous improvement. I wonder what kind of quality program Celstron has (lean, 6-sigma, etc.?) In my experience, most companies committed to a solid quality program would fix this kind of stuff and wouldn't have the assorted problems that we seem to regularly hear about with Celestron products.
John

#72 TCW

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Posted 29 May 2014 - 06:38 PM

Of course both Meade and Celestron have known about this issue for 40+ years and have done nothing.

Celestron started out with a better focus system and canned it.

#73 PowellAstro

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Posted 29 May 2014 - 10:08 PM

The new upper end units by Meade have corrected this issue. They use 6 roller bearings to create an internal Crayford style focuser. This also allows them adjustments now to align the primary mirror on the optical axis of the OTA.

#74 TCW

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Posted 29 May 2014 - 11:56 PM

Reputedly Celestron hand matches optical components until they get a good match. Perhaps they could do the same with the focusing?

#75 jhayes_tucson

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Posted 30 May 2014 - 01:45 AM

Reputedly Celestron hand matches optical components until they get a good match. Perhaps they could do the same with the focusing?


Having been in the business for a lot of years, I can tell you that hand matching optical components is sometime necessary unless you want to drive the cost into the stratosphere. That's not true with mechanical parts. CNC machines are really good an pumping out very nearly identical parts. There are also ways to implement a design to mitigate small variations and deal with zero clearance tolerances. That is basically what happens with Crayford focusers. I learned long ago not to underestimate the cleverness of a good mechanical engineer. In my view, if Celestron wanted to solve this problem, they could.
John






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