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Orion XT10 Mirror Cell Mod - Wow

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#1 RFeaster

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Posted 29 May 2020 - 11:47 PM

A few days ago, out of boredom, I modified my mirror cell.  I removed the 4 pieces of felt that were supporting the mirror edge and replaced them [2] #10 nylon screws 90° apart based on the "Mirror Edge Support Calculator".  I shaved the ends of the screws to an inside length of 1.5mm.  I also replaced the [3] cork pads with [3] #10 nylon countersunk washers which I shaved to 2.75mm thick.  Their locations were determined by the "PLOP" software.

 

I really wasn't expecting a noticeable improvement.  But when I took the scope out tonight for a test...wow.  The edge of stars are actually smooth now instead of the little cotton balls I saw before.  Didn't know what I was missing.  The best $6 I've spent in a long time. 

 

Thanks to all on CN for providing the information that allowed me to make such an improvement.

 

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#2 JohnBear

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Posted 30 May 2020 - 12:26 AM

I'm so glad you posted this! I had never seem a topic about this before (astronomy is a hobby with a LOT of learning involved) , but improved mirror mounting was a vague Idea in the back of my mind as a potential future project  Now you have really captured my interest!

 

I would be most grateful if you could post a few references that you found to be particularly helpful.  I have already googled "Mirror Edge Support Calculator" and added some new links t my project folder.  

 

Thanks!



#3 RFeaster

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Posted 30 May 2020 - 01:05 AM

JohnBear,

 

I purchased my XT10 last July which is the first scope I've owned and looked through since my first ever scope 48 years ago.  So needless to say, I've picked up a ton of information on CN in the last 9 months that's taken my understanding from zero to wherever I'm at now.    I guess I'm saying that every post I've read has contributed to my learning.  However, here's a few specific links that are related to my choices on this topic.

 

I like Mike Lockwood's study he made concerning edge supports and the use of nylon: Link

 

A post where Nils Olof Carlin indicates a target for PLOP which is 4.0e-06. Link

By the way, my 3 points only reached 4.1e-06.  I figured 'close enough'

 

PLOP download & How To:  Link

 

The shape of the pads per another Mike Lockwood study:  Link

This is why I chose countersunk washers.

 

Hope this helps.


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#4 RFeaster

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Posted 30 May 2020 - 01:15 AM

Oh, I forgot to mention I substantially reduced the size of the standard mirror clips (as seen in 2nd picture) with the aid of a hacksaw, razor knife, and file.  Picked up about 1.3 in2 of mirror.  Not much, but hey...why not?



#5 epee

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Posted 30 May 2020 - 09:07 AM

Aw, cr***!
More to do...


Thanks for the post!

Edited by epee, 30 May 2020 - 09:16 AM.


#6 JohnBear

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Posted 30 May 2020 - 09:42 AM

Ron - Thank you so much for the links!  This is definitely something doable that I want to try, and I'll also try to post the results here, with some before/after pictures, for other CNers.

 

It looks like you have done quite well for less than a year in the hobby!  Any tweaks on your XT10 that might be of interest to others?

For example, I refurbed an abandoned XT8 into a quite fine Dob about a year ago, and one of the tweaks I made was to improve the Alt tension by changing the springs (trampoline springs from the hardware store) and adding a turnbuckle for tension adjustment on the springs.  

 

Clear skies!


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#7 epee

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Posted 03 June 2020 - 06:58 AM

This has me modifying my XT12's cell. Unlike the XT10, the 12 has a 9-point, floating cell. The triangles rest on rounded studs with what seems to be oil ports and a pin which holds the triangle in orientation. Three nylon capped screws hold the mirror centered in the cell with three short strips of felt, presumably to help protect the mirror from the cell's edge are found at 60° from the nylon screws.

 

I'm going to replace the cork pads on the triangles with nylon countersunk washers and screws, very similar to what RFeaster has done. In addition, I'm contemplating adding two more nylon screws to the bottom edge of the cell to provide better edge support there.

 

I wish I could figure out how to mod the cell with rollers like Mike Lockwood uses in his study but I'm afraid that is beyond my abilities at this point.


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#8 RFeaster

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Posted 03 June 2020 - 12:15 PM

This has me modifying my XT12's cell. Unlike the XT10, the 12 has a 9-point, floating cell. The triangles rest on rounded studs with what seems to be oil ports and a pin which holds the triangle in orientation. Three nylon capped screws hold the mirror centered in the cell with three short strips of felt, presumably to help protect the mirror from the cell's edge are found at 60° from the nylon screws.

 

I'm going to replace the cork pads on the triangles with nylon countersunk washers and screws, very similar to what RFeaster has done. In addition, I'm contemplating adding two more nylon screws to the bottom edge of the cell to provide better edge support there.

 

I wish I could figure out how to mod the cell with rollers like Mike Lockwood uses in his study but I'm afraid that is beyond my abilities at this point.

After giving some thought as to what had the biggest impact, I would say it was the edge support.  I remember when removing the mirror to clean it a couple of times, it was difficult to get in and out of the cell.  The [4] felt pieces were the obstruction.  It was kind of like a "pressed fit".  Maybe some "pinching" going on?  

 

The next guess would be the mirror clips.  Even though I left the screws loose, they would still "lay" on the mirror.  There was a gap between cell and the rubber part of the clip.  In addition to cutting down the clips, I put small flat washers between the cell and the rubber part of the clip.  That way I could tighten the screw without the clip touching the mirror.

 

As far as what impact the nylon pads have vs. the cork pads, I have no idea.  That's above my pay grade.  I was just following Mr. Lockwood's lead on that one. 


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#9 epee

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Posted 03 June 2020 - 12:47 PM

From what I gathered, any pad material that produces unnecessary stiction on the mirror will produce subtle aberations. Lockwood named Delrin as best and Nylon as second best.

 

That is a great idea, using washers to insure that the clips don't touch the mirror at all.

 

I'm going to have to give the edge support some thought. The really large dobs that Lockwood speaks of, and the cells that he illustrates, only have support along the lower edge. Even so, the mirror will require some protection from the cell rim during transport. I may add other nylon support points but make sure those at the top of the cell do not touch the mirror at all.

 

That leaves the question, will the mirror be less constricted by more nylon screws along the bottom edge or fewer?



#10 RFeaster

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Posted 03 June 2020 - 01:15 PM

From what I gathered, any pad material that produces unnecessary stiction on the mirror will produce subtle aberations. Lockwood named Delrin as best and Nylon as second best.

 

That is a great idea, using washers to insure that the clips don't touch the mirror at all.

 

I'm going to have to give the edge support some thought. The really large dobs that Lockwood speaks of, and the cells that he illustrates, only have support along the lower edge. Even so, the mirror will require some protection from the cell rim during transport. I may add other nylon support points but make sure those at the top of the cell do not touch the mirror at all.

 

That leaves the question, will the mirror be less constricted by more nylon screws along the bottom edge or fewer?

To keep the mirror from touching the metal cell during transport, I placed small pad made from 3 layers of blue painters tape at the 12:00 position.  It doesn't touch the mirror edge when the mirror is resting on the [2] nylon points. 

 

If my understanding is correct, if you use [4] mirror edge supports rather than [2], you would have use a wiffle tree design where they pivot and act as [2] points keeping [4] equal pressure points. If you were to use just [4] independent points with  no pivot, I'm not sure how you could ensure equal pressure among the points. 


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#11 RFeaster

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Posted 03 June 2020 - 01:28 PM

epee,

 

I like your idea of a screw at the top that doesn't touch.  I think I'll ditch the painters tape and add a 3rd screw next time I take the mirror out for cleaning. 

 

By the way, don't forget to post pics of your progress!!



#12 epee

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Posted 03 June 2020 - 01:47 PM

If my understanding is correct, if you use [4] mirror edge supports rather than [2], you would have use a wiffle tree design where they pivot and act as [2] points keeping [4] equal pressure points. If you were to use just [4] independent points with  no pivot, I'm not sure how you could ensure equal pressure among the points. 

Yeah, That's why I'm still thinking this part through. I'd love to rig up the "wiffletree" design, but I don't know how I would execute that with the cell I have, or if there would even be room for such inside the tube. hmm.gif



#13 JHD

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Posted 20 September 2020 - 09:16 PM

Thank you for posting this! My 10XT is happy.  I removed the felt edge pads, added the 90 degree nylon edge supports at the CG depth, and added 'thinned' nylon screw heads where the cork pads were.  I really can see the difference.  The view is sharp and I'm very pleased. Reading "HIGH VOLTAGE" upside-down and backwards (telephone pole) clearly shows the exercise was worth the effort.   

 

I put two more studs on the top of the cell at 90 degrees (not touching the mirror), to keep the mirror movement tame during transport. I also cut the mirror clips down to the same triangular size shown, but I kept the two screw points so they can't twist.  

 

Lockwood and may others who have shared their finding, are greatly appreciated.  


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#14 coopman

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Posted 22 September 2020 - 12:01 PM

Were these changes made w/o drilling any extra holes in the cell?



#15 RFeaster

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Posted 22 September 2020 - 12:31 PM

For my version, I had to drill and tap 3 holes for the pads (#8-32) and 2 holes for the edge supports (#10-32).  My cell is all aluminum so it was pretty easy to drill and tap.  If you wanted to reverse the process, and go back to the cork pads and felt, I don't think the holes would cause a problem.



#16 coopman

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Posted 22 September 2020 - 12:58 PM

Are the edge supports supposed to be positioned at a certain angle with respect to the up-down axis?



#17 RFeaster

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Posted 22 September 2020 - 01:44 PM

Sorry, but I didn't save a copy of my calculations.  I made a quick sketch of where I put my edge supports.  Please excuse the crudeness of the drawing.

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#18 coopman

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Posted 22 September 2020 - 02:33 PM

Beautiful.  Thanks very much for your help.



#19 lakland5

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Posted 22 September 2020 - 03:54 PM

Sorry, but I didn't save a copy of my calculations.  I made a quick sketch of where I put my edge supports.  Please excuse the crudeness of the drawing.

Thanks for the diagram, that makes is all clear!



#20 lakland5

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Posted 22 September 2020 - 04:01 PM

Thank you for posting this! My 10XT is happy.  I removed the felt edge pads, added the 90 degree nylon edge supports at the CG depth, and added 'thinned' nylon screw heads where the cork pads were.  I really can see the difference.  The view is sharp and I'm very pleased. Reading "HIGH VOLTAGE" upside-down and backwards (telephone pole) clearly shows the exercise was worth the effort.   

 

I put two more studs on the top of the cell at 90 degrees (not touching the mirror), to keep the mirror movement tame during transport. I also cut the mirror clips down to the same triangular size shown, but I kept the two screw points so they can't twist.  

 

Lockwood and may others who have shared their finding, are greatly appreciated.  

Did you also drill and tap for your modification (as RFeaster reports in post #15)?  Would probably recruit help on this rather than trying to do it myself with my few hand tools--thanks for any info--



#21 JHD

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Posted 24 September 2020 - 12:49 AM

Did you also drill and tap for your modification (as RFeaster reports in post #15)?  Would probably recruit help on this rather than trying to do it myself with my few hand tools--thanks for any info--

 

Yes, I drilled and tapped with 1/4" nylon screws.  It is soft aluminum and easy to drill and tap.  The trimming/smoothing of the screws takes the most time.  The mirror cell really accommodates these mods well.  Shimming the mirror clips is important too.  The key is to have the 2 edge supports and 3 nylon pads supporting the mirror, but nothing else touching the mirror.  You want to have retention (one or two screws), so the mirror can only move a few millimeters in any direction before settling back exactly to where it should.  These are my assumptions based on the various studies regarding this subject.  I truly have no experience or expertise with this.   


Edited by JHD, 25 September 2020 - 12:17 AM.

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#22 KBHornblower

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Posted 24 September 2020 - 09:53 AM

Are the two edge supports at 90 degrees apart superior to Dobson's sling?  If so, I can easily install them in the square enclosure of my 17.5" Dob.  A 4-point wiffletree arrangement should be equally straightforward.



#23 RFeaster

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Posted 24 September 2020 - 12:18 PM

Are the two edge supports at 90 degrees apart superior to Dobson's sling?  If so, I can easily install them in the square enclosure of my 17.5" Dob.  A 4-point wiffletree arrangement should be equally straightforward.

Please refer to Lockwood's study on edge supports in post #3.  Its is a fascinating read.  Here's the Link



#24 Don H

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Posted 24 September 2020 - 12:42 PM

Are the two edge supports at 90 degrees apart superior to Dobson's sling?  If so, I can easily install them in the square enclosure of my 17.5" Dob.  A 4-point wiffletree arrangement should be equally straightforward.

You can see the effect of different edge support solutions here:

 

http://www.cruxis.co...ecalculator.htm


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#25 KBHornblower

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Posted 24 September 2020 - 07:03 PM

You can see the effect of different edge support solutions here:

 

http://www.cruxis.co...ecalculator.htm

The closest approach to my setup in the calculator is the 180 degree cable sling, and it returns negligible distortion.  My sling is a World War II-surplus canvas strap with a width about equal to the thickness of the mirror.  It holds the collimation adequately over most of the altitude range.  The low-budget Coulter mirror shows a triangular pattern just inside and outside of focus which rotates when I rotate the mirror.  This tells me that the fault is in the mirror rather than the support.  In moments of good seeing I have seen Izar B as a round spot of about 1 arcsecond at best focus.  I would say that introduced astigmatism, if any, is swamped by the limitations of the mirror.  It is nowhere Dawes' limit for the aperture, but is satisfactory for my primary use which is going deep on faint fuzzies.  My inclination now is to leave well enough alone.  If I were to go for broke and get a Zambuto mirror, then by all means I would reconsider.




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