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Advice/experience with refiguring Coulter 13.1" f/4.5

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#1 symbiosis

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Posted 11 October 2019 - 05:44 AM

This mirror is only 5/8" thick.

 

I spoke to the folks at Optical Wave Labs and they agreed to test it and (possibly) refigure. They didn't seem to think the glass thickness would be a problem and when I asked about possible flexing of the corrected mirror, their only concern was that it would need to be "properly supported" by the mirror cell.

 

If I use an 18 point cell, would that be adequate?


Edited by symbiosis, 11 October 2019 - 05:46 AM.


#2 TOMDEY

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Posted 11 October 2019 - 07:18 AM

If that 5/8" is the edge thickness, and if the mirror is flat on the back... then the center thickness would be a mere quarter of an inch --- which is impossible... or insane! Are you sure the mirror isn't 1.37 inches thick, at the edge?! I had a fine Coulter Odyssey 13.1, and that was the thickness.

 

CORRECTION: That's wrong... the center thickness is more like 0.45 inch --- very thin, but imaginable (thanx, Preston!)    Tom

Attached Thumbnails

  • 112 toms old 17.5 13.1 scopes.jpg

Edited by TOMDEY, 11 October 2019 - 08:20 AM.


#3 PrestonE

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Posted 11 October 2019 - 07:38 AM

If that 5/8" is the edge thickness, and if the mirror is flat on the back... then the center thickness would be a mere quarter of an inch --- which is impossible... or insane! Are you sure the mirror isn't 1.37 inches thick, at the edge?! I had a fine Coulter Odyssey 13.1, and that was the thickness.    Tom

I got 0.181 for the Sagitta...thus the center thickness 0.444 or there abouts???

 

Best Regards,

 

Preston


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#4 TOMDEY

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Posted 11 October 2019 - 08:18 AM

I got 0.181 for the Sagitta...thus the center thickness 0.444 or there abouts???

 

Best Regards,

 

Preston

Whoops! Yeah, I forgot the 2R = 4DF#... I was off by a factor of two. If the mirror is 5/8 thick... that's still wafer-thin, though. Ummm.... that, and the Provided Coulter Odyssey 13.1s were something like 1 and 3/8 at the edge. Where would a 5/8 thick one come from? Could his 13.1 it be a non-Coulter mirror? The 17.5 was an inch and a half at the edge... and the redoubtable 29 was also a (mere!) 1.5 at the edge!    Tom


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#5 gregj888

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Posted 11 October 2019 - 09:29 AM

I tested a Coulter mirror  with a Bath for the club.  It tested quite well but was thin.  To test I took groups of shots every 45 degrees, de-rotated and averaged.  It was a descent mirror but will need to be well supported in the cell.

 

I didn't run plop on it but I would guess 18 points or so.  3 blobs of RTV would not be my first choice.



#6 Tom Dugan

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Posted 11 October 2019 - 09:45 AM

That's weird, because my blue-tube 13.1" primary is just a hair under an inch. What could have happened to it in production, I wonder? Or maybe it was reground/figured already.

 

Hmmmm....



#7 Tom Dugan

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Posted 11 October 2019 - 09:46 AM

I tested a Coulter mirror  with a Bath for the club.  It tested quite well but was thin.  To test I took groups of shots every 45 degrees, de-rotated and averaged.  It was a descent mirror but will need to be well supported in the cell.

 

I didn't run plop on it but I would guess 18 points or so.  3 blobs of RTV would not be my first choice.

I ran Plop on mine years ago and IIRC, 6 points was sufficient. But then again, almost an inch thick.


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#8 Mark Harry

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Posted 11 October 2019 - 10:46 AM

I agree with Tom. I got sucked into one of those, and I plan to use it for a doorstop. Or perhaps test it, and cut out off axis segments.
These thin plate glass jobs were from that FL outfit, believe Murnaghan Instruments.


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#9 gregj888

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Posted 11 October 2019 - 12:05 PM

I wonder if there would be any benefit in adjusting the support points taking into account the surface deformation data (interferometric).  Not much use on a thicker mirror but on a thin one???

 

Basically put the supports under the low points  and adjust the offsets between the fulcrum points and distance to the mirror support pads. 

 

Hey, Plot guys... ;-)

 

Greg



#10 Steve Dodds

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Posted 11 October 2019 - 12:48 PM

Coulter over the years instead of increasing prices, dropped quality.  In the early 80's 13.1" mirrors were an inch thick and were usually of decent quality.  Late 80's they were 5/8" and you had to hunt to find a good one.  By 1990 they were .5" plate glass and I have never seen a good one from that era. 


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#11 mconnelley

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Posted 11 October 2019 - 02:43 PM

Hello:

   

  I have an 8" from Coulter that was the first telescope I ever bought.  I 'refigured' it by building a warping harness for it and just bending it into the right figure.  It's fairly thin plate glass, so the original mirror support (a few blobs of glue onto a hunk of particle board) just wasn't cutting it.  Happily, since it is thin plate glass, it's really easy to bend to the right figure.  My warping harness has 8 flex fingers around the edge of the mirror, with the goal of controlling astigmatism only.  However, by adjusting the screws appropriately, one could put the mirror into many other interesting figures.  I judged how to flex the mirror by looking at the inside-of-focus star image, and just tweaked the screws until the star was nice and round.  Starting with a randomly messed up mirror, I could fairly quickly get the mirror figure back and even compensate for the astigmatism in my eyes.  At the time I built this, I just had a jig saw and a drill press, so this sort of thing can be built with very rudimentary tools.   

Cheers

Mike


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#12 TOMDEY

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Posted 11 October 2019 - 06:16 PM

I wonder if there would be any benefit in adjusting the support points taking into account the surface deformation data (interferometric).  Not much use on a thicker mirror but on a thin one???

 

Basically put the supports under the low points  and adjust the offsets between the fulcrum points and distance to the mirror support pads. 

 

Hey, Plot guys... ;-)

 

Greg

Close, but no cigar! The forces exerted normal to the back of the mirror by (even a customized) whiffle-tree ensemble are each and every one proportional to sin(Alt). So, although their ratios are invariant, their magnitudes decrease as sin(Alt). But the forces needed to stress out the topological errors are constants... so it won't work. Mike's ~warping harness~ does that correctly by applying tuned forces that are independent of altitude. Also what one does not want are ~displacement actuators~

 

I came up with many ways to correct global constant (polished in) astigmatism and wrote a white-paper on that, an abbreviated version of that paper appearaing in Amateur Astronomy Magazine. Here's an image of one embodiment, that I successfully executed on a Coulter 29-inch mirror... that was otherwise decent. It brought the performance from annoyingly astigmatic to quite good (and free of astig). About the only aberration that you can reliably tune out is ~primary Zernike astigmatism~ None of the others gracefully yield to force actuators. The professionals found that out the hard way --- the very hard way... (I know, I was involved in a couple of those studies).   Tom

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  • 114 amateur astronomy dey astig article.jpg
  • 113 29-inch telescope force actuators tweaker clock astig 80.jpg

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#13 Pinbout

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Posted 11 October 2019 - 07:19 PM

Close, but no cigar! The forces exerted normal to the back of the mirror by (even a customized) whiffle-tree ensemble are each and every one proportional to sin(Alt). So, although their ratios are invariant, their magnitudes decrease as sin(Alt). But the forces needed to stress out the topological errors are constants... so it won't work. Mike's ~warping harness~ does that correctly by applying tuned forces that are independent of altitude. Also what one does not want are ~displacement actuators~

 

I came up with many ways to correct global constant (polished in) astigmatism and wrote a white-paper on that, an abbreviated version of that paper appearaing in Amateur Astronomy Magazine. Here's an image of one embodiment, that I successfully executed on a Coulter 29-inch mirror... that was otherwise decent. It brought the performance from annoyingly astigmatic to quite good (and free of astig). About the only aberration that you can reliably tune out is ~primary Zernike astigmatism~ None of the others gracefully yield to force actuators. The professionals found that out the hard way --- the very hard way... (I know, I was involved in a couple of those studies).   Tom

what's your edge support?



#14 TOMDEY

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Posted 11 October 2019 - 09:13 PM

what's your edge support?

On that particular little 29-inch baby scope, it's an old Astrosystems sling... which worked pretty well, provided I distributed/relaxed its stiction/grip, once I had the telescope aligned. BTW, on my new 36-inch scope, Ryan and I came up with the "Ladder-Sling", which applies 30 adaptive point loads equally to the front face plate and rear back plate, and of course, longitudinal rails, left and right. That indeed maintains both alignment and entirely astigmatism-free at all altitudes! Very happy with that. The bigger they are, the more thoughtfully conceived and executed... must be the supports. Even the 27-point whiffle tree(s) account for the curvatures of the plates. Anyway... it functions wonderfully!   Tom

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  • 119 36-inch diam x 5-inch thick mirror in Ladder Sling.jpg

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#15 gregj888

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Posted 11 October 2019 - 09:59 PM

Close, but no cigar! The forces exerted normal to the back of the mirror by (even a customized) whiffle-tree ensemble are each and every one proportional to sin(Alt). So, although their ratios are invariant, their magnitudes decrease as sin(Alt). But the forces needed to stress out the topological errors are constants... so it won't work. Mike's ~warping harness~ does that correctly by applying tuned forces that are independent of altitude. Also what one does not want are ~displacement actuators~

 

  Tom

Tom,

 

That was understood.  It was more the idea of using the mounting to help the figure than randomly holding the mirror in place.  Most of my observing is also near zenith (double stars) so my  personal situation is a little different.  Like your setup, that's slick...



#16 Mark Harry

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Posted 12 October 2019 - 05:34 AM

Pretty cool, Tom. Makes me think about slight toroidal deformations to tilted component mirrors.......



#17 symbiosis

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Posted 12 October 2019 - 05:45 AM

To be honest, the technical discussion is a bit beyond me. I can build any kind of mirror cell needed to support the mirror. Is it worth having it tested or refigured or should I just put it aside and move on? 



#18 Mike Spooner

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Posted 12 October 2019 - 08:27 AM

I made an 11" that was 5/8" thick borosilicate. Machine ground and polished with MOT only. It came out fine and I used a 9 point cell. For a 13" I'd probably use a ~15" lap if available.

 

Mike Spooner


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#19 TOMDEY

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Posted 12 October 2019 - 08:48 AM

To be honest, the technical discussion is a bit beyond me. I can build any kind of mirror cell needed to support the mirror. Is it worth having it tested or refigured or should I just put it aside and move on? 

Good point! The mirror is probably worth what you paid for it.

 

Having Cary evaluate and address the figure and coating seems entirely worthwhile. Building an entire telescope around an unknown, preexisting mirror is a risk of time and money... and this one (at the moment) has a three strikes against it: poor pedigree/history of similars, very thin, many negative disinterested testimonials. One common, reliable approach is to build the entire scope and just try it, as-is!

 

> If it works to your satisfaction --- enjoy it!

> If it's terrible, send it to Cary for evaluation.

     - if Cary finds the substrate workable, ask him to refigure and recoat, as needed.

     - if he deems it unworkable, ask him to make a (thicker) replacement mirror that will fit your scope.

 

It's more ideal to build a telescope around a known good mirror. At work, we occasionally took delivery of customer (meter-class) mirrors with mounts that were "almost finished, just needs a little tweaking", only to discover fatal flaws. Big, thin mirrors were notorious for that. Bad figure and terribly squirrely mount, stressed substrate... essentially nothing of value there, other than wistful hope. If the original shop went ultra-thin plate glass... just to save $$$ on the substrate cost... you can ~figure~ the whole process was --- cheap! To paraphrase Stephen Gosson (1579) "...you can't make a silk purse of a sow's ear".   Tom


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#20 mconnelley

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Posted 12 October 2019 - 04:52 PM

Hello:

 

  If you can build any kind of mirror cell, then you're in luck since the astigmatism correcting warping harness I made is fundamentally just a very weird mirror cell.  While most mirror cells are designed to support a mirror without stressing it, this one is designed to apply a lot of stress in a specific way.  I took a piece of plywood, and cut it into an 8 fingered 'star' shape.  The usual collimation screws go into this plate.  I put threaded inserts into the end of each 'finger'.  A thumbscrew goes through this insert to a fitting glued onto the back of the mirror that captures the screw.  So turning the screw changes the force on the flex finger, and the mirror bends in response.  It was really easy to make, as I said with a drill press and a hack saw.  The key is understanding why it works and how to tune it. Maybe I should write up something more detailed in case other folks are interested in trying this themselves.  

Cheers

Mike


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#21 symbiosis

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Posted Yesterday, 12:47 AM

I wonder if it would make sense for me to build the mirror cell for it first, then place it in the cell and then run a Foucault test, etc. to see what it looks like when it's properly supported ...?



#22 Dale Eason

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Posted Yesterday, 01:04 AM

I wonder if it would make sense for me to build the mirror cell for it first, then place it in the cell and then run a Foucault test, etc. to see what it looks like when it's properly supported ...?

Depends on at what angle you are going to place it in when testing it.  If the optical axis will be placed horizontally for the usual bench test setup, then it does not use any back support points like in the telescope.  In that case the proper edge support is must.  So that you don't add any astig to it with the edge support.  However if that induced astig is aligned with the vertical Foucault knife the Foucault test will not see it.  Then rotating the mirror 45 deg steps is essential in order for Foucault image to mirror only astig at some point.

 

If you want to have the back support be in use you need to tilt the mirror a lot toward the ceiling.  That can make it hard to test but not impossible if you have the vertical space to do it.  Remember that the Foucault tester will need to be 2 times the focal length away from the mirror.

 

It can be awkward.  I once did it in a stair well on my 16 F5.

 

Dale



#23 Mark Harry

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Posted Today, 06:55 AM

"...you can't make a silk purse of a sow's ear".   
*******

(I like to think of it as a pig with lipstick!)


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