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Finishing the 10" f/20 Buchroeder Trischiefspeigler

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#1 ccaissie

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Posted 10 November 2022 - 04:30 PM

Time to put this scope together.  The Covid episode dispersed the club's energy, but the arrival of Mars has made this a top project.

 

We've coated the optics (These were bought from Dick Wessling 10 years ago by Lenny Arsenault), and installed in a plywood OTA to confirm the design .

A redesigned aluminum angle frame has been built and the large fork mount is somewhere near completion.

 

I'm taking delivery of it from our treasurer's basememt, and a couple of items need tweaking before loading in the optics and setting it out under the stars. 

 

My earlier star tests on Polaris were extraordinary, and M42 using a 35mm Panoptic still sticks in my mind.

 

Pictures coming.

Attached Files


Edited by ccaissie, 10 November 2022 - 04:59 PM.

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#2 petert913

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Posted 10 November 2022 - 04:33 PM

Can't wait for lots of photos !



#3 Kitfox

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Posted 10 November 2022 - 04:49 PM

Do you have any photos of the mirrors?  I’ve always wondered how you keep the orientation of the asymmetrical primary correct. 


Edited by Kitfox, 10 November 2022 - 04:52 PM.


#4 Jim45157

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Posted 10 November 2022 - 04:58 PM

me and dick wessling was very good friends for 45 years i looked throught his 12-1/2 a lot i got a 6 inch set of optics from him which were great  really miss him great mirror maker i made part of the mount for his 12 inch you will like the scope great on planets jim45157



#5 Jim45157

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Posted 10 November 2022 - 05:02 PM

steve rismiller has the 8 inch set he made dont know if he put them in a tube goes back about 30 years jim45157


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#6 ccaissie

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Posted 10 November 2022 - 05:15 PM

I might add that this is the Richard Buchroeder design, much better than Kutter's design, but not as good as Mike Jones' adjusted design.

 

Anyone looking at one of these scopes should ask me about an OSLO file sent by Mike Jones.

 

These optics are part of a series built by Dick Wessling which included an 8" set and a 12.5" set.  Sadly, I can't report to Mr. Wessling, as he passed away in 2020.  These scopes were discussed in the old Telescope Making rag, way back when.


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#7 ccaissie

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Posted 10 November 2022 - 05:19 PM

Do you have any photos of the mirrors?  I’ve always wondered how you keep the orientation of the asymmetrical primary correct. 

No.  Not asymmetrical.  It's a 10" Ellipsoid, Conic of -.55 so like a half-parabolized primary.   The tilt of the primary and secondary cancels the Coma, and the thrid mirror is a very weak concave mirror that cancels the astigmatism while folding it.  You'll see the layout and the intelligent discussions on why this is a super design below

 

Here's the 2016 discussion when we started the project.  Had issues, but then resolved.

 

https://www.cloudyni...rking-out-well/


Edited by ccaissie, 10 November 2022 - 05:47 PM.

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#8 Kitfox

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Posted 10 November 2022 - 06:32 PM

That was an interesting read.  So the secondary is cut from the figuring tool?  How is the tertiary figured?  I’ll be lurking…



#9 starcanoe

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Posted 10 November 2022 - 07:10 PM

I might add that this is the Richard Buchroeder design, much better than Kutter's design, but not as good as Mike Jones' adjusted design.

 

Anyone looking at one of these scopes should ask me about an OSLO file sent by Mike Jones.

 

These optics are part of a series built by Dick Wessling which included an 8" set and a 12.5" set.  Sadly, I can't report to Mr. Wessling, as he passed away in 2020.  These scopes were discussed in the old Telescope Making rag, way back when.

I'd love to see the spot diagrams particularly for 2 to 3 inch diameter fields at the focal plane!

 

Was just looking at these two tri designs in my old ATM mags before I logged on here.



#10 SandyHouTex

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Posted 10 November 2022 - 07:34 PM

This thread brings back a lot of memories.  Prior to 2000, when I still owned my farm before my divorce, I was working on a 16 inch Buchroeder Tri-Schiefspiegler.  I still have the mirrors.  The 16 inch f/11 is almost completely polished out, with the 8 inch secondary and tertiary still just blanks.  Lately I've been working on a 10 inch Stevick-Paul.  The primary is completely polished out and just needs figured.  The secondary and tertiary are still blanks as well.

 

I wish you the best with your 10 inch.  It will be like having a 10 inch APO only color free.


Edited by SandyHouTex, 10 November 2022 - 11:46 PM.

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#11 Ed Jones

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Posted 10 November 2022 - 11:19 PM

Yea Dick was a great mirror maker and my boss for many years.  I got to look through his 12 inch many times, one awesome scope.


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#12 ccaissie

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Posted 11 November 2022 - 06:46 AM

I'd love to see the spot diagrams particularly for 2 to 3 inch diameter fields at the focal plane!

 

Was just looking at these two tri designs in my old ATM mags before I logged on here.

The tilted focal plane is a handicap in the case of an extended focal plane.  The Mike Jones version has a milder tilt, I believe.



#13 ccaissie

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Posted 11 November 2022 - 07:04 AM

That was an interesting read.  So the secondary is cut from the figuring tool?  How is the tertiary figured?  I’ll be lurking…

I'm not sure how the secondary was made.  I tried making a 5" f/20 and used the 3" secondary as the grinding tool for the 5" primary.  The primary was SO hyperbolic when polished.....still have the glass in a drawer....

 

Kutter Schiefs can be made with the secondary cored out of a matching diameter tool.  I have a classic 4.25 f/24 made by Norman Meyer that got a"honrable mention" @ Stellafane.

 

In this case the secondary is 6" IIRC.  I suspect it needs to be alternated with a full sized tool to make sure they both stay spherical and of the same radius.  Recent posts here re: creating a matching convex surface discuss the issues with making a match within a wave or two...special considerations.

 

The tertiary?  Good spherometer and LONG Foucault test, or as DAVEG shares, test it against a collimating scope..do the math re: focal shift.  



#14 starcanoe

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Posted 11 November 2022 - 07:22 AM

I've always wondered about coring secondaries/tertiaries or making a separate secondary/tertiary when a tilted/unobstructed design where there is a matching convex/concave pair that are are supposed to have the same radius.

 

The secondary/tertiaries are usually spherical.

 

Don't the core the tool/secondary. Don't make a separate optic. The surface is a sphere. Just use the whole thing uncored and mask off what you don't need....And this has the advantage that most of the surface you DO use won't be near an edge or near where the optical surface has been disturbed by coring.

 

This might not be so practical in bigger sizes...but for more modest apertures it seems reasonable to me.

 

And for that matter...in the larger sized scope I'd still be tempted to make moderately oversized secondaries and tertiaries to stay away from the edge more.



#15 DAVIDG

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Posted 11 November 2022 - 09:39 AM

 I have cored a number of convex secondaries from "tools" use to grind the primary when making a Schief. Using a diamond coring bit it is an easy job. When it comes to testing the ultra long focal length concave mirrors, the method I have used is to use a Newtonian with  known good primary. I use my 6" f/8  that the mirror was figured by Double Pass Autocollimation to show a clean null.  The long focus mirror is now used as a "flat"  in a double pass test.  Since the mirror is not optically  flat  it will change were the "object" comes to focus in the test and that will be longer since the "object" is seen as closer and not at infinity. One can setup the test in OSLO and see what the spherical aberration will be when the ultra long focus mirror is a perfect sphere.  Many times it is small amount of  under 1/8 wave  or much less The test will allow you to see zones clearly and figure the mirror to you get a smooth figure.  So when the mirror nulls the figure will be smooth and you'll know what the wavefront error is. This beat trying to do a Foucault test on a  mirror with radius of curvature of  say 100 feet which can be the case in certain TriSchiefspeigler designs.

  Here is a picture of how I tested a mirror with 85 foot focal length that I used in my  solar projection telescope.  It works great and I'm not 170 feet away trying to do a Foucault test.

 

              

                               - Dave 

 

          doublepass85footmirror.jpg     


Edited by DAVIDG, 11 November 2022 - 10:58 AM.

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#16 ccaissie

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Posted 11 November 2022 - 12:49 PM

The surface is a sphere. Just use the whole thing uncored and mask off what you don't need.

 

That would be great except the tilt angle of the system is minimized to clear the secondary, so smaller is more elegant.  You could shove most of it out of the way, but then the mounting, collimating, etc is a bit wonky.

 

In this 10f20 case, the tertiary is so far out of the way it's easy to make a 5" and mount it, even though it only needs to be 3".


Edited by ccaissie, 11 November 2022 - 12:51 PM.

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#17 Ed Jones

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Posted 11 November 2022 - 02:39 PM

Once I asked Dick how he tested the very long radius of the tertiary. He said he focused the image of the filament of a flashlight bulb on a white card. If it was nice and sharp it was good but not when a bit fuzzy.
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#18 Mike I. Jones

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Posted 11 November 2022 - 03:47 PM

I might add that this is the Richard Buchroeder design, much better than Kutter's design, but not as good as Mike Jones' adjusted design.

 

Anyone looking at one of these scopes should ask me about an OSLO file sent by Mike Jones.

 

These optics are part of a series built by Dick Wessling which included an 8" set and a 12.5" set.  Sadly, I can't report to Mr. Wessling, as he passed away in 2020.  These scopes were discussed in the old Telescope Making rag, way back when.

Yes, post it here?  I've forgotten which file it is! lol.gif lol.gif lol.gif lol.gif


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#19 ccaissie

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Posted 11 November 2022 - 05:05 PM

Yes, post it here?  I've forgotten which file it is! lol.gif lol.gif lol.gif lol.gif

Ya know, I can't find it.  It's in an archived post back in '16 or '17.  Will get it.



#20 Mike I. Jones

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Posted 12 November 2022 - 01:25 AM

I "think" this is the file you're talking about.

 

Attached File  10trischief.len   582bytes   26 downloads

 



#21 ccaissie

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Posted 12 November 2022 - 05:41 AM

I "think" this is the file you're talking about.

 

attachicon.gif10trischief.len

OK, that is the original file from DAVIDG using the Dick Wessling specifications for these mirrors.  

I think that the file I had in mind, that you created, was for a 10" Chief design using CVI lenes, and not the TS.  

All good.  

 

I am trying to determine the tolerances on the spacing and angles.  The prototype was built pretty closely to the Wessling specs, and I was able to collimate to a good degree, but who knows how close it was or how close it matters.  

 

DAVIDG states that it is super important to have all the angles and distances exact.  If I am within a half inch on spacing what does that do to the MTF?  Must it be closer than that?



#22 DAVIDG

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Posted 12 November 2022 - 09:36 AM

"DAVIDG states that it is super important to have all the angles and distances exact.  If I am within a half inch on spacing what does that do to the MTF?  Must it be closer than that?"

   There is a math trick that I use when building the Schief that makes it easy to get the angles correct enough that any error can be collimated out. You can measure the sides of a triangle to 1/16" without much trouble and even better. If you get the three sides of the triangle to  the needed length then the angles have to be correct. 

   To aligning the optics is easy with a laser collimator and a couple of paper masks to define the center of each mirror. You put the laser in the focuser and adjust the tip/tilt of the  focuser so it hits the mirror dead center. Now adjust that mirror so the  laser hits  dead center on the second mirror . Adjust the second mirror so it hits  the primary dead center. Now take the laser out the focuser and look in. Adjust the primary so the second mirror just moves out of the field and you can't see it in the primary. Now center up a star, you should just have to tweek the tilt the of primary a small amount to get perfectly round stars both in and out of focus. 

   When it comes to the tilt on the focal plane, you do NOT tilt the focuser, you tilt the eyepiece to match the tilt of the focal plane. This is discussed in Telescope Making. It is like when you use a magnifying glass to read text in a book. You look straight into the magnifier ( eyepiece) so you are on the optical axis and tilt the page in the book  ( focal plane) so the text is in focus across the whole page. 

 

    Here is Mike Jones OSLO file for his design of 10" Tri Schief.

 

                 - Dave 

Attached File  10trischief_MIJ.len   822bytes   18 downloads


Edited by DAVIDG, 12 November 2022 - 09:38 AM.

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#23 ccaissie

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Posted 13 November 2022 - 09:28 AM

Thanks David:   

 

I'm measuring the OTA as-built and plan to trig the angles etc.  It's too late to do the MIJ version of the Tri-S, not intnding to make another tertiary.  

 

Yes the focal plane is tilted and Lenny made an angled eyepiece adapter for that.   During my initial prototype tests, I did not feel the need to use that adapter, but perhaps in the future we will be more critical of the view.  For planetary work, the required FOV is small.  

 

My check in OSLO finds the spacing distances not as critical as the angles, so my findings and final assembly will emphasize the angles and let spacing float if necessary.

 

Here are some quick photos of the bits as unoaded in my garage.  The Orange paint is a Lenny Arsenault touch...He's a motorhead and it must be leftover from his Dodge Super Bee project.  His workmanship is superb.  His past duties included a maintenance gig at Vermont Yankee Nuclear Plant and required perfection.

 

Upon initial trial assmebly, it balanced and moved very niceley.  I'll work out some bugs, mount the optics and have it functional for the upcoming CMAS star party Nov 19th.

Attached Thumbnails

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Edited by ccaissie, 13 November 2022 - 09:43 AM.

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#24 DAVIDG

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Posted 13 November 2022 - 01:06 PM

"My check in OSLO finds the spacing distances not as critical as the angles, so my findings and final assembly will emphasize the angles and let spacing float if necessary "

   Just remember if you change one spacing all three angles will change that is just how triangles work. So to keep the angles close to  the the design specs all the spacing will need to be adjusted as well.

   It should be pretty easy to  make some radius rod from say pine molding to set the spacing at the needed separation and that will fix the angles. 

  I remember at Stellafane when I showed my Schief Alan French  he said it was the first one he had looked through that lived up to all the hype, all the other builders would say the they were still getting theirs dialed in.   So my advice is take the time to get the spacing right and the angle will have to be correct. The results will be the image in the eyepiece will match what it shows in OSLO.

 

           - Dave 


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#25 ccaissie

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Posted 14 November 2022 - 07:02 AM

 

   Just remember if you change one spacing all three angles will change that is just how triangles work. So to keep the angles close to  the the design specs all the spacing will need to be adjusted as well.

   It should be pretty easy to  make some radius rod from say pine molding to set the spacing at the needed separation and that will fix the angles. 

  I remember at Stellafane when I showed my Schief Alan French  he said it was the first one he had looked through that lived up to all the hype, all the other builders would say the they were still getting theirs dialed in.   So my advice is take the time to get the spacing right and the angle will have to be correct. The results will be the image in the eyepiece will match what it shows in OSLO.

 

           - Dave 

The issue with the 3 mirrors is that it is not a closed polygon, so a triangle does not naturally solve.  Working on it.

I'm making a couple of gauges to set the angles off a "centerline" that will also allow me to find the focal plane and the center of the field.

 

Will report in on all this.  I remember viewing Mars through your 4.25 Schief.  The view was similar to Lenny's !st Prize Optical  6"f/11 Newtonian that night.




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