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Folded Refractor Project

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#1 VVObserve

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Posted 28 November 2020 - 05:34 PM

 
 
IMG 3216

 

 

This is a new post to share my project to build an 8.25" folded refractor using a doublet of f/27 and two optical flats.

I purchased these optical components from the legendary Edward Byers a few months before his passing, a great loss to us all.

Ed had a pair of these 8.25" doublets and had planned to construct a folded binocular scope for solar observations.  He realized he did not have time to complete this dream project and sold the optics.  I knew the binocular project was far beyond my capabilities and so I obtained the optics for a single telescope.  Ed assured me the optical components are top notch.  He expected results comparable to a Clave lens...all I can say is holy moly yer kiddin me right?  The doublet was figured by a master optician and mounted in a cell by the Byers Company.

All the initial observations I have had with the folded refractor I have cobbled are like WOW.  Cassini...there. Mars surface detail...obvious. Jupiter Red Spot...well yeah.  The Moon...is that Niels golf ball.   Really exagerating on the last bit, haha.

 

Ed provided a layout for this folded refractor which I used as a model.

 

My OTA is built of baltic birch with old 6" primary cell mounts to hold the two flats, 6" for mirror 1 and 5" for mirror two.

A 2.5" Feathertouch completes the optical train.

Brandon eyepieces all the way and a Vernonscope binoviewer to boot.

 

Lets see $$$$$ but well worth it.  Accumulated over decades (hope I'm not too old to see by the time I get this completed!!!)

 

Questions:

The scope is not fully assembled since this is a trial and error project.

When I have looked through the scope...a most difficult operation as it is not yet properly balanced (will document all the counterweight issues at some point) and the drive is not connected, anyway...

The view through the focuser can be confusing. I made a baffle but I still see reflections from the second optical flat.

I have been reviewing Texereau and the Book cover has a diagram of a Cassegrain scope.  I have an 8" Cave Cassergain and my scope conforms to the Texereau diagram down to the long draw tube that limits the light exposure to the eyepiece.

Light bulbs are flashing...although not original...could I install a tube of pvc or somesuch inside the OTA to serve not as a drawtube but as a baffle for the focus end of the folded refractor?  I'm thinking, Why not?


Edited by VVObserve, 28 November 2020 - 05:39 PM.

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#2 ccaissie

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Posted 28 November 2020 - 05:51 PM

The Kutter Schiefspeiglers use a tube with an oval cutout at the far end to admit the rays from the primary, then shoot it down the tube to the ocular.

 

Have a look at Kutter diagrams to see if this suggests anything..  It works on the Kutter at f/24.7



#3 Couder

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Posted 28 November 2020 - 05:52 PM

Nice. I might try boxing it in first maybe with cardboard painted flat black to see if that improves it. It looks kind of like a Schiefspiegler.



#4 VVObserve

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Posted 28 November 2020 - 06:28 PM

Nice. I might try boxing it in first maybe with cardboard painted flat black to see if that improves it. It looks kind of like a Schiefspiegler.

Yes basically except the angles are only a degree or to off axis.

You are right about the open light test stages.  There is so many reflections bouncing around.

I have black flock paper to line the final OTA.



#5 PETER DREW

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Posted 28 November 2020 - 06:31 PM

I built several 8" folded refractors and a F25 folded binoscope. These were all in enclosed tubes so stray light wasn't a problem..

#6 VVObserve

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Posted 28 November 2020 - 07:53 PM

I built several 8" folded refractors and a F25 folded binoscope. These were all in enclosed tubes so stray light wasn't a problem..

step by step I am in a trial and error project. getting very close to final enclosure and I wanted to eliminate any issues before enclosure

Very interested in your projects. do You have them posted?



#7 dan_h

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Posted 28 November 2020 - 08:24 PM

To further test the scope on a board, why not use a shroud?  You could run wire from the corners of the primary board to the corners of the focuser board. Don't need anything fancy. Just a temporary support for a bedsheet or something similar.

 

To get a rough collimation, why not use a laser pointer in the focuser? 

 

1) Adjust/aim/shim the focuser with the laser on, so that the laser beam hits the centre of the first mirror. 

 

2) Adjust the first mirror so the reflection of the laser beam hits the centre of the second mirror. 

 

3) Adjust the second mirror so the laser beam strikes in the centre of the objective lens. 

 

4) At this point, collimation of the objective can be accomplished using typical achromat collimation procedures. 

 

If the mirrors are of adequate size, the primary aperture should be visible when looking through the focuser tube without an eyepiece inserted. 

 

Looks like a great project. Have fun!

 

dan



#8 VVObserve

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Posted 28 November 2020 - 09:55 PM

Fun times ahead!!!



#9 Mike I. Jones

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Posted 28 November 2020 - 11:20 PM

Some history on your objective lens: I designed that lens around 1983 or 1984, and Mike Marcario made two sets of objectives.  He wanted a long refractor, and that's exactly what he made.  Mike's description as a "master optician" just scratches the surface.  Mike was and remains the most fastidious, perfection-obsessed optician I have ever known in my nearly 50 years in optics.  These two objectives are as finely made as humanly possible.  I tested both in the optics lab at General Dynamics in 1984 and the imagery was spectacular.  Mike worked for Ed Byers in 1976-1979, and made the lens cells as well.  Sadly, Mike passed away in July of 1997.  I really wish I'd had him make me one of these lenses like you are fortunate enough to have purchased.  Ed sold the other one to someone, and I've never known who.

 

OSLO-EDU file of your lens attached.  It's a garden-variety BK7-F2 achromat, but at f/30 the secondary spectrum is almost semi-apo like.  Mike also made two flat mirrors so he could fold it into a Z to reduce the length.

Have fun with that beauty.

 

Attached File  8f30 achromat.len   551bytes   34 downloads


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#10 PETER DREW

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Posted 29 November 2020 - 12:04 PM

step by step I am in a trial and error project. getting very close to final enclosure and I wanted to eliminate any issues before enclosure
Very interested in your projects. do You have them posted?
[/

quote]This was a few years ago, the 8" Binoscope is shown on one of the DIY forums.



#11 VVObserve

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Posted 29 November 2020 - 01:37 PM

Some history on your objective lens: I designed that lens around 1983 or 1984, and Mike Marcario made two sets of objectives.  He wanted a long refractor, and that's exactly what he made.  Mike's description as a "master optician" just scratches the surface.  Mike was and remains the most fastidious, perfection-obsessed optician I have ever known in my nearly 50 years in optics.  These two objectives are as finely made as humanly possible.  I tested both in the optics lab at General Dynamics in 1984 and the imagery was spectacular.  Mike worked for Ed Byers in 1976-1979, and made the lens cells as well.  Sadly, Mike passed away in July of 1997.  I really wish I'd had him make me one of these lenses like you are fortunate enough to have purchased.  Ed sold the other one to someone, and I've never known who.

 

OSLO-EDU file of your lens attached.  It's a garden-variety BK7-F2 achromat, but at f/30 the secondary spectrum is almost semi-apo like.  Mike also made two flat mirrors so he could fold it into a Z to reduce the length.

Have fun with that beauty.

 

attachicon.gif8f30 achromat.len

Your reply and the attached data on this lens has made my day!!!

 

Thank you so much Mike.

As you say, the lens is spectacular.


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#12 VVObserve

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Posted 29 November 2020 - 01:56 PM

The lens data confirms my measurement of the focal length, thanks again!

 

Actual focal length 239.41"

my measurement   238.75 =/- 0.5"


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#13 VVObserve

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Posted 29 November 2020 - 02:36 PM

Here is the beauty we are talking about
 
8.25" achromat

 


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#14 VVObserve

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Posted 29 November 2020 - 09:29 PM

Well it is a cloudy night here in Pennsylvania so I must postpone taking a peek through the folded refractor.

 

Collimation of the mirrors:

 

The Folded Refractor's 6" and 5" mirror flats are mounted in lens cells with collimation screws.  

I achieved the gross angles of reflection needed by tilting the entire cells when I attached the cells to the plywood face plates of the OTA.

Finer collimation is done with the adjustment screws on the cells.

So far I have done only rough collimation looking down the focus tube to align the mirrors.

I need to progress to fine collimation.

Several collimators are in my tool box - a peep tube with cross wires, a cheshire collimator, and an autocollimator - all 1 1/4"

I also have an odd 2" red dot collimator that has a red led mounted in the center of a clear plate...at least I think it is some sort of collimator.

 

Does anyone have experience collimating a folded refractor?

Thanks for any advice.

 



#15 Alrakis

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Posted 30 November 2020 - 12:37 AM

Dave Trott is here on cloudynights and could help. 

 

Check out his website on adjusting a folded refractor. 

 

http://davetrott.com...lded-refractor/

 

Chris 


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#16 PETER DREW

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Posted 30 November 2020 - 04:52 AM

I made my folded refractors before specialist tools and lasers were available.  Initial collimation was judged using a black and white bullseye target,  The target was set up with the bullseye facing the objective, a few feet away and centred on the objective.  You looked through a small hole in the target which revealed a reflection of the target from all of the optics which were then adjusted until the images were concentric.  Final collimation was done on a real or artificial star.


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#17 VVObserve

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Posted 30 November 2020 - 10:25 AM

Collimation of the Folded Refractor

 

Viewing through the first collimation instrument, a simple pinhole tube with cross wires, I discovered the first issue.  

The back plate holding the focuser is slightly out of square with the baseplate causing the focus tube to be a degree or so off axis.

Correction will be to square up the back plate assembly with the baseplate of the OTA.  This should bring the focuser and the lens onto parallel axes.  Once this construction error is corrected I can proceed with collimation.

 

Thanks for the collimation tips...this seems to be somewhat uncharted territory...



#18 DAVIDG

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Posted 30 November 2020 - 01:36 PM

 I have made  a number of tilted optical system. It is very easy to collimate them with a laser. The laser is placed in the focuser and the focuser is adjusted so the spot falls on the exact center of the first folding flat. Now  that flat is adjusted so the laser spot falls on the center of the second flat. Now the second flat is adjusted so the laser spot falls in the exact center of objective. Now the scope is pointed at a wall  about 6' to 10' away.   You should see the bright spot on the wall that is coming directly from the center of the lens. If the lens is tilted you will also see a couple of other spots around the main one. Adjust the tip and tilt of the lens so all spot converge into the one in the middle. Your folded refractor is now perfectly collimated.  Making a couple of circular masks the size of the flats and objective with the center marked makes adjust them so laser is hitting the exact center a bit easier.

 

                            Happy Holidays !

 

                                  - Dave 


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#19 VVObserve

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Posted 30 November 2020 - 05:07 PM

 I have made  a number of tilted optical system. It is very easy to collimate them with a laser. The laser is placed in the focuser and the focuser is adjusted so the spot falls on the exact center of the first folding flat. Now  that flat is adjusted so the laser spot falls on the center of the second flat. Now the second flat is adjusted so the laser spot falls in the exact center of objective. Now the scope is pointed at a wall  about 6' to 10' away.   You should see the bright spot on the wall that is coming directly from the center of the lens. If the lens is tilted you will also see a couple of other spots around the main one. Adjust the tip and tilt of the lens so all spot converge into the one in the middle. Your folded refractor is now perfectly collimated.  Making a couple of circular masks the size of the flats and objective with the center marked makes adjust them so laser is hitting the exact center a bit easier.

 

                            Happy Holidays !

 

                                  - Dave 

Perfect,  I just ordered a Farpoint laser collimator.

 

Thanks Dave for the tip!



#20 VVObserve

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Posted 30 November 2020 - 06:29 PM

Maybe a dumb idea.

Thinking of  placing a center spot on the objective for laser collimation.



#21 dan_h

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Posted 30 November 2020 - 07:03 PM

Maybe a dumb idea.

Thinking of  placing a center spot on the objective for laser collimation.

Cut a paper mask to fit the outside of the objective. If you use white paper you can see the laser dot through it.  If you want a more permanent tool, cut it out of milky transparent plastic.  Use to be able to get sheets inside binders often provided as dividers.  Probably get something suitable at Staples etc. 

 

Don't mark that beautiful lens. 

 

dan



#22 VVObserve

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Posted 30 November 2020 - 07:05 PM

Cut a paper mask to fit the outside of the objective. If you use white paper you can see the laser dot through it.  If you want a more permanent tool, cut it out of milky transparent plastic.  Use to be able to get sheets inside binders often provided as dividers.  Probably get something suitable at Staples etc. 

 

Don't mark that beautiful lens. 

 

dan

Of course.

I guess I knew it was a dumb idea.

I have white paper perfect for a mask.



#23 DAVIDG

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Posted 30 November 2020 - 07:22 PM

Maybe a dumb idea.

Thinking of  placing a center spot on the objective for laser collimation.

 I just use  standard  printer paper and draw the circle with a compass to make a mask.  The compass point will  mark exact center of circle  on the mask and the laser will easily show on the paper and through it. 

  Be sure to check that your laser collimator is correctly aligned. My own and others I have checked have been off right out the box. You should place the laser collimator in a V block and  rotate it while observing the laser spot on a wall about 10 feet away. The spot shouldn't move. One of mine made a circle about 12" diameter ! I personally use the  lathe in observatory's machine to hold the collimator and shine the laser beam out the tail stock with the collimator mounted in a three jaw chuck.  

               - Dave 


Edited by DAVIDG, 01 December 2020 - 09:51 AM.

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#24 VVObserve

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Posted 01 December 2020 - 09:51 PM

Quote "The laser is placed in the focuser and the focuser is adjusted so the spot falls on the exact center of the first folding flat. "from DavidG.

 

I had assumed the focuser would be parallel to optical axis of the objective.  should the optical parts be angled toward one another?

 

I placed the objective perpendicular to the base and then used the base plate as my measuring platform for other components.

The first reflective mirror after the objective is Mirror 1 at an angle of about 7 degrees from the lens optical axis tilted toward the second optical flat Mirror 2.

I have the focuser aligned on the same axis as Mirror 2 center parallel to the lens optical axis.

Mirror 2 is angled at 1 degree

It took me a bit to think this one through.

I think I have to obtain certain mirror angles of tilt  while keeping the objective and focuser aligned on parallel axis but also aligned to the center spots of the mirrors 1 and 2 respectively.

This is how I (hopefully) constructed the OTA

It seems there will be a slight off center visual appearance of axes through the focuser to the mirrors even though the scope is collimated.


Edited by VVObserve, 01 December 2020 - 09:54 PM.


#25 DAVIDG

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Posted 02 December 2020 - 09:52 AM

 The mechanical axis of the focuser needs to be aligned with the optical axis of the telescope. If not the eyepiece will be tilted with respect to  the focal plane and you will get astigmatism. So my method of aligning the system using a laser and starting at the focuser ensure that all the optical elements are precisely aligned.  When finished the focuser will be aligned to the optical axis of the objective.

   Here is a picture of my friends 6" Schupmann refractor that I refigured the optics for. The layout of the optics is very similar to your folded refractor forming a |/|   shaped path like yours.  It uses a singlet objective that feeds  a field mirror that reflects to a mirrored corrector and then to the eyepiece.  The design is a bit more complex then your folded refractor and by  using a laser it took a few minutes to collimate perfectly.

 

                  - Dave 

 

6schupmannmattandme.jpg


Edited by DAVIDG, 02 December 2020 - 11:17 AM.

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