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Mike I. Jones
Carpal Tunnel


Reged: 07/02/06
Posts: 2874
Loc: Fort Worth TX
Herrig double-bounce telescope
      #1127895 - 09/02/06 10:46 AM Attachment (92 downloads)

This latest discussion on schiefspieglers reminded me of a very interesting 8" aperture double-bounce TCT design Edward Kutter published in the November 1997 issue of S&T. Light enters a defined entrance 8" aperture stop, bounces off of an inclined 12.6" diameter convex mirror M1, angles up to another inclined 11.2" concave secondary M2, reflects back to another part of M1, reflects back to another part of M2, then slides by under the edge of M1 again to an inclined focal plane. I entered the design in ZEMAX and optimized the angles a little. Better than 1/10 wave RMS performance over a 1/2 degree FOV. Hard to believe it works but it does (see attached plots). From an interesting optical design viewpoint it gets five stars. From a practical standpoint, it ain't. Make 12.6" and 11.2" mirrors for only an 8" aperture? Bounce twice off each mirror with 12% reflection loss each time, giving 0.88^4=60% transmission? You build one of these for fun and to satisfy curiosity's sake, not for any practical reason. Am I going to build one? You Bet!

Attachment

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56 mirrors, lenses, 16" f/6 Newt, 6" f/10 refractor, TOA-130S, Tinsley 5" f/15 Mak, 6" f/4 RFT, Coronado PST. Still to build: 24" f/10 Modified Dall-Kirkham, 10" f/26 Mak, 8" f/12 apo, spectrohelioscope, Herrig, Schupmann, and others.


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Mike I. Jones
Carpal Tunnel


Reged: 07/02/06
Posts: 2874
Loc: Fort Worth TX
Re: Herrig double-bounce telescope new [Re: Mike I. Jones]
      #1127902 - 09/02/06 10:48 AM Attachment (76 downloads)

Here are the spot diagrams from the Herrig system I optimized. If anyone wants the ZEMAX file or for me to just post the design I will.
Mike

Attachment


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refractory
Pooh-Bah


Reged: 02/05/05
Posts: 1337
Re: Herrig double-bounce telescope new [Re: Mike I. Jones]
      #1127936 - 09/02/06 11:20 AM

A couple of months ago I thought of making a telescope utilizing the phi-based spiral curves (such as in the mollusk Nautilus). Also too much light loss (and a LOT more reflections than you have here). Every ray keeps the same initial angle of incidence at every reflection, even though each ray has a different one originally.

Given the difficulties one would have in actually constructing one of these (such as how would you grind, polish, or better yet even test such as mirror?), I doubt that we'll ever see one (unless its built from the ground up, or biologically grown...)

Jess Tauber

--------------------
"Keep an open mind, but not so open that your brain falls out." - Richard Feynman


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highertheflyer
sage


Reged: 07/08/05
Posts: 456
Loc: Ft. Worth, Texas
Re: Herrig double-bounce telescope new [Re: Mike I. Jones]
      #1127980 - 09/02/06 11:49 AM

Hi Mike:

Just finished the polishing of a spherical 8" f/5 and will be working today to polish towards that parabola.

Not an easy task for a beginner like me, so gonna keep my fingers crossed and hope I work slowly enough to correct that parabolizing stroke and keep the shape going in the right direction!

The scope you are describing may make a relatively easy grab and go project for an amature mirror grinder like me.

It would be interesting to see the methods used to polish the correct shapes for the telescope you are describing.
To me, just a beginner, it looks like the second bounce mirror uses the same radius of curvature as the "tool" used for the first bounce mirror.

One concaved and one convex....

Kewl!!!

So the "tool" could be used as the second bounce mirror, and should grind and polish as spheroid shapes with the same radius of curvatures.

If that's the case, then using a quality glass blank for the tool should lead towards the second bounce mirror.

That means less HOGGING OUT...

And then assuming a parabolic figure for the first mirror, the "tool" used to figure that parabolic mirror would polish to a strange shape.

So how would ya take that "tool" and shape it for the second bounce mirror?
Back to a sheriold again?

Fun to think about and really a neat idea,
Jim

12.5 F/5 dob
5" Unitron


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Scott Milligan
super member


Reged: 09/08/05
Posts: 116
Loc: MA
Re: Herrig double-bounce telescope new [Re: Mike I. Jones]
      #1128401 - 09/02/06 05:09 PM

Mike:

A Herrig would be a very interesting project; please keep us posted if you decide to go for it.

Scott Milligan


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Mike I. Jones
Carpal Tunnel


Reged: 07/02/06
Posts: 2874
Loc: Fort Worth TX
Re: Herrig double-bounce telescope new [Re: highertheflyer]
      #1128896 - 09/02/06 11:40 PM Attachment (68 downloads)

I wish M1 and M2 could be a matched pair but they aren't. Attached are the construction parameters for this optimized version of Herrig's S&T published design. M1 is made from a 12.5" Pyrex blank, and spherical and convex with a very long 648" (54 foot) radius. To make this convex sphere the matching 648" radius concave grinding tool (ideally also of Pyrex) is first made into a test plate by grinding the back flat and dummy shining it so interference fringes can be seen through it, then figured to a very smooth 648" concave radius sphere with Foucault testing. M2 is easier, with a more manageable radius of 205.6" and concave so it can be directly Foucault tested. As both mirrors are struck twice at different footprints, both spheres need to be very accurately and smoothly made.

The Herrig is not going to be an easy telescope to make well, but of course that's the challenge. I may scale the 8" design down to 6" to save cost on the blanks. Optimization of a Herrig involves a fairly complex ZEMAX merit function to keep the two different used portions of the spheres coincident in 3D, to keep M2 clear of the incoming beam, and M1 clear of the cone of light coming to focus below it. I'm going to make the M1 primary first and get its radius and sphericity good, then tweak the M2 design accordingly before making it.

I'm still working on the McDonald 18" f/8 mirror, then I have a 10" f/7.5 and other mirrors to finish, so I probably won't start on the Herrig until mid '07 or so. I'll definitely keep CN statused as it progresses.

Mike

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Ed Jones
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Reged: 04/06/04
Posts: 2232
Loc: Sin-sin-atti
Re: Herrig double-bounce telescope new [Re: Mike I. Jones]
      #1129591 - 09/03/06 12:36 PM

What do you like about it? Four reflections, 2 oversize optics, doesn't have much going for it. What is the f number and field tilt?

--------------------
Ed Jones




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Art Bianconi
Post Laureate


Reged: 03/06/06
Posts: 4659
Loc: Delaware River Valley, New Jer...
Re: Herrig double-bounce telescope new [Re: Mike I. Jones]
      #1129691 - 09/03/06 01:25 PM Attachment (69 downloads)

Every time I chance upon one these dialogues and see the complexity of optics and fabrication of alternate designs, I find myself more and more appreciative of the simplicity and effectiveness of Dave Groski's optical systems.

Here is the mechanical system I designed to support his optics. How much easier can it get?

Art
==================

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Mike I. Jones
Carpal Tunnel


Reged: 07/02/06
Posts: 2874
Loc: Fort Worth TX
Re: Herrig double-bounce telescope new [Re: Ed Jones]
      #1130114 - 09/03/06 04:59 PM

Quote:

What do you like about it? Four reflections, 2 oversize optics, doesn't have much going for it. What is the f number and field tilt?




Yep - I agree, I made those points in my opening post. But to me the intrigue of successfully making some of these wack-job tilted component unobstructed pupil systems such as Herrig, Stevick-Paul, Yolo, etc. outweighs any practicality issues. As I said previously, I don't need an 8" to be satisfied, 5" or 6" is enough to scratch the itch.

Field tilt is 4.33 deg. oriented as shown in the last figure. EFL is 91.8", or about f/11.5.
Mike


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camvan
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Reged: 05/02/05
Posts: 2142
Loc: British Columbia
Re: Herrig double-bounce telescope new [Re: Mike I. Jones]
      #1130652 - 09/03/06 09:31 PM

*sigh* it's times like this I was better at math and doing constructive things...I could only imagine the fun one would have in figuring something like this out!!! but Art, that Schief diagram is sweet! if I showed something like that to my brother, he'd have it figured out n built pretty quick me thinsk!

--------------------
Cameron

"Aperture can only be replaced by even more aperture. Dark transparent skies cannot be replaced by anything else." - Stathis Kafalis

Intes MN66
Meade SN8
handfull of cheap ep's


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