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Equipment Discussions >> ATM, Optics and DIY Forum

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Mike I. Jones
Post Laureate
*****

Reged: 07/02/06

Loc: Fort Worth TX
Re: 17 inch spotplots new [Re: mark cowan]
      #4979519 - 12/22/11 12:05 PM

Right Mark, it's not a challenging conic to make. Based on Kevin's project and our recent telephone conversation, Carl has already modified his analysis spreadsheet to enable him to properly figure mirrors with conic constants other than paraboloids. Knife edge shifts are simply multiplied by the conic constant, and Carl figures the mirror to have the scaled shifts between zones. A paraboloid has a conic constant of exactly -1. A hyperboloid with conic of -1.4 just means that the KE shifts for the equivalent paraboloid are scaled up by 1.4X. The same goes for Dall-Kirkham primaries, with conic constants ranging between zero and -1. This opens up a mirror maker's capabilities to Rosin, Ritchey-Chretien and Dall Kirkham primaries as well as traditional paraboloids.

Mike


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Benach
Pooh-Bah


Reged: 01/24/08

Re: 17 inch spotplots new [Re: Mike I. Jones]
      #4979631 - 12/22/11 01:09 PM

Mike: To answer your request, I have worked a bit on ESA's COROT mission and the primary telescope is also a TMT with a correction system, so there you have another example of a weirdo scope.

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mark cowan
Vendor (Veritas Optics)
*****

Reged: 06/03/05

Loc: salem, OR
Re: 17 inch spotplots new [Re: Mike I. Jones]
      #4979697 - 12/22/11 01:44 PM

Quote:

What I've seen so far tells me that for both elements, radii should be held to 0.1% either side of nominal, thicknesses should be within ±0.005", and element wedge should be less than 0.001".




Is this absolute or can the spacings be adjusted to accommodate as-manuf actual specs? Both Kevin (I think) and myself are just planning to have these lenses made commercially.

My other question is (obviously) going to be about coatings - we've all seen the kind of charges for small qty coatings on lenses. With 4 surfaces what would any tradeoff look like?

Finally, since I'm already planning to manufacture large fast hyperboloids the quartz primary will be more like a vacation.

Best,
Mark


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MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: 17 inch spotplots new [Re: Mike I. Jones]
      #4979753 - 12/22/11 02:19 PM

Quote:

s Kevin will almost exclusively use this as a visual instrument, there wasn't a need to design for a field wider than 1" diameter. It could be done, but at the expense of sharpness over the central 1" format. I have experimented with wider FOV's using a third element, but so far I have encountered diminishing returns



Oh that goes without saying, Mike. Ed's configuration is awesome, even if the angular image field is somewhat restricted. It should be mentioned that its performance off axis is better than a classic paraboloid of the same aperture and focal length. Likewise, Ed's design is more compact than a classic Newtonian version.

The only thing I see as being somewhat more of a challenge is (1) the hyperboloidal primary, and (2) the fact that Ed's design is a compound system. As such it would have to be figured at the focus by autocollimation or interferometry. So, a lot more work than an ordinary Newotnian.

Mladen


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kfrederick
Carpal Tunnel


Reged: 02/01/08

Re: 17 inch spotplots new [Re: MKV]
      #4980152 - 12/22/11 06:22 PM

Much thanks for the cool telescope .The best image I have seen was in my 20 f8 chief[ED Jones design] .Mark Harry is helping with testing of the secondary and lens .I have the best in the design and optics .The bigest problem is the perfect structure .Thanks to the "Jones" for this unreal unobstructed reflector design. It does work!

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Mike I. Jones
Post Laureate
*****

Reged: 07/02/06

Loc: Fort Worth TX
Re: 17 inch spotplots new [Re: mark cowan]
      #4980192 - 12/22/11 07:04 PM

Right Mark, I plan to be standing by as Kevin gets his optical elements finished, starting with Carl's finished primary radius and conic, and then as the lens elements are finished. The convex secondary should likely be finished last, as its radius and tilt angle can be modified slightly to peak up the final design performance.

The tightest tolerances are required if one were wanting to build perhaps 100 or 200 of these Chiefs in production, and didn't want to have to spend labor hours mixing and matching lens elements and secondaries in each assembly. But, 0.1% on radii, ±0.005" on thicknesses and wedge <0.001" are not unreasonable specs for quality lens suppliers, provided the manufacturer has test plates and metrology to guarantee precision and quality control.

These tolerances can be relaxed by perhaps 2X or 3X as long as the secondary is done last and can be tweaked to compensate, then all the tilts and decenters optimized using all finished components.

Mike


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gatorengineer
Carpal Tunnel
*****

Reged: 02/28/05

Loc: Hellertown, PA
Re: 17 inch spotplots new [Re: Mike I. Jones]
      #4980320 - 12/22/11 08:46 PM

Guys,

I admire the energy and enthusiasm, and investment. But I really have to wonder if a scope like this is truly buildable..... I was ready to go on a traditional 8 F8 chief, until I really got into the necessary tolerances for fabrication, which were less than 1/8".... Which being a plywood guy is about the limit of my skills.

A scope requiring 0.01, or thereabouts isnt even aluminum because of thermal expansion issues, its Carbon Fiber..... Then when you figure in the need to hold that from horizon to zenith, well.... Its an impressive undertaking in a large scope....

Perhaps I am just not understanding....

To me studying all of the alternate designs out there, I continually wonder why there arent alot more large (10" and bigger) Schupmans out there....


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Ed Jones
Carpal Tunnel
*****

Reged: 04/06/04

Loc: Sin-sin-atti
Re: 17 inch spotplots new [Re: Mike I. Jones]
      #4980364 - 12/22/11 09:19 PM

Mike,
Great design as usual! It looks to be a Chief version of a hyperbolic Newtonian except with a convex secondary. This goes to show that there are a bunch of designs that have both a Newtonian and an unobstructed Chief analog. This design has good correction for coma. My only criticism is the 7 degree field tilt which is higher than most Chiefs.

The tolerances may not as severe as you think. I haven't done any tolerancing but guessing that it shouldn't be a lot different than the Chief's I've built. When you do tolerancing you can use the primary tilt as a compensator as well as rotation of the lenses as a group. The Y centration of the lenses relative to each other is critical and needs an accurate cell but I make this adjustable to simplify things. In the Chief small lens radius errors cause mainly a little bit of CA.


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kfrederick
Carpal Tunnel


Reged: 02/01/08

Re: 17 inch spotplots new [Re: Ed Jones]
      #4980805 - 12/23/11 07:10 AM

I think building the structure will be fine The top piece is small 4.5x28 inches It has to hold The 4 inch secondary two lens and focuser and laser target .If that is made correct then it is like making a newtonian .The f8 optical system is very easy to keep in focus and that has to be seen to understand f8 rocks This will do deep sky great. Black sky .Only two small lens and go unobstructed .Very excited and thankfull for the design work. What you think of calling it a "MTT" Mike Tilted Telescope??

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kfrederick
Carpal Tunnel


Reged: 02/01/08

Re: 17 inch spotplots new [Re: kfrederick]
      #4982498 - 12/24/11 08:43 AM

Mike Was looking at testplates at http://www.optimaxsi.com/Resources/TestPlateLibrary.php and they have some close I thought if i ask for a quote it it would be less if we match there testplates work .For the second lens they have a 973.212 mm and a 534.228 That is the hardest lens to make .The first lens they have a 1232.70 .Mark Harry and I can do them if it doesnot work out ,Having them made. Any body know of other lens makers that might be usefull .Thanks hope more will try one.

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wh48gs
Carpal Tunnel
*****

Reged: 03/02/07

Re: 17 inch spotplots new [Re: Mike I. Jones]
      #4984793 - 12/25/11 09:07 PM Attachment (49 downloads)

My OSLO version doesn't have neither of the two Ohara glasses used for this system. The close Schott analogs, LASF3/PSK52 produce similar result (with a bit of tweaking). Even simpler Schott glasses work well, including combinations with one lens being BK7 (example attached). So there is some flexibility with respect to glasses that can be used.

This type of corrector is probably the best ATM choice for catadioptric tilted elements telescope, but there is a price to pay. Achieving unobstructed design by adding more elements is never a clear gain. It is not only the added fabrication requirements, it is also the increased difficulty of achieving and maintaining near-perfect alignment. In this particular system, axial polychromatic Strehl for perfectly fabricated and aligned lenses in the 450-660nm range is 0.96 (BK7 version), comparable to 14% central obstruction (diameter). Less than perfect fabrication/alignment could easily make it comparable to ~20% c.o., which is achieavable in this aperture size. The likelihood of larger average alignment error for monochromatic aberrations comes extra. That's the reality of it.

Vla


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kfrederick
Carpal Tunnel


Reged: 02/01/08

Re: 17 inch spotplots new [Re: wh48gs]
      #4984915 - 12/25/11 11:36 PM

Via You have to admit. Mikes design is very good.Maybe better than anything in 17 inch .All for two little lens and a fancy holders .Thanks for the work .All good

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MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: 17 inch spotplots new [Re: kfrederick]
      #4985022 - 12/26/11 02:19 AM

Quote:

Mikes design is very good.Maybe better than anything in 17 inch .All for two little lens and a fancy holders .Thanks for the work.



No one questions Mike's design. But Vla makes a valid point. It's one thing to compare theoretical designs; it's an entirely different thing to make valid comparisons of finished products.

The only way to truly compare two configurations realistically is to make them of the same quality and test them next to each other under identical conditions - at the focus.

The rest is just theory that cannot predict all the complicating factors of fabrication that will determine the actual final image quality. And, the shape of the wavefront converging to a focus is what matters in the final analysis.

Clearly, "Chiefs" have a distinct advantage of being somewhat more compact than your ordinary Newtonians. Beyond that I am not aware of anyone making a direct aperture per aperture, same quality, direct scientific comparison in the field.

Mladen


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kfrederick
Carpal Tunnel


Reged: 02/01/08

Re: 17 inch spotplots new [Re: MKV]
      #4985110 - 12/26/11 07:01 AM

Mladen I have a 20 f8 Chief for 2.4 years and have a 20 f5 newt for over 20years .That compairson has been done .This design CAN give a better image than anything .The unobstructed has many good points that is not reflected in the plots .F8 optical system Great stray light control /no dew problems .lots of back focus .I understand what your saying about it being harder . There are many designs that are worth trying for the ATM .Thanks for your time

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Dave O
sage
*****

Reged: 12/21/11

Loc: Sri Lanka
Re: 17 inch spotplots new [Re: kfrederick]
      #4985162 - 12/26/11 08:29 AM

I think one thing that should not be forgotten is that Mike's design offers a well corrected field of 1 inch diameter; better than that of an uncorrected Newtonian of similar aperture and focal length. When you add correcting lenses to the Newtonian, you will likely have to increase the central obstruction to accommodate them.

Another advantage of Mike's design is that access to the eyepiece is much better. A 17" f/8 Newt with minimum obstruction will place the diagonal about 10' from the primary mirror -- a pretty good reach for most of us.

What Mike has done is:

1) provided an excellent usable field of 1 inch diameter;
2) eliminated the Central Obstruction; and
3) provided improved access to the eyepiece in a 17" f/8 telescope...

... if everything can be kept in alignment ...

Dave O

Edited by Dave O (12/26/11 09:13 AM)


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kfrederick
Carpal Tunnel


Reged: 02/01/08

Re: 17 inch spotplots new [Re: Dave O]
      #4985247 - 12/26/11 10:06 AM

Dave O thanks for puting Mikes Zemax design in OSLO . Not sure if that has been done .The Oslo spot plots are done by Dave O using Mikes numbers .Great to have that tool .

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Mike I. Jones
Post Laureate
*****

Reged: 07/02/06

Loc: Fort Worth TX
Re: 17 inch spotplots new [Re: wh48gs]
      #4985668 - 12/26/11 02:48 PM

If you don't have the glasses used in this scope, you need to upload the latest version 6.5.4 of OSLO-EDU at http://lambdares.com/education/oslo_edu. It's freeware, so there's no reason to have an out-of-date version of it.
Mike


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Mike I. Jones
Post Laureate
*****

Reged: 07/02/06

Loc: Fort Worth TX
Re: 17 inch spotplots new [Re: Dave O]
      #4985670 - 12/26/11 02:53 PM

Thanks Dave, but credit should be given where properly due. Ed Jones invented the basic Chief design concept of tilting the primary, folding with a flat, and correcting the beam with two weak tilted/decentered figure-of-revolution lens elements. I merely improved it slightly to open up the spectral band and sharpness over the FOV by (1) going to more exotic glasses and (2) allowing the fold flat to take on a spherical radius.
Mike


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DAVIDG
Post Laureate
*****

Reged: 12/02/04

Loc: Hockessin, De
Re: 17 inch spotplots new [Re: Mike I. Jones]
      #4985709 - 12/26/11 03:28 PM Attachment (20 downloads)

I think credit needs to be given to Dick Buchroeder as well. Dick published an article back in 1971 Jan. Sky and Telescope "CHT: A Catadioptric Herschellian Telescope with Tilted Components". Dick's design uses the same concept with two lenses to corrector for the astigmatism and both lens have equal but opposite internal radii, they are also decenterd slightly. Dick's design also uses a flat to bend the light path. Dick's design differs slightly in the lenses are weak menicus in shape vs having plano backs.

- Dave


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kfrederick
Carpal Tunnel


Reged: 02/01/08

Re: 17 inch spotplots *DELETED* [Re: DAVIDG]
      #4985731 - 12/26/11 03:44 PM

Post deleted by kfrederick

Edited by kfrederick (12/27/11 01:20 PM)


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