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Folded 12" Alvan Clark

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#1 jragsdale

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Posted 15 April 2024 - 09:34 PM

Greetings ATMers! I'm a regular on the Classics Forum but I thought this would be better posted here, since I'm basically starting from scratch building my first folded refractor. And of course, as ambitious as I am, it's a 12" f/15.6 Alvan Clark, the 1868 one formerly owned by Leslie Peltier of Starlight Nights fame. The rest of the scope and mount was unsalvageable (within financial reason), so I only have the original lens, which was in safe keeping and is in great shape. This lens is notoriously high performing, even for an Alvan Clark. I'd love to honor it by making a straight through classic refractor, but alas, I don't have a permanent observatory large enough to house such a beast. Folding will allow this to be usable/portable/shareable in many more ways than a permanent installation could. And with today's technology, I know a folded refractor can be made in a way that doesn't negatively affect the original quality of the lens doing the refracting. So far I have a rough idea on what I'd like to build, a N (or Z, which is a sideways N) fold design with 2 internal mirrors. I estimate a 10" and 6" are about right for this design and would yield a scope about 5 feet long and around 100lbs. I already have a mount sufficiently large to handle the scope (12" Schaefer with a 250lb capacity). The next step is designing and starting construction on the OTA. I did already find the smaller flat; which is 6.3" diameter, 3/4" thick and 1/13 wave PV (1/67 wave RMS). Just need to get it aluminized. Also still on the hunt for a 10" x 2" flat, 1/10 wave or better. Preferably fused silica or zerodur (if that matters). Any tips or advice would be appreciated, thanks!
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#2 CharLakeAstro

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Posted 15 April 2024 - 10:25 PM

TMB (Thomas Back) - 10 inch f/9 Triplet, mounted in a Matthias Wirth designed folded scope. Some ideas...?

 

 

Edit (Pics removed - not relevant to OP design)

 


Edited by CharLakeAstro, 15 April 2024 - 11:24 PM.


#3 jragsdale

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Posted 15 April 2024 - 10:41 PM

TMB (Thomas Back) - 10 inch f/9 Triplet, mounted in a Matthias Wirth designed folded scope. Some ideas...?

Pretty scope, and looks like an easy to use mount. I'm a Thomas Beck fan as well. But I don't see anything that would help me on my project. Thanks for posting though!



#4 starspangled

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Posted 15 April 2024 - 11:00 PM

Folding refactors is a bad idea. Every aluminized surface scatters light and reduces the advantage of a refracting telescope .

 

I did a really simple experiment with some high quality binoculars and a lamda/20  aluminized  12"flat and alternated between a direct view and bouncing off the mirror and the loss of contrast was really obvious.


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#5 Bob Campbell

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Posted 15 April 2024 - 11:09 PM

Emile Schaer:

 

http://astrosurf.com/ursa/saes.htm

 

Foldedrefractor.jpg

 

Maybe with a deeper dive, you could dig up some construction notes/posts. Probably in French smile.gif

 

He seems like he was one of the experts in Z folded refractors of significant aperture and focal length

 

Bob


Edited by Bob Campbell, 16 April 2024 - 12:07 AM.

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#6 JohnH

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Posted 15 April 2024 - 11:12 PM

Large telescopes have used mirrors in them notably Helio stats
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#7 CharLakeAstro

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Posted 15 April 2024 - 11:13 PM

Looking at the size of the single-folded 10" f9, and given your much longer focal length, your idea of an N (Z) fold using two mirrors would be better. Dave Trott's site has this one... which I assume represents the direction you are looking

 

https://davetrott.co...lded-refractor/

 

Six-Inch-Folded-Refractor-Schematic-Smal

 

Pretty scope, and looks like an easy to use mount. I'm a Thomas Beck fan as well. But I don't see anything that would help me on my project. Thanks for posting though!

 


Edited by CharLakeAstro, 15 April 2024 - 11:14 PM.

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#8 jragsdale

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Posted 15 April 2024 - 11:20 PM

Folding refactors is a bad idea. Every aluminized surface scatters light and reduces the advantage of a refracting telescope .

 

I did a really simple experiment with some high quality binoculars and a lamda/20  aluminized  12"flat and alternated between a direct view and bouncing off the mirror and the loss of contrast was really obvious.

Light scatter is all due to surface irregularities and smoothness. So if you get a flat that's made to be a reference for tooling (which prioritize flatness, not smoothness) that doesn't mean that it has a smooth surface and is suited for optical use. Zambuto talks ad nauseum about the topic. And I've already talked to several optical experts who said that this should be a very doable project given smooth proper optical flats. Maybe someday I can build a 2nd tube (or make a replica of the original) that's straight through, and have a place to mount and use it, but that's just not realistic at the moment. It's 2 choices; sit in a box (which it's already been sitting in a box for 44 years), or build this into a folded refractor. My question is not; should I fold this? My question is, how do I BEST fold it.


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#9 CharLakeAstro

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Posted 15 April 2024 - 11:22 PM

Archived German paper...

https://web.archive....refraktoren.pdf


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#10 jragsdale

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Posted 15 April 2024 - 11:23 PM

Looking at the size of the single-folded 10" f9, and given your much longer focal length, your idea of an N (Z) fold using two mirrors would be better. Dave Trott's site has this one... which I assume represents the direction you are looking

 

https://davetrott.co...lded-refractor/

Yeah, that's a good example of what I have pictured in my mind. Just a lot bigger, haha. An oblong shape comes to mind, about 21" wide and 14" tall.


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#11 starspangled

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Posted 15 April 2024 - 11:56 PM

Light scatter is all due to surface irregularities and smoothness.

Aluminium coatings scatter light . The flat I did my experiment with had been polished on the same system that polished the original LIGO optics . It was flat and smooth . 
 


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#12 jragsdale

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Posted 16 April 2024 - 12:14 AM

Aluminium coatings scatter light . 

In a hypothetical perfect mirror with perfect coatings layer; no they don't. Good debate for a different post but let's stay on topic please.


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#13 Oregon-raybender

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Posted 16 April 2024 - 02:01 AM

I would fold the optics. I seen many folded optics at RTMC, with little

or no issues. A basic AL and good flat will do fine. The tube will

have air currents, vents will be needed, to cool down.

Flats 1/10 to 1/8 wave is all that is needed. You have to remember

the AC optics will have color and perhaps other issues.

Keep it simple.

 

Do you have any test reports to share?

 

Just enjoy the lens for what it is.gramps.gif

 

Starry Nightswaytogo.gif


Edited by Oregon-raybender, 16 April 2024 - 02:02 AM.

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#14 jragsdale

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Posted 16 April 2024 - 05:16 AM

Do you have any test reports to share?

Here's a test report of the smaller flat. Still looking for the 10" flat.

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#15 Oberon

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Posted 16 April 2024 - 05:29 AM

In a hypothetical perfect mirror with perfect coatings layer; no they don't. Good debate for a different post but let's stay on topic please.

Yeh well Ligo is about as perfect as it gets, and you’re arguing with the expert who knows it better than anyone else on the planet, so probably best not to contradict as though you know better. It was good advice, and a rare treat.

 

But you couldn’t know that, and still want folded optics irrespective, which is OK too, so fair enough to request a stay on topic. 


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#16 PETER DREW

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Posted 16 April 2024 - 06:00 AM

I've built several 8" aperture Z type folded refractors including a pair for a binoscope project and folded a 6" AP Super Planetary for a client so that it would fit into his observatory.  None with any noticeable issues.  I once had a 12" F15 achromat in a brass cell that I was going to fold but ended up selling it as the objective for a Camera Obscura.

Any mechanical/optical means that permits an objective to be used rather than not is a good plan in my opinion. 


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#17 jragsdale

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Posted 16 April 2024 - 08:18 AM

Yeh well Ligo is about as perfect as it gets, and you’re arguing with the expert who knows it better than anyone else on the planet, so probably best not to contradict as though you know better. It was good advice, and a rare treat.

 

But you couldn’t know that, and still want folded optics irrespective, which is OK too, so fair enough to request a stay on topic. 

I don't mean to come off as dismissive. I'm sure that he's right then to a certain degree. And I like the information and the sharing of experiences, I just really don't want to get sidetracked onto a "better to do nothing than something" narrative. (e.g. why build a car with as low of rolling resistance possible when you can just build an airplane that has none?)


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#18 elstargazer12

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Posted 16 April 2024 - 09:36 AM

jragsdale,

 

Like you I've also posted here and there in many forums here on CN over the years, with some projects (not as cool as yours!) here in ATM.  I can tell you, I've been surprised by the amount of naysaying and negativity that's on this forum compared to the others.  Several times I've posted an idea that's clearly not absurd and replies come back to the point of "that's crazy and will NEVER work!!!".  Guess what, they all worked, to an acceptable extent at least.  And most of all, I had fun doing them including the trial and error, and that's why we do ATM.  And even those replies sometimes have good points, you just gotta ignore the negativity and drama.  

 

Of course, there are always a LOT more replies that have a healthy tone and are encouraging and helpful.  So I'd say stick with it, it's worth engaging with this community, just don't get baited to countering to every dismissive reply you get. 

 

I wonder if it's because, by its nature, ATM means people have put more effort into forming their opinions, and so they are expressed more strongly.  But what do I know!

 

Anyways, wishing you good luck on the folded refractor and I've been reading your post on Classics- wonderful thing you're doing. 

 

To the point at hand, I'm sure your folded refractor will perform just fine.  Does each aluminized surface cause some scatter?  Yes of course.  Is it enough to destroy the view even if you do it with the right parts?  No, of course not.  Have you SEEN how many reflective surfaces the light does through on the newest professional optical observatories?  Especially adaptive optics and interferometers (search Keck interferometer layout, for example), but even just more "standard" optical trains.  Granted your flats and coatings aren't going to be as good as the ones on Keck, but anyone can do the test removing the diagonal from your fav refractor and comparing the view.  With a good diagonal, I've never seen a difference.  

 

Have fun and don't let anyone discourage you!  I'll repeat my main point: it's still a great community with lots of helpful people.


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#19 jragsdale

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Posted 16 April 2024 - 10:13 AM

Have fun and don't let anyone discourage you!  I'll repeat my main point: it's still a great community with lots of helpful people.

Thanks for the words of encouragement. I'm not so easily discouraged by a challenge or obstacles (as you've probably gathered after reading my post on how I acquired this lens). In that sense I'm trying to keep the same can-do spirit that Alvan Clark & Sons had; they had no formal education with optics, didn't really know what they were doing when they started, never even toured another optician's shop! So they made up their own techniques and tools and guess what? They ended up being superior to their peers at the time by not just doing it the same as everyone else. I've got a vision of this project in my mind, but just would like to avoid common pitfalls and misconceptions if possible.


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#20 duck

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Posted 16 April 2024 - 11:40 AM

That you can make this telescope weight only 100 lbs is surprising!



#21 jragsdale

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Posted 16 April 2024 - 11:57 AM

That you can make this telescope weight only 100 lbs is surprising!

It was napkin math, but should be pretty close if I can build it like I want to.



#22 Oregon-raybender

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Posted 16 April 2024 - 01:36 PM

The flat looks fine. If you can get another like it,

you are good to go. A simple box would do for

the tube. One side with solid support, like a flat

optical rail or table, then just add covers.

 

Do you have any test reports on the

12 inch lens ?  Even a simple star test with a Ronchi screen,

80 or 100 lines.

 

I have been following this on ATS.

 

Starry Nightswaytogo.gif


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#23 Astrojensen

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Posted 16 April 2024 - 01:55 PM

Folding the scope to make it actually usable is the right thing to do. I've seen a 12" f/10 in straight, and it was ginormous! The observatory was the size of a small villa. And that was just an f/10! 

 

 

Clear skies!

Thomas, Denmark


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#24 jragsdale

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Posted 16 April 2024 - 01:58 PM

Do you have any test reports on the

12 inch lens ?  Even a simple star test with a Ronchi screen,

80 or 100 lines.

No but I'd like to if possible. I have a friend with a big flat we will try to DPAC it once it's all setup and installed.

 

I'm very curious because even Alavan Clark himself said this lens specifically was the best he had ever seen. And he had quite the discerning eye!


Edited by jragsdale, 16 April 2024 - 02:04 PM.

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#25 Oregon-raybender

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Posted 16 April 2024 - 02:08 PM

If you check the ATS site, you will see that many of the members

have experience with the testing. (Peter C, Roger C, and me )

 

Should be interesting.

 

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