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Equipment Discussions >> Refractors

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Sky Muse
sage


Reged: 10/26/12

Loc: De Soto County, MS
The Demise of the Great Refractor
      #5525469 - 11/18/12 03:08 AM

An interesting read...

http://www.aip.org/history/cosmology/tools/tools-first-telescopes.htm

First came the refractor, then only a few years later, the reflector. It was all very simple, as the refractor never vied for the reflector's position; nor the reflector for the refractor's, until the creation of the Yerkes forty-inch objective crafted by Alvan Clark...

http://www.google.com/imgres?q=alvan+clark&start=96&um=1&hl=en&am...

...the beginning of the end.

Sadly, optical lenses above forty inches in diameter tend to sag in the center, thus rendering them impractical for serious observation, with the creation of the forty-inch Yerkes objective sounding the death knell for the great refractor in unwillingly nominating its successor to the scientific community, the great reflector...

http://www.catchersofthelight.com/catchers/image.axd?picture=%2F2012%2F07%2F6...

Today, the Keck twins are the largest reflectors on the planet, with whichever one through which one chooses to observe being a toss-up.

It is most regrettable that the superior refractor, and due to said diametrical limitation, has been supplanted by mirrored contraptions, but no matter, as modern-day scientists have known no other by which to conduct their studies, and are satisfied, for the time being.

The dominance of the reflector through unfortunate chance has also resulted in a proliferation of large-apertured reflectors in the hands of the public, with said possessors substituting quantity over quality, an obstructed, indirect primary over that unobstructed and direct, and born, sadly, of far less expensive glass and metallic coatings.

Nothing is impossible, therefore there is a way to create an efficient, unobstructed objective surpassing even the greatest of the great reflectors, and beyond, the formula laying alongside cures for cancers and other seemingly uncontrollable diseases, and the eventual realisation of travel to the nearest planet outside our solar system, thus a fitting end to the dominance of inferior mirrored arrangements.

Alan


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Astrojensen
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Reged: 10/05/08

Loc: Bornholm, Denmark
Re: The Demise of the Great Refractor new [Re: Sky Muse]
      #5525488 - 11/18/12 03:40 AM

We often hear the reason for not making bigger refractors than 40" is that their objectives would sag and render them useless. I am not sure about that. I do remember some rumours that the late Thomas M. Back had calculated that a 60" objective (of to me unknown design) would be well within the possible. Then there's the 50" Paris lens. While the telescope as a whole was a spectacular failure, the objective has never been tested with modern equipment and there's a real possibility that it would work well. And it still exists! Yes, the Yerkes objective is *not* the largest in existence!

But apart from the sagging, there are many reasons for the demise of the refractor as a large telescope. The first is the perfect achromatism of the mirror. The importance of this cannot be overstated. Second is that it is possible to support a mirror all across its back. This is important for two reasons: 1) You can make it extremely large 2) You can correct its shape in real time with actuators, thus giving us a way to fight bad seeing.
A third reason is that you can make mirrors of glass that is not optically transparent. This is a MAJOR advantage, once you reach meter-class sizes. Today it's possible to make clear crown and flint glass in large sizes relatively easily, but a half-meter chunk of A-grade optical quality is still going to cost you a pretty penny. A raw blank for a half-meter mirror, on the other hand, is quite inexpensive. Special glass for apochromats in this size is running into astronomical price levels.

But let's say that we, as a thought experiment, want to build a three-meter refractor. Let's play with the idea for a while. Any ideas on how we fight gravity and false color? These are going to be the main issues, I think. Let's leave out the issues of cost. We can also assume that we can get crown and flint glass in the required size.

I have some ideas, but I'll wait and see what people come up with first.


Clear skies!
Thomas, Denmark


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mayidunk
Don't Ask...
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Reged: 02/17/10

Loc: Betwixt & Between...
Re: The Demise of the Great Refractor new [Re: Sky Muse]
      #5525492 - 11/18/12 03:43 AM

Imagine, all the engineering and fabrication that went into the Kecks just to hold several grams of aluminum in the proper position! BTW, my first view of a planet (Jupiter) through any telescope was through a 9" Alvan Clark. It was spectacular!!

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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: The Demise of the Great Refractor new [Re: Sky Muse]
      #5525582 - 11/18/12 06:32 AM

Some stuff to think about:

Refractors make good small scopes but unlike a reflector, their inherent limitations and aberrations increase as aperture increases so they do no make very good large telescopes. Reflectors are just the opposite, their inherent aberrations do not increase with aperture and their limitations decrease with increasing aperture so they are the tool of choice for larger telescopes.

It is important to understand thsee scaling issues when comparing the optimal design. Here's a few comments.

- All telescopes are obstructed and by far the most important obstruction is the outer obstruction, i.e. the aperture. This is what determines the resolution and contrast limits of a telescope. The effect of a central obstruction is small by comparison. This is why a 10 inch reflector can out perform a 6 inch reflector.

- Coma, the primary aberration in a Newtonian, is a function of focal ratio and does not increase with increasing aperture. It is also easily corrected. Chromatic aberration, the primary aberration in a refractor increases with increasing aperture and to scale a design, to maintain comparable levels of false color, the focal ratio of the refractor must be scaled as well. Thus, a 60 mm f/17 achromat is able to bring all the colors nearly to focus at a point but the f/17 Yerkes focus separates the colors by a quarter of an inch... And while better glasses can reduce chromatic aberrations, apochromats are subject to the same scaling. A 25 inch f/5 Newtonian is considered slow, a 25 inch f/5 refractor is an impossibility. Even a 10 inch f/5 without serious color issues is not possible.

- Reflectivity is not a function of aperture, transmission is. The reflectivity losses are the same in a large reflector as in a small one but as refractors increase in aperture, the transmission losses increase. The homogeneity of the glass also is important in a refractor, not in a reflector.

So, to my mind the notion of "the great refractor" is based on the concept that the design compromises that make the refractor the ideal design for a 3 or 4 inch telescope scale in a nice way... But like most things, refractors do not scale linearly, a hummingbird is a marvelous creature. If a human had the aerobic capacity of a hummingbird, a 165 lb human would be capable of producing 8 horsepower continuously for 30 hours. But like the refractor, the hummingbird, as amazing as it is, does not scale well.

As an observer, that is someone who enjoys looking at the night sky with a telescope, understanding some of the basic optical principles, contrast, diffraction, dispersion, resolution, airy disks, coma, etc is useful in optimizing ones choices of equipment...

Refractors are fine telescopes, reflectors are fine telescopes. Both are capable of great magic. Why not build a large (30inch) refractor? A little math guides us to see why that is not a good idea. Why not build a 60mm reflector? A little math guides us in seeing why this is not a good idea.

Bottom line: refractors get better the smaller they are, reflectors get better the larger they are....

Jon " slobbers over both reflectors and refractors" Isaacs


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Sky Muse
sage


Reged: 10/26/12

Loc: De Soto County, MS
Re: The Demise of the Great Refractor new [Re: Astrojensen]
      #5525805 - 11/18/12 10:20 AM

The late, honourable Mr. Back was obviously on to something before he passed.

The Paris refractor was never mounted, however I wouldn't be interested in resurrecting it, given Yerkes' extancy. Companioned to the Yerkes, the two would exhibit an identical negligibility in monument as that between a four and five-inch refractor in miniature.

Imagine a 25-metre refractor, and observing from the opposite end not as one would through an ocular with the eye, but by entering a short walkway up and before an eight-foot lens.

In that it may be imagined, it may exist.

"Today it's possible to make clear crown and flint glass in large sizes relatively easily..."

Yes, as Alvan Clark made a forty-inch over a hundred years ago, but this is the 21st century, and the dawn of far greater possibilities.

"I have some ideas, but I'll wait and see what people come up with first."

Transparent metals..."transparent aluminum"?

Again, in that it may be imagined...

Cheers,

Alan


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SeattleScott
Pooh-Bah


Reged: 10/14/11

Re: The Demise of the Great Refractor new [Re: Sky Muse]
      #5525821 - 11/18/12 10:32 AM

Jon, congratulations on hitting post # 30,000!

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Sky Muse
sage


Reged: 10/26/12

Loc: De Soto County, MS
Re: The Demise of the Great Refractor new [Re: SeattleScott]
      #5525848 - 11/18/12 10:50 AM



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Starhawk
Space Ranger
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Reged: 09/16/08

Loc: Tucson, Arizona
Re: The Demise of the Great Refractor new [Re: SeattleScott]
      #5525864 - 11/18/12 11:00 AM

There is something unstated, here- there are design approaches one may be willing to take with a reflector, which if applied to refractive elements, would solve many of the scaling issues.

What I am talking about are segmented mirrors- the path for reflectors of extraordinary size is clearly to put in an array of mirrors, so telescopes like Keck and the larger Magellan telescope are made up of several mirrors, and though the approach causes new aberrations, the fact the central obstruction and spider already cause some has the odd effect of making users feel this is more acceptable.

Now, imagine if one made a frame to mount 6" objective cells, with the ability to start over on thickness issues and a stout frame gained by going from a unitary objective to an array. You coud see how 7 across could yield a 45" telescope. But refractor folks are not going to like it, probably, because there's now an optical artifact from the frame, even though there would be no central obstruction.

-Rich


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jrbarnett
Eyepiece Hooligan
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Reged: 02/28/06

Loc: Petaluma, CA
Re: The Demise of the Great Refractor new [Re: Jon Isaacs]
      #5525897 - 11/18/12 11:19 AM

In way do refractor aberrations increase with aperture and reflector aberrations do not?

While it is true that in a Newtonian reflector coma stays constant so long as focal ratio stays constant, focal ratio rarely stays constant, but instead (by necessity - no one wants a fire truck ladder arrangement for their 30" Dob) reduces, as reflector aperture increases.

Now sure, the Great Lick (36" f/17.37) is very colorful, with different wavelengths coming to focus *inches* of focuser travel apart. But we're talking "refractors" without limitation on design. What if the 36-incher were a TMB triplet design, made by LZOS, instead of its Clark Doublet design? How much less "aberrated" would it have been?

I think the demise of the great refractors was not caused by reflectors as much as it was caused by a "stall" in the evolution of refractor-related technologies; glass-making, optical design, etc. Had Clark been able to cook up and execute a TMB triplet type design, and Schott been able to conceive and execute on low dispersion glass types in the heyday of the great refractor, the type might have had a longer life in professional circles than it did. The Newtonian reflector, too, was rapidly abandoned by professionals as inefficient for serious scientific work. Had someone cooked up a Paracorr and well-corrected wide field eyepiece design back in the day, the Newtonian, too, might have enjoyed a longer scientific relevance than it did.

In fact, had imaging technologies advanced more quickly, we might never have seen scopes like Mt. Wilson or Mt. Palomar. Today a 10" RC with a decebt CCD camera is capable of going deeper and showing more than Palomar did with emulsion plates.

Regards,

Jim

Edited by jrbarnett (11/18/12 11:44 AM)


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Astrojensen
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Reged: 10/05/08

Loc: Bornholm, Denmark
Re: The Demise of the Great Refractor new [Re: jrbarnett]
      #5526113 - 11/18/12 01:08 PM

Quote:

I think the demise of the great refractors was not caused by reflectors as much as it was caused by a "stall" in the evolution of refractor-related technologies; glass-making, optical design, etc. Had Clark been able to cook up and execute a TMB triplet type design, and Schott been able to conceive and execute on low dispersion glass types in the heyday of the great refractor, the type might have had a longer life in professional circles than it did. The Newtonian reflector, too, was rapidly abandoned by professionals as inefficient for serious scientific work. Had someone cooked up a Paracorr and well-corrected wide field eyepiece design back in the day, the Newtonian, too, might have enjoyed a longer scientific relevance than it did.

In fact, had imaging technologies advanced more quickly, we might never have seen scopes like Mt. Wilson or Mt. Palomar. Today a 10" RC with a decebt CCD camera is capable of going deeper and showing more than Palomar did with emulsion plates.




Good observations and thinking. But I do wonder, whether it is possible even today, to build a 40" triplet apochromat? Any insight from optical designers and glass makers? How about a corrector for already existing refractors? Sort of a chromacorr for Yerkes?

Thomas M. Back also designed a 21" f/12 triplet apo and from what I understand, APM/LZOS are actually building one. The design was extremely well corrected for color, given its huge aperture (for a refractor). I think it's safe to assume that a 36" f/17 with identical design would perform extremely well and far better than the current Lick refractor.


Clear skies!
Thomas, Denmark


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mayidunk
Don't Ask...
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Reged: 02/17/10

Loc: Betwixt & Between...
Re: The Demise of the Great Refractor new [Re: Astrojensen]
      #5526157 - 11/18/12 01:30 PM

Doesn't supporting the weight of the objective at the end of the long OTA also have a direct bearing on the feasibility of building larger refractors? It can rightly be said that a truss design would be all that's needed to counter that. However, wouldn't there still be inherent cost savings to building large reflectors what would still mitigate against building refractors of similar aperture? In this day and age of ever shrinking budgets, just because it could be done doesn't mean it ever will be done.

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jrbarnett
Eyepiece Hooligan
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Reged: 02/28/06

Loc: Petaluma, CA
Re: The Demise of the Great Refractor new [Re: Astrojensen]
      #5526264 - 11/18/12 02:42 PM

"Sort of a chromacorr for Yerkes?"

Yep, same idea. What if Alvan Clark had discovered how to make Chromacors?

Regards,

Jim

Edited by jrbarnett (11/18/12 02:43 PM)


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blueman
Photon Catcher
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Reged: 07/20/07

Loc: California
Re: The Demise of the Great Refractor new [Re: jrbarnett]
      #5526269 - 11/18/12 02:45 PM

I think the strength of the refractor is that it works well in smaller sizes. This make is easier to transport and place on a mount and still have good performance.
Plus, the refractor has made astrophotography blossom and become a huge part of our hobby.
But, aperture fever is best treated with the reflector design.
Blueman


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Calypte
Pooh-Bah


Reged: 03/20/07

Loc: Anza, California
Re: The Demise of the Great Refractor new [Re: jrbarnett]
      #5526295 - 11/18/12 02:59 PM

I'm reading through all of this, and a basic assumption seems to be that suitability for visual observation is the criterion for a telescope's usefulness. Can even an APO refractor bring all wavelengths, even into the near-infrared, to focus at the same point? Can it be made to work at f/3.3, which is the prime focus of the 200-inch Hale Telescope at Palomar Observatory on Palomar Mountain? Most of the work currently being performed with the 200-inch Hale Telescope is in the near-infrared, although most that work uses the Cassegrain focus (f/16).

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csrlice12
Postmaster
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Reged: 05/22/12

Loc: Denver, CO
Re: The Demise of the Great Refractor new [Re: Sky Muse]
      #5526304 - 11/18/12 03:03 PM

Quote:

An interesting read...

http://www.aip.org/history/cosmology/tools/tools-first-telescopes.htm

First came the refractor, then only a few years later, the reflector. It was all very simple, as the refractor never vied for the reflector's position; nor the reflector for the refractor's, until the creation of the Yerkes forty-inch objective crafted by Alvan Clark...

http://www.google.com/imgres?q=alvan+clark&start=96&um=1&hl=en&am...

...the beginning of the end.

Sadly, optical lenses above forty inches in diameter tend to sag in the center, thus rendering them impractical for serious observation, with the creation of the forty-inch Yerkes objective sounding the death knell for the great refractor in unwillingly nominating its successor to the scientific community, the great reflector...

http://www.catchersofthelight.com/catchers/image.axd?picture=%2F2012%2F07%2F6...

Today, the Keck twins are the largest reflectors on the planet, with whichever one through which one chooses to observe being a toss-up.

It is most regrettable that the superior refractor, and due to said diametrical limitation, has been supplanted by mirrored contraptions, but no matter, as modern-day scientists have known no other by which to conduct their studies, and are satisfied, for the time being.

The dominance of the reflector through unfortunate chance has also resulted in a proliferation of large-apertured reflectors in the hands of the public, with said possessors substituting quantity over quality, an obstructed, indirect primary over that unobstructed and direct, and born, sadly, of far less expensive glass and metallic coatings.

Nothing is impossible, therefore there is a way to create an efficient, unobstructed objective surpassing even the greatest of the great reflectors, and beyond, the formula laying alongside cures for cancers and other seemingly uncontrollable diseases, and the eventual realisation of travel to the nearest planet outside our solar system, thus a fitting end to the dominance of inferior mirrored arrangements.

Alan




You go astronomizing (is that a word?) with the scope you have, not the scope you wished you had.....


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neotesla
Pooh-Bah
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Reged: 11/18/10

Loc: Canada
Re: The Demise of the Great Refractor new [Re: csrlice12]
      #5526371 - 11/18/12 03:49 PM

It still comes down to cost... An big refractor versus a Dob, SCT or Newt, which one is going to be cheaper? As well think of the mounting necessary and observatory size. Also when you factor in the need to make it a triplet for colour correction, think of the massive weight that would be involved at the front end of the scope that would need to be balanced, and the moment arm that it would entail. When you need massive light gathering capability, you can't beat a large mirror. Especially in this age of adaptive optics, how do you propose to do that with a refractor?

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Eddgie
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Reged: 02/01/06

Re: The Demise of the Great Refractor new [Re: Sky Muse]
      #5526379 - 11/18/12 03:57 PM

Yet another "Refractors are beautiful" post?

There is little justifiction for making a large refractor, and the reduction of performance caused by a central obstruction is vastly over-rated.

But all that aside, what will stop large refractors from becomeing a reality for all but the eccentric, super wealthy is simply the cost.

To get the color correction required for decent performance as the apeture get larger (the CA can be more dibilitating than the contrast loss from a modest sized obstruction), it will be necessary to have a minimum of a triplet, and it may be more practical to just use a four lens design.

And the cost of procuring the four giant blanks of optically excellent glass, then grinding and polishing the 6 to 8 surfaces to curves that vary less than 1/50th of the wavelenght of light, then polishing them and coating them would prove to be so enormously expensive that very few would ever be produced (and I was on the AP wait list for five years, so what does that tell you).

So, this is what keeps the large refractor from being the "Perfect" scope. It costs too much to build them. And if you can't afford it, it matters little that one might exist other than the novelty of it.

And while imperfect per inch of apeture, it is all to easy to get better performance with a slighly larger reflector.

And when professional astronomers figured this out they dumped the refractor and they never looked back.

At small apertures (below about 8") a refractor can indeed be quite superrior to any other design and if one is willing to stomach the price, can give a fantastic amount of pleasure to the owner,

But I can't afford a 14" APO and a building to put it in. Few people could.

And it is not perfect if it is not affordable, which is always been the giant flaw in refractor performance.

Edited by Eddgie (11/18/12 04:04 PM)


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neotesla
Pooh-Bah
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Reged: 11/18/10

Loc: Canada
Re: The Demise of the Great Refractor new [Re: Eddgie]
      #5526610 - 11/18/12 06:47 PM

Just for reference... The Lick telescope's objective, two pieces of glass alone weighs in at 225Kg. The Great Lick refractor is 57 feet long, 4 feet in diameter, and weighs over 25,000lbs.

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DAVIDG
Post Laureate
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Reged: 12/02/04

Loc: Hockessin, De
Re: The Demise of the Great Refractor new [Re: Astrojensen]
      #5526669 - 11/18/12 07:25 PM

Quote:

Quote:

I think the demise of the great refractors was not caused by reflectors as much as it was caused by a "stall" in the evolution of refractor-related technologies; glass-making, optical design, etc. Had Clark been able to cook up and execute a TMB triplet type design, and Schott been able to conceive and execute on low dispersion glass types in the heyday of the great refractor, the type might have had a longer life in professional circles than it did. The Newtonian reflector, too, was rapidly abandoned by professionals as inefficient for serious scientific work. Had someone cooked up a Paracorr and well-corrected wide field eyepiece design back in the day, the Newtonian, too, might have enjoyed a longer scientific relevance than it did.

In fact, had imaging technologies advanced more quickly, we might never have seen scopes like Mt. Wilson or Mt. Palomar. Today a 10" RC with a decebt CCD camera is capable of going deeper and showing more than Palomar did with emulsion plates.




Good observations and thinking. But I do wonder, whether it is possible even today, to build a 40" triplet apochromat? Any insight from optical designers and glass makers? How about a corrector for already existing refractors? Sort of a chromacorr for Yerkes?

Thomas M. Back also designed a 21" f/12 triplet apo and from what I understand, APM/LZOS are actually building one. The design was extremely well corrected for color, given its huge aperture (for a refractor). I think it's safe to assume that a 36" f/17 with identical design would perform extremely well and far better than the current Lick refractor.


Clear skies!
Thomas, Denmark




There is already a refractor design that is perfectly achromatic and of 1 meter in diameter. That is the Swedish solarschupmann.
http://en.wikipedia.org/wiki/Swedish_Solar_Telescope
Since the Schupmann design uses a singlet objective, it can be made thin so weight deflection becomes less of issues so it should be possible to make a 50" or 60" diameter one.
The Schupmann corrector can be applied to already existing refractors to greatly correct the chromatic aberration. In fact James Baker in his famous paper on the future of planetary telescopes which was published in the late 1950's purposed just that.
- Dave


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chboss
professor emeritus


Reged: 03/24/08

Loc: Zurich Switzerland
Re: The Demise of the Great Refractor new [Re: DAVIDG]
      #5527110 - 11/18/12 11:59 PM Attachment (47 downloads)

Unfortunately large refractors are a thing of the past in professional astronomy. I personally love the look of these classic instruments although they are not very practical to use. Below the image of the 65cm Zeiss refracotr of the National observatory of Japan. It is unused and serves only as backdrop of the museum housed in the dome.

There are some special application scopes as the mentioned solar telescope but they are rare these days.

best regards
Chris


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