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photonovore
Moonatic
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Reged: 12/24/04

Loc: tacoma wa
Re: Stranger than fiction! new [Re: wh48gs]
      #4091919 - 10/04/10 12:46 PM

Quote:



Thanks, but I still don't see where this 0.000512 wavelength comes from. The cutoff frequency is D/Lambda, normalized to 1. The frequency corresponding to Rayleigh limit is D/1.22Lambda, or 0.82 normalized.

Vla




0.000512 is the light frequency I happened to use in the formula. If you use 0.000538 for the frequency, the results would agree with 0.82 normalized. (the average between 505nm and 555nm happens to be 530.5nm, so it appears that location of the Rayleigh limit at 0.82 normalized is predicted upon some sort of rather arbitrary mesoptic peak--as there is no consensus upon peak sensitivity for mesoptic vision.... )


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Astrojensen
Post Laureate
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Reged: 10/05/08

Loc: Bornholm, Denmark
Re: Stranger than fiction! new [Re: BillP]
      #4092224 - 10/04/10 02:36 PM

Quote:

In the example of your 85mm f/19 Zeiss isn't it still just a conventional crown-flint doublet? So an "acromat" of sorts even though at that focal length it's color blur should be low enough to classify it as near-apocromatic, if not fully visually apochromatic in performance.




Hi Bill

No, my 85/1600mm Zeiss is a true apochromat. According to the 1916 Zeiss "Astro 30" main catalogue, it has three times better color correction than a conventional air-spaced achromat of the same focal length. The f/ratio has to be kept at f/17 or longer, due to the strong internal curvatures of the lens. Mine is even longer than that, because it was designed as a 90mm, but due to a fracture in the edge of one of the lenses, it was ground down to a 85mm (this is at least the guess of the previous owner, who was extremely knowledgeable about the early history of Zeiss). There is still a small clamshell in it.

The color correction is really stunning. Visually, there is no hint of any color in or out of focus. It can be taken to ridiculous powers. I have used 800x on the sun, using my projection box, to observe the umbrae of large sunspots.

Here is a raw, unprocessed video of how it performs on the moon. Notice that it was taken with a color webcam.

http://www.youtube.com/watch?v=f9MKUE0WBis

It is truly a shame that no one is making telescopes such as this anymore. People that claim their f/7 apos to be just as good simply have not had a chance to observe with my 85mm f/19, so they don't know what they're talking about. Really. Stopped to 75mm it completely and utterly destroyed a 76mm TeleVue apo on Mars, Saturn and the moon.


Clear skies!
Thomas, Denmark


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Clive Gibbons
Mostly Harmless
*****

Reged: 05/26/05

Loc: Oort Cloud
Re: Stranger than fiction! new [Re: Astrojensen]
      #4092298 - 10/04/10 03:11 PM

Wow, that's a super image Thomas.
The Moon's lack of yellowish or greenish cast also indicates how well focused the colors are.

I agree.
Long focus apos (f/15 or longer) are exceptional performers.
While a long achro is really good, an f/15 (or slower) ED doublet or triplet would be phenomenal.
Hopefully a "niche" vendor will run with the idea.


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jrbarnett
Eyepiece Hooligan
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Reged: 02/28/06

Loc: Petaluma, CA
Re: Stranger than fiction! new [Re: Clive Gibbons]
      #4092386 - 10/04/10 03:56 PM

"Getting back to an automotive analogy, promoting the long focus achro vs. shorter apos reminds me somebody extolling the virtues of a big ol' Buick compared to modern sports cars having the latest in multi-valve engines. It's a tough sell to the auto enthusiast crowd. Well, most of 'em."

True, but it's a pretty easy sell to the other 90% who have double-knit butts of large proportion and aging frames that love the dyna-ride float-o-matic suspension. There's no doubt that the boat...er...Buick is a better tool for certain automotive jobs than the Porsche 911. Coincidentally, it just so happens that the long focus achromat is a better tool for certain tasks than a fast achromat.

- Jim


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astroneil
Carpal Tunnel
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Reged: 07/28/09

Loc: res publica caledoniae
Re: Stranger than fiction! new [Re: Clive Gibbons]
      #4092641 - 10/04/10 05:25 PM

Quote:

Wow, that's a super image Thomas.
The Moon's lack of yellowish or greenish cast also indicates how well focused the colors are.

I agree.
Long focus apos (f/15 or longer) are exceptional performers.
While a long achro is really good, an f/15 (or slower) ED doublet or triplet would be phenomenal.
Hopefully a "niche" vendor will run with the idea.





I guess there's a number of ways to do that;

Option A: design an ED F/15 objective (FPL 51 would be nice ) http://www.aokswiss.ch/ scroll down page and click on red long focus ED.
Option B: design a chromacorr which corrects colour without introducing spherical aberration
Option C: Use a conventional achromatic doublet and fold the design for something like a 4-6" F/20 or some such. Not true Apo but near enough. It wouldn't look as nice though IMO

Neil.

Edited by astroneil (10/04/10 05:46 PM)


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Wes James
Postmaster
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Reged: 04/12/06

Re: Stranger than fiction! new [Re: astroneil]
      #4092754 - 10/04/10 06:11 PM

Quote:

ist eines der besten Planetenteleskope seiner Grössenklasse.




(quote from that AOK ad- for the Schief)

Ah, HA! Like I said... Schief's a great planetery scope!!
All right.... I'll shut up now!
Wes


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DAVIDG
Post Laureate
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Reged: 12/02/04

Loc: Hockessin, De
Re: Stranger than fiction! new [Re: astroneil]
      #4093416 - 10/04/10 11:24 PM

Quote:

Quote:

Wow, that's a super image Thomas.
The Moon's lack of yellowish or greenish cast also indicates how well focused the colors are.

I agree.
Long focus apos (f/15 or longer) are exceptional performers.
While a long achro is really good, an f/15 (or slower) ED doublet or triplet would be phenomenal.
Hopefully a "niche" vendor will run with the idea.





I guess there's a number of ways to do that;

Option A: design an ED F/15 objective (FPL 51 would be nice ) http://www.aokswiss.ch/ scroll down page and click on red long focus ED.
Option B: design a chromacorr which corrects colour without introducing spherical aberration
Option C: Use a conventional achromatic doublet and fold the design for something like a 4-6" F/20 or some such. Not true Apo but near enough. It wouldn't look as nice though IMO

Neil.




Option D : Make a Schupmann refractor which has PERFECT color correction out of two piece of cheap BK-7 and enjoy one the best views possible. No false color, polychromatic Strehl of 0.99+, no coma, no astigmatism, no obstruction and it can easily corrector for atmospheric dispersion.

- Dave


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jrbarnett
Eyepiece Hooligan
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Reged: 02/28/06

Loc: Petaluma, CA
Re: Stranger than fiction! new [Re: DAVIDG]
      #4093548 - 10/05/10 12:29 AM

If the Schupmann design is so simple and wonderful, why in its 100+ year history has it never caught on commercially? Markets and competition are usually pretty efficient. If the Schupmann was offered competitive advantage over other designs, why are they so rare? There must be a catch. What is it?

- Jim


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Wes James
Postmaster
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Reged: 04/12/06

Re: Stranger than fiction! new [Re: jrbarnett]
      #4093849 - 10/05/10 07:38 AM

I think that unusual telescope designs are a niche item even amongst a group of dedicated, die-hard astro enthuasiasts like we have here on C/N. It is fairly well accepted that off-axis telescopes offer the best of both worlds- better color correction with no obstruction. Yet Orion offered a 3.4" off axis scope- it failed commercially... For its aperture, it was a fairly large scope. DGM offers them, but I doubt if they sell a lot... AOK in Europe still offers Schief's, and they have a European following, other than a few enthuasiasts here, the more unusual designs just don't get their due. Not enough people are willing to accept them, even though they offer definite advantages. While your statement "Markets and competition are usually pretty efficient" is true, customers are pretty traditional. I think that even things such as appearance can put people off... if it doesn't look like a telescope, they tend to steer clear of it. And if people steer clear of it, manufactures won't stay in the market long if they can't sell them. The car market has had many revolutionary designs that offered many advantages, but for one reason or another, they failed. The Schief's and the Schupmann's definitely fall into the "unusual appearance" telescope category.
Just my humble opinion??
Wes

Edited by Wes James (10/05/10 07:45 AM)


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DAVIDG
Post Laureate
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Reged: 12/02/04

Loc: Hockessin, De
Re: Stranger than fiction! new [Re: jrbarnett]
      #4094027 - 10/05/10 09:43 AM

Quote:

If the Schupmann design is so simple and wonderful, why in its 100+ year history has it never caught on commercially? Markets and competition are usually pretty efficient. If the Schupmann was offered competitive advantage over other designs, why are they so rare? There must be a catch. What is it?

- Jim




If you read Jim Daley's book it gets into some of this. The largest Schupmann is 40" and used for solar research. http://en.wikipedia.org/wiki/Swedish_Solar_Telescope It has provides some of the very best ground based solar images taken so far. There is also a 13" at Stellafane. Having built three of them they are not difficult to make and actually much easier then achromat or APO. I'll be glad to provide spot diagrams that show how extremely well these telescopes are corrected. World Class ATM's like Gerry Logan, who has made just about every optical design known has stated that his Schupmanns give the best image of all his scopes. http://www.siderealtechnology.com/atm/Logan_Gerry/
All I can say is both theory and practice both bare out the fact that Schupmanns provide extremely good images with no chromatic abberation and they are made for the inexpensive optical glass.

- Dave


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astroneil
Carpal Tunnel
*****

Reged: 07/28/09

Loc: res publica caledoniae
Re: Stranger than fiction! new [Re: DAVIDG]
      #4094049 - 10/05/10 09:59 AM

You can also get a modest reduction (~18%) in secondary spectrum using Bak2-F1 glass at little additional cost to standard achromat optical glasses.

Some interesting reading here:

http://www.rfroyce.com/refractor%20spots.htm

Cheers,

Neil.


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Starman1
Vendor (EyepiecesEtc.com)
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Reged: 06/24/03

Loc: Los Angeles
Re: Stranger than fiction! new [Re: astroneil]
      #4094138 - 10/05/10 10:47 AM

Long focal length achromats have better color correction than short ones.

Long achromats have greater tolerances for mispacing on the lenses than short focal length triplet apochromatic refractors.

Long focal length achromatic refractors can outperform short apochromatic triplets in some situations.

Is this really news?

Unfortunately, long focal length achromatic refractors that have any aperture require large, heavy mounts that are very difficult to transport. And if they are not so mounted, they shimmy in the wind like the branches of a willow tree.
For people who live in the world of wind and weight, the 127mm f/7.5 triplet may outperform the 127mm f/15 achromat simply because it is portable and more immune to conditions of wind.

Having owned the long skinny scope before (a 4" f/15 Unitron), I doubt I will ever be tempted to go back to that long skinny scope on the high mount again. It was only stable when there was no wind, which was 5 days a year where I lived when I owned that scope.


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Covey32
Pooh-Bah
*****

Reged: 12/09/04

Loc: Georgia
Re: Stranger than fiction! new [Re: Starman1]
      #4094147 - 10/05/10 10:53 AM



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astroneil
Carpal Tunnel
*****

Reged: 07/28/09

Loc: res publica caledoniae
Re: Stranger than fiction! new [Re: Starman1]
      #4094181 - 10/05/10 11:08 AM

Quote:

Long focal length achromats have better color correction than short ones.

Long achromats have greater tolerances for mispacing on the lenses than short focal length triplet apochromatic refractors.

Long focal length achromatic refractors can outperform short apochromatic triplets in some situations.

Is this really news?

Unquote:

Yes it is!:)

Quote:

Unfortunately, long focal length achromatic refractors that have any aperture require large, heavy mounts that are very difficult to transport. And if they are not so mounted, they shimmy in the wind like the branches of a willow tree.
For people who live in the world of wind and weight, the 127mm f/7.5 triplet may outperform the 127mm f/15 achromat simply because it is portable and more immune to conditions of wind.
Unquote

Not necessarily,

Lookie here:

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/4063058/page/3/view/collapsed/sb/5/o/all/fpart/1

As you can see, they needn't be heavy or expensive!



Cheers,

Neil.



Edited by astroneil (10/05/10 12:46 PM)

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jrbarnett
Eyepiece Hooligan
*****

Reged: 02/28/06

Loc: Petaluma, CA
Re: Stranger than fiction! new [Re: DAVIDG]
      #4094268 - 10/05/10 11:55 AM

Thanks Dave. I caught the performance superiority claims from the initial post. What's missing is that if there's no "catch" (i.e., no downside with the design) then the lack of commercial examples makes no sense. Folks that build telescopes to make a living tend to be very quick to adopt better designs, methods and materials, and yet none of them have opted to make commercial Schupmanns. Why?

"Beats me" isn't a very satisfying (or credible) answer. Think of it this way. Back when Celestron Pacific was in its infancy and looking for a great design to produce commercially, and selected the Schmidt-Cassegrain, it faced several tough design problems foremost of which was how to cost-effectively figure the complex correctors. By investing time, resources and ingenuity into cracking the code for mass-production of correctors, Celestron was able to profitably mass produce the design (unchallenged) for decades.

There's got to be some very apparent, very practical reasons no commercial entrant has bothered with the Schupmann. I'm wondering what they are. I understand thet there are variations on the design, so not every comment regarding the design will apply to every derivative of the design, but here are some of the issues one source cites for the design:

"The final step in designing Schupmann is making the image accessible by tilting all three elements. Small concave mirror typically needs 4 to 5 degree tilt, catadioptric element about half as much, and front lens a fraction of it. Tilting the mirror doesn't induce appreciable aberrations, but tilting the CE does. This is why the front lens needs to be tilted as well, in the plane perpendicular to CE's tilt, in order to offset astigmatism induced by tilting the CE. If done correctly, tilting doesn't appreciably affect the mid-field aberration level. It does, however, increase off-axis astigmatism, and produces asymmetric, tilted image field typical for tilted systems. It also presents additional difficulty in achieving and maintaining proper system alignment ."

http://www.telescope-optics.net/schupmann_medial_telescope.htm

Could those highlighted issues partially explain the lack of commercial examples?

Regards,

Jim


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jrbarnett
Eyepiece Hooligan
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Reged: 02/28/06

Loc: Petaluma, CA
Re: Stranger than fiction! new [Re: Starman1]
      #4094301 - 10/05/10 12:07 PM

Sonoma county is very windy.

I've never had any wind-related problems with my 4" f/14.4 that weren't also problems for any of my other scopes. The scope actually has a smaller wind profile than a solid tube Dob, truss Dob with a shroud or large SCT. If well-mounted (in this case an Atlas is more than enough for the 10# 6-foot-long OTA, provided that a long dovetail is used and a second long dovetail joins the ring tops), a 4" f/15 is really not much different to mount or use than any other scope that could ride on the same mount.

I think a 4-inch long focus refractor isn't a bad choice scope for die-hard double star hunters. On the other hand, if your observing diet is more diverse, then it likely wouldn't be a satisfying "only scope".

Now if we're talking bigger such as a 5" f/15...yikes. That means carrying the OTA on a roof rack and not in the car and a G-11 class mount. 6" f/15? I wouldn't put it on less than an A-P 900 or better yet A-P 1200. Without folded designs or permanent installations, I think the audience for big long focus achromats (larger than 4") is pretty small.

Regards,

Jim


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freestar8n
Post Laureate
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Reged: 10/12/07

Re: Stranger than fiction! new [Re: Starman1]
      #4094644 - 10/05/10 02:31 PM

Quote:

Is this really news?





This was basically my question. If the result is surprising or disagrees with anyone's experience - then theory alone has to be handled carefully. But if no one disagrees - and it's just a matter of the small, newer expensive thing not being as good as the long, simpler, old design thing - then it's more a matter of theory supporting the evidence. Separately there are issues of personal preference, usability, and ungainliness - but it sounds like the relative visual performance isn't a controversy.

Frank


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DAVIDG
Post Laureate
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Reged: 12/02/04

Loc: Hockessin, De
Re: Stranger than fiction! new [Re: jrbarnett]
      #4094703 - 10/05/10 02:53 PM Attachment (41 downloads)

Jim,
I have seen that quote, but I personally haven't had any problems keeping any of my Schupmanns aligned and they have been more robust then my Newtonians. If you look at the spot diagrams of many of them, you will see that the amount of off axis abberation are BELOW diffraction limited. My own 4" f/12 has over 1/2 degree field of view that is diffraction limited. If you compare this to many other designs, they are far worse.
I can't get explain why Schupmanns have not caught on commerically. The data shows that they are excellent.
One the other hand there are commerical optical design that fall far short, yet they are made.

- Dave


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astroneil
Carpal Tunnel
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Reged: 07/28/09

Loc: res publica caledoniae
Re: Stranger than fiction! new [Re: DAVIDG]
      #4094877 - 10/05/10 04:16 PM

The article has now been updated to correct for the aforementioned errors.

Cheers,

Neil.


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jrbarnett
Eyepiece Hooligan
*****

Reged: 02/28/06

Loc: Petaluma, CA
Re: Stranger than fiction! new [Re: freestar8n]
      #4094884 - 10/05/10 04:19 PM

That's just it. The premise of the report runs counter to the majoritarian position on the question of whether a long focus achromat produces steadier images than a faster apochromat. The majority of posters on multiple Refractor forum threads indicated that they believed there was no correlation between focal length and image stability. In fact, there's a very well-known article on a vendor site claiming precisely the contrary; that focal length/ratio has no bearing on image stability.

http://www.fpi-protostar.com/bgreer/seeing.htm

"Conclusion

Telescopes of equal aperture are affected the same by atmospheric turbulence, regardless of focal ratio. "

Given that this article contradicts that one and is also better documented and reasoned than that one, in my opinion it's timely, informative, valuable and important reading for anyone interested in the correlation between focal ratio and image stability in refractors.

It's also one of the best written, most transparently researched and documented articles on CN. In that regard it sets a new, higher standard for such articles.

Regards,

Jim


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