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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

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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.

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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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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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ist eines der besten Planetenteleskope seiner Grössenklasse.

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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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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.

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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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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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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?

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Yes it is!:)

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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.
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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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Is this really news?

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