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APO doublet?


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I'm drooling over the WO Megrez 88 on the TS website...

I have two questions.

1. Is the Megrez 88 any good?

2. What is an Apo doublet? I thought an Apo needed to be a triplet to correct for three wavelengths? Being a doublet, is the Megrez 88 really just a very good Achromat?

Edited by Ags
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I'd not experienced enough or brave enough to attempt to provide a definitive answer to your question. This article by someone who really knows about the subject might provide some insight:

Color Correction in Refractors

I've recently been trying a 6 element refractor and that controlled chromatic aberration very well, but not perfectly.

My Vixen with it's ED 102mm F/6.5 doublet does a pretty good job of that as well - certainly good enough for my eyes.

But I'm not going to attempt to define either of the above scopes as apochromatic - which my newtonian is of course :D

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Technically an APO should be a triplet. but with the advances in glasses and coatings today a good doublet can perform like one.

Of course there are different qualities of APOs so I look at it as the top quality APOs are indeed triplets but the lower quality triplets are comparable to the best doublets.

Let the fight begin :D:D:D

John

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A Celestron ED100R is visually apochromatic as far as I am concerned. :)AP may be different but as I don't do AP, I don't care!!:p

An I sold my ED100R in favour of my Tal 100RS which is a more pleasing all around scope that I always look forward to using. So there.:D

Dave

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Wikipedia has a good explanation on definition of achromatic, apochromatic and superachromatic (super apo) lens. Basically, achromatic lens has to bring 2 colours to focus and has a quadratic colour shift curve. Apochromatic lens needs to bring 3 into focus with a cubic shift curve. Super apo needs to bring four colours into focus and has a quartic colour shift curve.

Achromatic lens - Wikipedia, the free encyclopedia

Apochromat - Wikipedia, the free encyclopedia

Superachromat - Wikipedia, the free encyclopedia

The shift depends on the lens design and it may be possible to achieve a third order correction with the right selection of materials and lens curvature.

What I don't get is what make a scope semi-apo or neo-achromat.

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Thomas Back definition:

After designing, testing and selling many different apochromatic lenses I can state this: There is no "definite" line where a lens becomes "apochromatic" in the world of commercial apochromatic lenses.

But any lens, be it a doublet, triplet, quad, air-spaced or Petzval, that has a peak visual null (~5550A - the green-yellow) with a Strehl ratio of .95 or better, coma corrected and is diffraction limited from C (red) to F (blue) with 1/4 wave OPD spherical or better, has good control of the violet g wavelength with no more than 1/2 wave OPD P-V spherical and optical spot sizes that concentrate the maximum amount of photons within the diffraction limit - a result of the low spherical aberration, which can be seen with modern optical design programs, as the "spot rays" will be seen concentrated in the center of the spot, not evenly or worse, concentrated outside the center - will satisfy the modern definition of "Apochromatism."

Lenses of this quality do not satisfy the Abbe definition, but for all intents and purposes, will be color free and will give extremely sharp and contrasty images.

Edited by great_bear
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I see quite a bit of false colour in my ED120 when viewing Jupiter and (especially) Venus. On most other objects its colour free and "apo"enough for me. The cost of removing the colour on those 2 objects (ie. going to a 120mm triplet) is too high for me to consider.

Edited by GazOC
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I have read some reviews of the megrez 88 and they indicate red fringing is visible at powers over 50 and photographically. Honestly my mak sounds better optically - no chromatic aberation, more aperture - at the price of a field 2.6 times smaller of course.

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