Ags Posted March 24, 2011 Share Posted March 24, 2011 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? Link to comment Share on other sites More sharing options...
John Posted March 24, 2011 Share Posted March 24, 2011 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 RefractorsI'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 Link to comment Share on other sites More sharing options...
johninderby Posted March 24, 2011 Share Posted March 24, 2011 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:DJohn Link to comment Share on other sites More sharing options...
Peter Drew Posted March 24, 2011 Share Posted March 24, 2011 In the "Red" corner......My Vixen 102mm Flourite doublet is as colour free as I could wish for. . Link to comment Share on other sites More sharing options...
Moonshane Posted March 24, 2011 Share Posted March 24, 2011 In the "Red" corner......My Vixen 102mm Flourite doublet is as colour free as I could wish for. .If I were ever tempted by a refractor again I think this would be it. Link to comment Share on other sites More sharing options...
AndyH Posted March 24, 2011 Share Posted March 24, 2011 Tag teaming in the red corner for Peter's vixen, is my Burgess Fluorite doublet. A quite wonderful scope. Andy. Link to comment Share on other sites More sharing options...
F15Rules Posted March 24, 2011 Share Posted March 24, 2011 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!!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.Dave Link to comment Share on other sites More sharing options...
E621Keith Posted March 25, 2011 Share Posted March 25, 2011 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 encyclopediaApochromat - Wikipedia, the free encyclopediaSuperachromat - Wikipedia, the free encyclopediaThe 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. Link to comment Share on other sites More sharing options...
great_bear Posted March 25, 2011 Share Posted March 25, 2011 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. Link to comment Share on other sites More sharing options...
Moonshane Posted March 25, 2011 Share Posted March 25, 2011 one thing I hope to do at SGL6 is look through a good quality APO/ED scope (never done so) and compare it with my 6" f11 (if I can fit this in the car!). Link to comment Share on other sites More sharing options...
Ags Posted March 26, 2011 Author Share Posted March 26, 2011 Thanks all; I've always wondered if 'apo' had a well-defined meaning, but it seems that as usual it has sunk into the muddy waters of marketing. Link to comment Share on other sites More sharing options...
dweller25 Posted March 26, 2011 Share Posted March 26, 2011 In the Blue corner .... My Takahashi FS-128 F/8.1 Flourite doublet meets my needs Link to comment Share on other sites More sharing options...
Kef9 Posted March 26, 2011 Share Posted March 26, 2011 My C100ED is colour free to my eyes. Link to comment Share on other sites More sharing options...
GazOC Posted March 26, 2011 Share Posted March 26, 2011 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. Link to comment Share on other sites More sharing options...
angusb1 Posted March 26, 2011 Share Posted March 26, 2011 In the "Red" corner......My Vixen 102mm Flourite doublet is as colour free as I could wish for. .I've had a hankering for one of these for a while now too. Link to comment Share on other sites More sharing options...
Ags Posted March 26, 2011 Author Share Posted March 26, 2011 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. Link to comment Share on other sites More sharing options...
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