Whitepeak Observatory, Tacoma, WA

Comparative Color Correction of Apochromat, Moderate F/Ratio achromats and High F/Ratio achromats


Notice there are three examples of the range of .80 strehl across the visual spectrum as related to degree of correction for CA. Notice the top example of a Conrady standard achromat is wholly equivalent in corrected range to that of the larger apochromat. Now compare those spectrums to the bottom, that of an achromat having a very modest f-ratio/D ratio of 1.9. Note the difference in the diffraction limited range across the spectrum and how it decreases (and by how much) as compared to the top example. By using a bit of interpolation here you can see that a 4" f/13 or 15 is going to at least split the difference in diffraction limited spectral range between the top & bottom examples--a difference that is not all that massive to begin with. To my mind this puts such a scope (4 f/15 with a f-raio/D of 3.82) as approaching apochromatic correction.

There is more observational evidence that the above representation is quite accurate in practice as well. In comparing the evidence of chroma between a 5" 1.86 f-ratio/d to a 80mm of 3.5 f-ratio/d, the latter displays virtually no color as tested upon white doubles at high magnification-- the entire diffration pattern is quite color-free as compared to that of the former scope, which display much more color in comparison--just as theory predicts. In fact a cheaper apo (such as the Orion 80ED) displays no less appreciable color when compared to that displayed by a good 80mm f/11 achromat, IMO, having observed through both.

Also, I am not the only one to draw such conclusions as to high f-ratio/d ratio achromats as having virtual (or even equivalent) performance to that of an apo. Rodger Gordon (professional optical designer who worked for Unitron in the good old days) flatly states, "Image quality of a 4-inch f/20 doublet O.G. is on a par with a 4-inch f/6 apochromat." This statement is in entire agreement with the graphic representations of the comparative diffraction limited range of the Conrady standard refractor and the apo above. A 3" f/15 achromat meets this criteria of apo correction equivalence.

So I feel it is not a stretch to say that the slightly less corrected 4" f/15 or so approaches the quality of correction of an apochromat of half that f/ratio (if it's well made &-designed which is, of course, a big 'if'.)

A table comparing the f-ratio/D ratios of various objective diameters vs. various f/ratios can be found here.

Why is this realtionship and it's ramifications not more widely recognized? I can think of several factors, all market related. For one, it is difficult to find a 3" refractor well mounted and available commerically. So this class of refractors are widely dismissed as 'department store' (as their mounts undoubtably are) and not taken seriously and therefore little used today. The market tends to go directly to the 3" apo class from the low f-ratio achromat "wide field" class with little investigation between. Long refractors are just not popular and their advantages of excelelnt color correction and concurrent image definition thus go unnoticed by virtually all but the fans of so-called 'classic' refractors. More's the pity IMO.

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