Much prejudice has the achromatic refractor received over the last decade; prejudice that is entirely unjustified, especially in the case of binary stars. In the gladiatorial arena that is the internet forums, I’ve seen men of great education say the most astonishing things concerning the ability of achromats to resolve double stars compared with the apochromats they so ardently defend.
“An apochromat ought to be able to resolve closer doubles than an achromat of the same aperture,” posted one learned man, bobby fae Canada, a professor of astronomy. “Its colour correction ought to allow finer splits of tricky binaries!,” he continued.
That kind of statement is known in the industry as a ‘logical construct.’ It was based not on hard observations but on what seemed reasonable. But I’ve learned before that such statements are not to be trusted wholeheartedly, unless one finds out for oneself.
So, away I went to test that theory, and after a few years of trying, came back thinking, “that cannae be right!” When I compared Apos to Achros I couldn’t for the life of me see any advantages in terms of the sheer resolving power of a number of doublet and triplet apochromats in comparison to their simpler siblings, the achromats. Indeed, ironically, it was the latter that often faired better in my field tests.
Intrigued, I consulted the history books and found out for myself how men of the ilk of Hussey, Aitken and Burnham regularly reached 0.1″ with the Great Lick Refractor. Yep, that’s 100 milliarcseconds folks. Indeed, there are entries made by Aitken that record separations of an incredible 50 milliarcseconds! Furthermore,because these observations were made well before the age of adaptive/ active optics, that kind of angular resolution was truly astonishing!
All this from a 36 inch f/19 Clark achromatic doublet. Now if you divide its f ratio by its aperture in inches, you get a so called Chromatic Aberration (CA) index of ~0.5.
Now according to the widely used table below, the levels of secondary spectrum thrown up by that telescope ought to make observations next to useless.
And yet E.E. Barnard discovered Amalthea with the Lick Refractor, the tiny, fifth satellite of Jupiter,and in a field of view that must have been swimming in a morass of unfocused purple light all round the planet. And only a handful of folk have seen it since.
Ergo, there is a contradiction between the extraordinary achievements of this telescope and the prognostications of the cognoscenti!
The commonly held notion that the residual chromatic aberration is a serious limitation to the defining power of classical refractors in comparison to reflectors and also, by implication, apochromatic refractors, particularly in relation to their double star resolving power, actually has no theoretical underpinning.
Here is a passage from the book; Observing and Measuring Double Stars (Springer) written by Christopher Taylor (pp 98-99), a mathematical physicist and keen double star observer based at Oxford;
” That there is, in fact, no theoretical justification for this view in the case of any refractor of sufficiently long focus to be used for high-resolution imaging (say f/10, at least for smaller apertures, rising to f/18 for larger apertures) has been known at least since the work of Conrady. It was shown there that moderate levels of defocusing such as may be induced by the secondary spectrum in such a refractor, that is up to one quarter or even one half wavelength phase lag does not significantly alter the diameter of the Airy disk formed by the telescope, despite its intensity declining noticeably.
Effectively, the chromatic dispersion of focus is lost in the depth of focus naturally allowed by the wave theory; this is the reason why image defintion is so good in refractors despite secondary spectrum. The result is that resolution of high constrast targets such as double stars is fully maintained, even if some low-contrast fine detail may be lost in planetary images. That this conclusion is fully borne out by practical experience is convincingly demonstrated by the magnificent achievements in high resolution double star astronomy of the best visual observers using the big refractors……..Indeed Paul Couteau [a professional double star astronomer who used the 74cm refractor at Nice, France] , seems from the remarks in his well known book to consider the secondary spectrum of refractors to be a positive advantage. Clearly, three colour or apochromatic correction, whatever its benefits for the use of relatively short focus instruments in planetary imaging, is for the double star observer an expensive and dispensible luxury – the classical long focus doublet objective is more than equal to the task required.”
That’s sterling advise from a man who knows his stuff. It is supported by theory, a wealth of history and above all else, personal experience in the field.
Having just received the second edition of Bob Argyle’s book on double stars, I turned to Christopher Taylor’s chapter discussing, in the main, the reflecting telescope in double star astrometry. On page 118, he reasserts what has been quoted above, namely that secondary spectrum has little or no detrimental effects on the resolution of double stars. Furthermore, on page 122 of the same chapter he asserts the following in regard to the classical achromat:
What all of this amounts to in practice is that a reasonably well-made Fraunhofer achromat is a hugely more robust instrument than a typical reflector in the face of the thermal variations, mechanical flexure and shifting collimation which commonly arise in real observing conditions, and can be relied upon far more than the comparitively fickle reflector to deliver critical definition at a moment’s notice with minimal cosseting and adjustment. It is also more likely to meet the optical tolerances necessary for such diffraction-limited performance. These are the reasons why the refractor has often been the first choice for observers of close visual binaries.
Long live the classical refractor!
Post scriptum: The quotations were taken from Chapter 11 of the aforementioned book, written by Taylor. The reader will note that Taylor has spent several decades observing and measuring double stars with a 12.5″ f/7.1 reflector (Calver speculum). The chapter is, in the most part, dedicated to what can be acheived with an optimised Newtonian (not your ubiquitous f/3-f/5 commercial Dobsonian), which he continues to use to this day.
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