Apochromatic Refractors : The Quest for Perfect Optics
Stephen Reb
The 400 year quest for the perfect telescope continues with no end in sight. At the individual level the search narrows, particularly if one eliminates such criteria as cost, size and practicality, and if one accepts the premise that telescopic configurations will never significantly improve beyond their existing state. If you're reading this page you're likely familiar with such phrases as "apo-like views" or "refractor-like images." Over the last twenty-five years apochromatic refractors have become the measuring stick by which all visual instruments are judged.
At the risk of making enemies so early into this article, it should probably be noted that stalwart proponents of the apo-as-measuring-stick notion are very often the same people who paid roughly 1500 dollars per inch of aperture for their six-inch telescopes. In their defense, until recently those high priced apos provided the only means of acquiring such flat, wide-field images. For the serious astrophotographer they really were necessary.
A reasonable justification for the exhorbitant prices might be that, prior to Takahashi's release of the first apochromatic triplet, no one had seen a refracting telescope produce such color-free views. In fact, the astronomer's lexicon included neither "apochromat" nor the abreviated "apo." The author's name escapes me, but a reviewer at the time said that achromatic means color-free, while apochromatic means color-free, but this time we mean it.
But despite providing photographic superiority, are they really the only means of bringing such flat, asymetrical clarity to the eyepiece? Owners of certain Russian or American Maksutov-based telescopes would disagree. And, though it may surprise newcomers to the hobby, it is possible for the common reflector, one of the simplest optical designs of all, to equal the performance of the finest apochromatic refractors, as long as certain conditions are met. In fact, given their capacity for greater aperture, it could be argued that no other similarly sized telescope can compare with the larger high-end reflectors.
By the time that Alvan G. Clark finished the 40" Yerkes Telescope, it had already become apparent to optical engineers that refracting telescopes were nearing their practical limit at 40 inches. Even at that size there was always a danger of the primary objective sagging and losing its shape under the great weight of its glass. Alvan Clark understood the limitations but did not agree with a 40-inch limit. He died before putting into motion his plan for a 60-inch refractor. Early in the twentieth century there were several attempts to exceed the 40-inch barrier. None were successfull.
Fluorite crystals embedded in calcite
For twentieth century stargazers requiring both an apochromatic refractor and aperture, Germany's APM now offers a 20 " apo at roughly 1.5 million U.S. Then, of course, you would have to find a mount for it. To save money, you could always forgo the specially ordered fork mount and propietary software. I'm guessing that Astro Physics' 400 pound "el Capitan" might suffice. By suffice I mean to suggest that the AP equatorial would probably carry a 20-inch F/11 refractor as sturdily as a CG5 mount holding a C11 SCT. By now, anyone who reads the telescope reviews has heard that a CG5 will support the weight of a Celestron 11" SCT, just as long as no one attempts a photographic exposure of longer than 2 seconds. Visual use or webcam imaging? Yes. Anything else? No.
The Niche that Dmitri Maksutov Built
Dmitri Maksutov 1896 - 1964
But back to apo substitutes, compound instruments such as Maksutov-based scopes are probably the most popular, or at least the best known and most controversial. If there could only be one mirror-based telescope judged to be the equal of the best refractors, it would have to be Astro Physics' 10-inch Maksutov-Cassegrain. This is a telescope many consider to be as close to perfection as is possible to attain. The optical design itself began some fifty years previous.
Dmitri Maksutov was a Soviet optical engineer responsible for many valuable and lasting contributions to the fields of astronomy and medicine. He developed optical surgical tools, as well as large reflecting telescopes for state observatories, many of which are still used. But Maksutov is best remembered for the meniscus lens, the most distintive component of the telescopes which bear his name.
Maksutov used spherical, nearly symmetric surfaces-- a primary mirror with a full diameter convex lens, or meniscus, placed close to the point of focus. The configuration was designed to eliminate coma, astigmatism, aberations, both off-axis and spherical, while at the same time preventing chromatic aberation. The secondary mirrors were slightly smaller than those found in Schmidt-Cassegrain telescopes, allowing them to provide more contrast. A closed tube meant an absense of tube currents. Maksutov telescopes tended to have high focal ratios and therefore provided higher levels of magnification. This, coupled with the higher contrast, made them better suited for planetary and lunar viewing.
Mak Evolution and Two Legends
If you had to chose a single example of a telescope that became legendary within the lifetime of its designer, this would be it, hands down. When AP shipped off one of the last of these maksutovs from the final run, its new owner immediately listed the scope in an auction witht a well-known website featuring classified ads for astronomy equipment. AP sold these telescopes at a little over 9K-- a hefty but reasonable price considering the time and workmanship that went into them. The auction winner paid just under twenty thousand dollars for his.
Anyone with more than a cursory interest in telescopes knows about the Astrophysics 10-inch Maksutov-Cassegrain. No other telescope has attracted so much attention, or gained so much respect. Nor has any telescope been a coveted as this ten-inch compound scope. Just months after the line was discontinued, one was sold at auction for just under twenty thousand dollars, roughly twice the original price.
Anyone with more than a cursory interest in telescopes knows about the Astrophysics 10-inch Maksutov-Cassegrain. No other telescope in recent years has
Continued soon - 06/01/2011...