ACF optics

Features of this Telescope's Optical System . . .

  • Advanced Coma-Free catadioptric designed to emulate the optical performance of a Ritchey-Chrétien telescope: The traditional two-mirror Ritchey-Chrétien (R-C) design uses two large hyperbolic primary and secondary mirrors to produce images that are free from coma over a wide field. Because of this wide coma-free field and a relatively fast focal ratio, the Ritchey-Chrétien design is particularly well suited to astrophotography. The R-C is the design of choice for most of the major professional observatory telescopes built in the last half-century. For example, the Hubble Space Telescope and the twin 10-meter Keck telescopes in Hawaii are Ritchey-Chrétiens.

        However, because of the complexity of fabricating and testing a large aperture hyperbolic mirror (just ask the people who built the initially-flawed, but not discovered until it was in space, Hubble Space Telescope), traditional large aperture two-mirror Ritchey-Chrétiens are very expensive to manufacture and purchase, too expensive for many amateur astronomers.

        To emulate the coma-free performance of a true R-C telescope, while keeping the cost very much within reason, each Meade Advanced Coma-Free (ACF) catadioptric optical system uses a full aperture aspheric corrector lens in conjunction with a simple spherical primary mirror. This creates a two-element primary mirror system that performs like an R-C's single hyperbolic primary mirror from the optical point of view of the system's secondary mirror. The hyperbolic secondary mirror itself is mounted directly on the rear of the corrector lens, rather than in the traditional R-C's conventional spider vane assembly. This eliminates the image-degrading diffraction spikes of the secondary mirror support structure visible in commercial R-C scope images. The result is R-C-class coma-free wide-field performance, at only a fraction the cost of most true R-C systems.

        The corrector-modified design would itself be expensive to fabricate were it not for Meade's more than a quarter-century of experience making Schmidt-Cassegrain correctors, which are in the same optical family as the corrector needed for the coma-free design of the ACF scopes. An additional benefit of the full aperture corrector in the ACF design is slightly better correction for astigmatism than the traditional R-C design.

        In addition, the ACF design, due to its front corrector plate, is a closed tube design. This keeps the primary optical components protected from dust, moisture and other contaminants that might fall on the optical surfaces of the primary and secondary mirrors as can happen with the traditional open-tube R-C design.

        While the ACF scopes may not be traditional R-C designs, their performance is R-C-like in all important characteristics. A review in Sky & Telescope magazine of the ground-breaking predecessor of the Meade ACF optics said the bottom line is that the optics do "indeed perform like a Ritchey-Chrétien." Another such review, in Astronomy magazine said, "This scope delivers Ritchey-Chrétien-like performance at a fraction of the cost."

    • Low thermal expansion mirrors: The primary and secondary mirrors are made of low thermal expansion Pyrex to limit any possible focal length change as the temperature drops. This reduces the possibility of the focus changing during critical through-the-scope CCD imaging.

    • Oversized primary mirrors: The diameter of the primary mirror of each ACF scope is larger than the diameter of the corrector lens at the front of its optical tube that admits the light. For example, the primary mirror of the 8" scope is actually 8.25" in diameter, compared to the 8" diameter of the corrector lens. Oversizing the primary mirror in this way gives you a wider fully-illuminated field than a conventional catadioptric scope whose corrector and primary mirror are the same size. The result is a gain of 5% to 8% more off-axis light available to your eye or camera, depending on the telescope model.

    • Fully multicoated UHTC (Ultra High Transmission Coatings) optics: The primary and secondary mirrors are vacuum-coated with aluminum, enhanced with multiple layers of titanium dioxide and silicon dioxide for increased reflectivity. A final layer of durable silicon monoxide (quartz) assures long life.

          A series of anti-reflective coatings of aluminum oxide, titanium dioxide, and magnesium fluoride are vacuum-deposited on both sides of the full aperture corrector plate. These antireflection multicoatings provide a high 99.8% light transmission per surface, versus a per-surface transmission of 98.7% for standard single-layer coatings. Overall light throughput (the amount of light collected by the objective lens that actually reaches your eye or camera) is approximately 89% at the focal plane.

          UHTC multicoatings provide a 15% increase in light throughput compared with standard single-layer coatings. . For example, they effectively add the equivalent of a little more than four-tenths of an inch of extra light-gathering aperture to the performance of the 6" optical system - but with no increase in actual size or weight. The UHTC multicoatings also improve contrast, for lunar and planetary images that appear sharper and more crisply defined.

    • Fully baffled optics: A cylindrical baffle around the secondary mirror, in combination with the cylindrical baffle tube projecting from the center of the primary mirror, prevents stray off-axis light from reaching the image plane. In addition, a series of field stops machined into the inner surface of the central baffle tube effectively eliminates undesirable light which might reflect from the inside surface of the tube. The result of these baffle systems is improved contrast in lunar, planetary, and deep space observing alike.
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