LX200-ACF UHTC 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 (RC) design uses approximately 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 RC 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 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 RC telescope, while keeping the cost within reason, the LX200-ACF 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 RC's single hyperbolic primary mirror from the optical point of view of the LX200-ACF secondary mirror. The hyperbolic secondary mirror itself is mounted directly on the rear of the corrector lens, rather than in the traditional RC's conventional spider vane assembly. This eliminates the image-degrading diffraction spikes of the secondary mirror support structure visible in commercial RC scope images. The result is RC-class coma-free wide-field performance in the LX200-ACF, at about a fourth the cost of most true RC 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 LX200-ACF. An additional benefit of the full aperture corrector in the ACF design is slightly better correction for astigmatism than the traditional RC design.
        In addition, the LX200-ACF, 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 RC design.
        While the LX200-ACF may not be a traditional RC design, its performance is RC-like in all important characteristics. A review in Sky & Telescope magazine of the 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."

  • Oversized primary mirror: The diameter of the primary mirror of each LX200-ACF is larger than the diameter of the corrector lens at the front of its optical tube that admits the light. The primary mirror of the 8" scope is actually 8.25" in diameter, compared to the 8" diameter of the corrector lens. The 10" primary is 10.375" in diameter; the 12" is 12.375"; the 14" is 14.57"; and the 16" primary is 16.375" in diameter. 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 overcoating 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. They effectively add the equivalent of 15% extra light-gathering area to the performance of a scope with standard coatings. It's the equivalent of three-quarters of an inch of extra aperture in the case of a 10" scope, for example, but with no increase in actual size or weight. UHTC coatings 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.

  • Mirror lock: A progressive tension lock knob on the rear cell locks the telescope's primary mirror rigidly in place once an approximate manual focus has been achieved. The standard equipment electric focuser, described below, is then used for fine focusing. Locking the mirror eliminates the possibility of mirror shift (the image moving from side to side while focusing, caused by the primary mirror tilting on the central baffle tube as the mirror moves fore and aft along the tube). Mirror shift, once the bane of CCD astrophotographers because it could easily move the image off a small CCD chip, is non-existent with the Meade system.

  • Electric focuser: The supplied zero image-shift electric microfocuser is controlled by the Autostar II computer hand control. It moves an externally-mounted eyepiece or camera to focus, rather than moving the primary mirror. This eliminates mirror shift during precise image centering and focusing for CCD applications. The microfocuser has four different operating speeds, from very fast down to an extremely slow creep, giving you focusing accuracy to a truly microscopic level during critical visual and astrophotographic applications.
        The focuser is designed to hold 2" star diagonals and eyepieces. A supplied 1.25" adapter allows the use of 1.25" diagonals and eyepieces in the 2" focuser. Another supplied adapter duplicates the 2" rear cell thread used on Schmidt-Cassegrain telescopes to allow the use of off-axis guiders, T-adapters, etc. A 1.25" visual back is not supplied with the scope. If you want to do high magnification eyepiece projection photography of the Moon and planets, you will have to add an optional 1.25" visual back #9135 and a tele-extender to the focuser's supplied 2" rear cell thread adapter.
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