LX400-ACF optics

Optical 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 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 Hubble Space Telescope), traditional two-mirror Ritchey-Chrétiens are very expensive to manufacture and purchase.

        To emulate the coma-free performance of a true RC telescope, while keeping the cost within reason, the LX400-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 LX400-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 LX400-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 quarter-century of experience making Schmidt-Cassegrain correctors, which are in the same optical family as the corrector needed for the modified RC design. An additional benefit of the full aperture corrector in the modified RC design is slightly better correction for astigmatism than a traditional RC scope. In addition, the RCX, due to the front corrector plate, is essentially a closed tube design (however, due to the front cell focusing method discussed below, there is an small opening between the tube wall and the periphery of the corrector, which prevents the tube from being totally sealed). This essentially closed tube design reduces the amount of image-degrading dust, moisture and other contaminates that would otherwise fall on the optical surfaces of the primary and secondary mirrors as is the case with traditional open-tube RC designs.

        While the LX400-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 . . . Some hobbyists mistakenly believe that a product review without equal doses of praise and criticism is biased or unbalanced. Truth is, I can't find many negative things to say." Another review, in Astronomy magazine said, "All things considered, this scope more than lived up to my hopes. It is well-made, easy to use, and has high-quality optics . . . This scope delivers Ritchey-Chrétien-like performance at a fraction of the cost."

  • 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 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 Cassegrain focus.
        UHTC multicoatings provide a 15% increase in light throughput compared with standard 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.

Mechanical features of this Telescope's Optical System . . .

  • Fixed primary mirror with computer optimized primary and secondary baffling: Unlike traditional catadioptric designs that move the primary mirror fore and aft along the central baffle tube in order to achieve focus, the LX400-ACF primary mirror is fixed and independent of the baffle tube. The primary mirror is laser aligned to the true optical path, then float-bonded in place on the rear cell. Although fixed in position in the optical tube, it literally floats on a layer of adhesive instead of resting on the baffle tube. This results in zero stress to the glass and no distortion in the optics (unlike conventional mirror cells, which can cause pinched optics if not properly assembled). This fixed and independent mirror allows a no-compromise baffle design with full stray light cut-off, producing the maximum possible contrast. A series of field stops machined into the inner surface of the baffle tube effectively eliminates undesirable light which might reflect from the inside surface of the tube.
        The secondary mirror baffle, which is attached directly to the rear of the full aperture corrector plate, is machined of aluminum with a series of distinctive knife-edges around its outside. These prevent stray off-axis light from reaching the image plane. The result of these primary and secondary baffle systems is improved contrast in lunar, planetary, and deep space observing alike.

  • Encoder-measured digital front cell focusing: The patented LX400-ACF electric front focusing system produces a razor sharp focus, with no image shift. With the laser-aligned primary mirror fixed in position, focusing is done by using three encoder-controlled motors to precisely move the entire corrector lens, with its attached secondary mirror, fore and aft in the front cell. It moves in increments as fine as 1/100 of a millimeter, at any of four different focusing speeds. There's a digital readout of the focus position on the telescope's AutoStar II hand control. Since focusing is accomplished without sliding the primary mirror along the baffle tube, image shift (once the bane of CCD astrophotographers because it could easily move the image off a small CCD chip) is virtually non-existent.

  • Electronic focus presets: The LX400-ACF allows the observer to preset up to nine individual focus positions to customize the focus for observers with differing eyesight characteristics (similar to the custom settings in luxury cars that change the mirror, seat, and steering wheel settings from one driver to another). The feature is also very useful when switching between various eyepiece and Barlow combinations, or when switching from a visual setup to a camera setup.

  • Electronic collimation: Precision collimation adjustments to the secondary mirror are made electronically by using the arrow keys of AutoStar II hand control while observing, rather than by using a screwdriver or hex-head wrench to adjust small collimating screws in the dark on a trial and error basis. The LX400-ACF observer sees the results of a collimating adjustment instantly as it is made, shortening the time needed to collimate the scope by as much as a factor of ten. Collimating a Cassegrain telescope has never been easier.
        In addition, Meade precisely collimates the optics at the factory and sets that position as the default setting in the AutoStar computer. So, if a newcomer to astronomy succeeds in accidentally decollimating the scope, rather than collimating it, he or she can always return to the correct factory default setting by simply pushing a button.

  • Built-in dew heater: Aftermarket dew heaters wrap a heating element around the telescope's optical tube to send heat through the scope's metal front cell to warm the corrector. This prevents dew from forming on the corrector plate. Having to heat the entire metal cell in order to slightly warm the lens can drain a battery quickly. The LX400-ACF incorporates a standard equipment nickel-chromium wire heating element that is in direct contact with the glass of the corrector plate. This quickly, efficiently, and safely warms the lens using the lowest power drain possible.
        There are two onboard temperature sensors, one inside the fork arm to measure the ambient temperature, and one to measure the temperature of the corrector plate itself. By using the information from these two sensors, the built-in dew heater can be set to keep the corrector plate warmed to a user-defined setting just above ambient temperature. By automatically using the dew heater only when needed, battery drain is kept to a minimum. All functions to operate the dew heater are controlled by the AutoStar II hand control.

  • Advanced front and rear cell architecture: The front and rear cells of the LX400-ACF are designed to allow the maximum amount of air-flow around the optics to achieve the quickest "cool down" times. To accelerate the cool-down, a built-in fan on the rear cell can be turned on and off through the AutoStar II hand control.
        The rear cell incorporates a panel with eight electronic ports - three USB 2.0 ports to connect auxiliary equipment, an autoguider input, the input for the AutoStar II hand control, an output to power an illuminated reticle eyepiece, an RS232 communications port, and a port for future "smart" accessories. Some equipment moves with the scope, such as the AutoStar II hand control and Deep Sky Imager. Attaching the cords and cables of this equipment directly to the tube, rather than stretching them to attach to the drive base, virtually eliminates cord wrap. This protects CCD cameras and other accessories from accidentally pulling out of the telescope as the scope slews. The rear cell is flatter than conventional scopes to maximize the clearance between the rear cell and the fork mount. Two tube positioning handles are built into the rear cell.

  • Carbon fiber and Kevlar optical tube: The optical tube is fabricated from a woven carbon fiber and Kevlar composite that forms a unique, light-weight, and high strength material. It has ultra-low thermal expansion characteristics. This maintains the critical spacing between the optics so that the focus does not change due to tube expansion and contraction as the ambient temperature changes, a critical feature to astrophotographers.
        Instead of forcing the mechanics of the scope to fit into a perfectly round optical tube assembly, the tube is shaped to conform to the internal mechanisms and drive system of the front focus and collimation assembly. This "form follows function" philosophy gives the LX400-ACF optical tube its unique look and style.
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