By the middle of the first decade of the twenty-first century SCT-using amateurs were getting a mite antsy. What would Meade do next? Celestron was playing things fairly safe with the CPC. Would Meade up the ante as far as technical innovation, something they have been known to do frequently? Indeed, they did, with the LX400-ACF (initially called the RCX400). This innovative CAT, unfortunately, was a little too much for most amateurs—too much money and too many radically new features. Meade did not forget the rank and file, however, and soon replaced their former top-kick scope LX200GPS with the $2,700 LX200-ACF (Plate 19).
"Replaced" is probably too strong a word. All Meade did to update its LX200GPS SCT into an LX200-ACF (originally sold as the LX200R) was change the optics in the OTA. Meade's Advanced Coma Free optics are different, but not tremendously different, from those used in the company's other and earlier SCTs. The focal ratio is still f/10, and the coatings are Meade's advanced UHTC recipe (standard). What is changed is the secondary mirror and the corrector. The "traditional" Meade (or Celestron) SCT has heretofore been equipped with a spherical convex secondary. The ACF-type SCT replaces this with a secondary mirror that is figured as a hyperbola (or a parabola, depending on which optics guru you listen to), a deeper curve, instead. The primary remains a sphere. The corrector may be slightly altered in figure for the new optical prescription but is much the same.
What benefits do these "optimized" optics confer on the amateur? Not many, not for the visual observer, anyway. Their main benefit is that they flatten the naturally curved SCT field, making stars look "tighter" away from the center of the field, delivering sharper stars and less "coma" (although field curvature is a far more serious problem for SCTs, and that is mainly what the ACF fixes rather than true coma) than normal SCT optical sets. Amateurs have been achieving this same effect for a long time, however, by using one of Meade's or Celestron's inexpensive f/6.3 reducer/ correctors, which have the added advantage of making the scope's field wider.
One group of amateurs will benefit from the new optics: astrophotographers using digital single-lens reflex cameras (DSLRs) or astronomical CCD cameras with large chips. While reducer/correctors can be useful for cameras with smaller imaging chips, using a reducer/corrector with a DSLR tends to result in vignetting. The entire frame is not evenly illuminated; the resulting picture gives the appearance of looking through a porthole. That can be cured or at least improved with flat-field frames and other processing tricks, but it is always best to work with an image that does not require much cleaning up. ACF-type images are flatter, mostly free from vignetting, and require less postprocessing.
Other than the optics, what is the LX200-ACF like? There is a built-in GPS receiver like the one on the LX90 that makes alignment in alt-azimuth mode a joy. The fork? It is sturdy, if not overkill. The LX200-ACF replaces the standard 497 Autostar with the Autostar II, which amazingly adds even more computer-
Plate 19. (LX200-ACF 8-inch) Meade's latest LX200 features the company's "Advanced Coma-free" ACF optics. Credit: Image courtesy of Meade Instruments Corporation.
ized features, including an overwhelmingly huge library of 147,541 objects (if you cannot see many of these, you can at least image some of them). One of the more important features of the LX200-ACF for imagers is the mirror lock. Once focus is achieved, the mirror is locked down with a rear cell knob to prevent mirror "flop" during long exposures. What if a focus touch-up is needed after the lock is engaged? Meade provides an Autostar-controlled motorized Crayford-style focuser that attaches to the rear port.
What accessories are included with the ACF? In addition to UHTC coatings, there is the standard Meade field tripod, which is both heavy enough to hold the scope fairly steady but still light enough to spare middle-aged astronomers' backs. Power is provided via eight C cells that are as useless with this scope as they are with the LX90 (optional AC supplies and DC cords are readily available). The finder is the same good 8 X 50 shipped with the LX90. The included eyepiece is Meade's 25-mm Series 5000 Plossl and is paired with a better-than-average 1.25-inch diagonal. Meade usually also throws in a copy of the Autostar Suite planetarium software.
The drive system on the ACF is Meade's good worm-spur gear set, which, unlike the LX90 drive, features PPEC. Record a guiding run, striving to carefully guide out the occasional fluctuations the LX200-ACF's drive—like any telescope drive— displays, and periodic error will be drastically reduced.
Unlike the PEC system used in the LX90, this PPEC recording is not lost when power is turned off at the end of the evening. The ACF's drive is also blessed with a feature called "Smart Mount." This is a software utility accessed from the Autostar II HC that allows the scope's go-to pointing accuracy to be refined by sighting multiple alignment stars (more than 40) following a "normal" go-to alignment. This procedure is probably mainly of interest to observers with permanently mounted telescopes since those 40 stars must resighted if the telescope is moved. Frankly, Meade's normal go-to accuracy is good enough that visual observers and most imagers will not need to bother with Smart Mount.
The bring-downs associated with the LX200-ACF are few but need to be mentioned. While reasonably priced, this is not a cheap scope at $2,700. It has a lot of features and frills, but many amateurs would be just as happy with the similar and cheaper LX90-ACF. Also, while LX200-ACF's go-to accuracy is very good, its tracking accuracy is average at best. At the scope's native focal length of f/10, do not expect unguided exposures longer than 30 seconds even with a careful polar alignment. The scope can be autoguided with CCD cameras, but it may take considerable tinkering with autoguide software settings and PPEC "training" before the LX200-ACF's mount behaves well enough for long exposures. The addition of a reducer/ corrector can help, but Meade has not released a reducer/corrector designed for the ACF's slightly different optics. A "stock" f/6.3 reducer/corrector can be used, but it may not provide results as good as those on a standard SCT.
The altitude lock on this SCT, like the one on the LX90, does not have a firm feel when tightened. This has driven some users to invest in the aftermarket mod kits sold by Peterson Engineering, which makes several interesting accessories and mod kits for these scopes, that allows the declination axis to lock firmly without requiring the knob to be cranked down hard. As is the case with the LX90, this is probably not needed; the scope is usually held firmly enough with the standard lock finger tight. Finally, small hardware—nuts and bolts on the OTA and tripod—is another minor issue. As with its other CATs, Meade does not use high-quality stainless steel hardware, so screwheads may begin rusting after several dew baths. Some users replace these bolts and screws with a better grade of hardware, but a little rust does not do harm beyond the cosmetic.
There are a few negatives, true, but not enough to steer CAT buyers away from the LX200-ACF. This is a sophisticated scope with very good optics, perhaps the finest optics available in a production SCT. If you are after a fork-mounted SCT for general use, a top-of-the-line model with tons of features, you should give strong consideration to the ACF. Although many folks look on this as a scope for imagers, its real strength may be for visual observing. Mounted in alt-azimuth fashion, the scope is extremely solid and a joy to use.
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