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Turbocharging the Losmandy G11: 3.5 arc seconds peak to peak

Posted by Greg Nowell   06/24/2008 00:00:AM

Turbocharging the Losmandy G11: 3.5 arc seconds peak to peak
The Losmandy G11 equatorial mount has been around for so long that it has even been derided as a "dinosaur." Nonetheless, this workhorse of the astronomical community keeps on expanding its user base in the astronomical community.

There appear to be some good reasons for this longevity. The Losmandy G11 and G8 are the last of the well known, good quality German equatorial mounts that are still sold in both a go-to and a stepper motor version. They still offer analog setting circles, which are useful "when all else fails" in the field, and also for evaluating the accuracy of electronic systems. Its large user base has promoted the development of a significant aftermarket. The user community has "appropriated" the G11 in ways that give it a life independent of the manufacturer, and this general purpose equatorial mount is seen carrying virtually every known optical tube design. Much like car tinkerers, the user community adds bearings, changes the power train, and makes other alterations. Meantime the manufacturer is not idle: Options such as the heavy duty folding tripod are now available. Cosmetically the anodized Losmandy finish keeps its appeal even after years of heavy use. The electronics are extremely durable in all climate conditions, and the only major design which intelligently bolts the computational brains of the system to the mount where they won’t get dropped. Ergonomically the altitude adjustment design, a real problem in many GEM configurations, is one of the best in the business. The ability to track past the meridian one of the features that distinguishes the mount from competitors such as Celestron's CGE.

Just as the B52 bomber has undergone major modifications without changing its basic shape, the Losmandy G11 has been through a number of iterations since its first development. Early on the altitude lock was improved and this upgrade is available for the older mounts. The mount was one of the first on the market with go-to, whose software has been updated many times since its first release. While Celestron and Meade have the resource base to incorporate many software driven functions in their paddles, Astro-Physics and Takahashi have lagged by not incorporating such capabilities as mount modeling and polar alignment correction. The Losmandy Gemini system by contrast is fully competitive with the major companies' software, and has been adopted by two other makers of equatorial mounts. For those not interested in go-to gadgetry the stepper version has also been improved with the conversion from Hurst to SAIA motors, which reduce vibration and are very quiet.

As astrophotography has become a more widespread and affordable passion, the periodic error of the mount has become of increasing concern in the user community. The original mount had typical periodic error in the range of 15 arc seconds peak to peak. This was basically a "visual" mount whose periodic error could be controlled for film. Lobbied by the user community, the company released a high precision stainless worm in the early 2000s that was followed by the precision brass worm. User reports of the brass worm's performance are good, with some reports showing periodic error under 5 arc seconds peak to peak, out of the box.

Nonetheless the user community has continued to report dissatisfaction with the bearing block configuration on the mount. The individual bearing blocks are difficult to adjust and have a fair amount of play, that has led to reports of cyclical errors with a frequency below the four minute period of the worm itself. These errors are difficult to guide out. Fastidious users have been able to shim the blocks to reduce this problem, but most recently, the French company Ovision ( has released a single-unit worm and worm block that makes the mount a breeze to adjust and reduces periodic error to well under 5 arc minutes. (On the G8, periodic error will be 2x the G11, because it has 180 teeth instead of 360).

Although I personally am not an imager, I was lucky to get the help of local club member Harvey Patashnick with a preliminary evaluation of the G11 worm. Harvey not only did the photographic set up and capture, he generated the graphics below and was the sole source of tech support for this project. Harvey guided me through two tests using an FS128 with ST7. Software Bisque's "track and accumulate" function was used to evaluate trailing star images with the old worm gear and the new Ovision NS worm gear. (Trailing star images are acquired by throwing the mount out of polar alignment and then doing a timed exposure on a star’s drift at the celestial equator.) Harvey then graphed the data using a least squares function. These tests were not done with the precision that is possible with products such as PEMPRO. When we tested the Ovision worm, the backlash was much looser than it should have been. I had misunderstood the directions about the desired level of backlash. Extra backlash can lead to a bit more wobble in the PE, as seen in the final cycle of the test results on the right. Even so, results were under 4 arc seconds peak to peak. The two test durations were 750 seconds, or three complete 240 second revolutions and thirty seconds extra. These test results are not definitive, but hugely indicative: If a person can plop down his money, turn four screws to slap on a worm (two for the worm, two for the Oldham coupler), and get results like these, it is a remarkable development. Cautiously put, the test below serves the basic purpose of alerting the user community of the major potential benefit that the $500 Ovision NS worm retrofit offers. In the coming months additional results from other users are expected on the Losmandy Yahoo! group.

The results, in the range of 3.5 to 4 arc seconds peak to peak, confirmed Ovision's own certification of the worm's performance (which were done with considerably more sophisticated software and protocols). An Ovision-retrofitted mount (which I dub the G11-O) appears capable of meeting the 7 arc second peak to peak performance that is the standard for such high end brands as the Astro-Physics Mach 1 or the Paramount ME. In short, the G11-O is likely to satisfy most imaging requirements in its weight class, and is capable of unguided exposures in the 2 minute range. Combined with the superior paddle performance, the G11 is reborn as a serious competitive alternative to mounts that cost two and three times as much, taking into account that the G11 comes equipped with tripod, counterweights, and mounting plate, and that the paddle's modeling capabilities makes the need for laptop software control optional. Moreover, since the Ovision worm retrofit can be used on all the G11s post-1992, one could pick up a used G11 for $500 and retrofit the Ovision worm to achieve under 5 arc second peak to peak periodic error for, at typical used prices, as little as $2300 (stepper version) or $2800 (servo-Gemini version). Most of the G11 mounts of produced over the past two decades are now in a position to compete, as astrophotography platforms, with contemporary high end mounts.

The Berlebach wooden tripod, configured not just to hold the G11 and G8 equatorial head but also hold its electronics, is an attractive option for users who prefer wood's dampening characteristics and relative ease of use in extreme cold. For many years the Losmandy G11 was considered the "low end of the high end," the German equatorial mount one got when one first got serious about good quality. Limitations on the performance of the worm, however, often frustrated serious imagers and caused them to move on to more expensive alternatives, even though their load requirements stayed the same. With the availability of the G11-O, the Losmandy mount may be not only astoundingly cost effective but the last German equatorial many astrophotographers will ever need.