S & T TEST REPORT

The JMI MOTOTRAK V

by Dennis di Cicco

     Aperture Envy. While many amateurs first caught this bug when it reached near-epidemic proportions during the Dobsonian revolution of the late '70s and early '80s, astrophotographers were afflicted with it long ago. For years I've collected countless items as part of a master plan to build ever bigger telescopes. But the digital revolution has altered my thinking.
     The dream of having a bigger telescope hasn't died; it just dropped a few notches on the priority list. My 11-inch Schmidt-Cassegrain that once struggled to record 16th-magnitude stars photographically now routinely reaches 19th magnitude with CCD exposures that are not only shorter but also more affected by growing light pollution.
     While CCDs put my aperture envy into remission, they brought about a new problem, one a friend aptly described as "drive envy." A good drive certainly makes an astrophotographer's life easier, even though long-exposure photographs still have to be guided. Autoguiders have lightened the burden, but they take time to set up properly. Fifteen minutes spent readying an autoguider is a minor price to pay for a 1-hour exposure, but it's a lot of overhead for a 3-minute CCD image.
     Exposures that short don't need guiding, if the telescope drive is first rate. Technological advances first gave us better gears. By the late 1980s I was testing production telescopes with periodic errors smaller than 20 arc seconds. Then the electronic revolution came to amateur telescopes. Both Celestron with its PEC and Meade with its Smart Drive reduced errors into the 5-arc second range. Edward R. Byers Co. has even come out with a mounting and computerized tangent drive that one astrophotographer says has "no discernible periodic error."
     But where does all this leave those of us who use old telescopes?

JMI to the Rescue

     Almost as soon as the term PEC (periodic-error correction) became main-stream jargon, JMI introduced its Mototrak V and the promise of periodic-error correction for many old telescopes. These include ones with worm-gear drives having 360, 359, 144, and 96 teeth, as well as the unusual 216-tooth gears found in the 11- and 14-inch Celestron. (You must identify your drive when ordering the Mototrak V, since its electronics are customized for the rotational period of the worm.) The dual-axis drive corrector (it operates a declination motor too) has additional outputs to control a variable-speed, focusing motor and make an illuminated guiding reticle blink. Other Mototrak V features include selectable drive rates, one of which can be programmed by the user, and an optional rechargeable battery for self-contained operation.
     All this fits neatly into a hand-held package weighing a mere 12½ ounces (the optional battery adds 7 ounces more). I found the unit easy to operate when holding it with two hands, but it is a bit awkward for single-hand operation. This is especially true if it is powered by an external 12-volt DC source (a wall transformer and cigarette-lighter adapter are standard accessories), since wires then extend from the top and bottom of the unit. There is a modular jack for inputting signals from an autoguider. JMI also offers an optional hand control that plugs into this jack so you can guide a telescope without having to hold the Mototrak itself.
     The feature that most intrigues me is the PEC. It really works! My 1982-vintage, 11-inch Celestron has a periodic error of 35 arc seconds. Unfortunately, much of this error occurs in a small segment of the worm's 6.7-minute period. Yet my very first attempt to "train" the Mototrak by manually guiding the telescope during one revolution of the worm reduced the periodic error to about 5 arc seconds. To quote from my notes: "Wow!"
     How much you can reduce a drive's periodic error depends on how well you guide the telescope during the training session. It is possible to have an autoguider do the work, but there's a catch. The Mototrak can "remember" only a maximum of 125 corrections during a revolution of the worm. For my "slow" worm, this works out to no more than about one correction every 3 seconds, which to too leisurely for the quality of guiding I want during the worst part of my worm's cycle. Setting the autoguider to make more frequent corrections can exceed the 125 limit, ruining the training session. (This will be even more of a problem for people whose drives have 144- and 96-tooth worm wheels and worm periods of 10 and 15 minutes, respectively.) My solution was to train the Mototrak manually, being careful to make minimal corrections during the part of the worm's cycle that had little error. Finally, Mototrak V has user-selected high- and low-speed guiding rates. Either can be used for training the PEC, but switching to the other mode renders the training useless.
     PEC training is retained even when the drive is switched off. Over several seconds the electronics slow the motor and remember where in the worm cycle it stopped. Over three weeks and a dozen on-offs, the PEC seemed to maintain its accuracy.
     Another Mototrak feature I like is the user-programmed drive rate. Any frequency from 40 to 80 hertz can be set with a precision of 0.002 Hz. This is great for homemade drives that have odd gearing, or for achieving unusual rates for special applications. Unfortunately, the PEC does not function with these non-standard rates.
     The optional rechargeable battery pack is a standard Radio Shack item, and it drove my scope for about 5 hours, long enough for a typical evening under the stars. The pack requires 14 to 16 hours to fully recharge with a small wall transformer.
     With the Mototrak V, JMI has once again developed a product that gives old telescopes the ability to keep pace with state-of-the-art instruments that are redefining the way amateurs do astronomy.