What is a Motor Drive Corrector? A Motor Drive Corrector converts the output of a battery, usually 12 volts DC (direct current), to AC (alternating current). It also provides slightly different rates to "correct" the telescopes slew rate for the different apparent speed at which the celestial objects move. The idea is to automatically keep the object steady in the eyepiece as it moves across the sky. Why is a Motor Drive Corrector Needed? Telescope mounts are often supplied with AC synchronous motors, which move the telescope in Right Ascension (RA) so that it may be kept pointed at a target as the earth rotates on its axis. These motors are designed to run on "wall clock" time, the same as kept by your wristwatch. Celestial objects, however, actually move slightly faster or slower. In fact, only the Sun appears to move on this time and it is technically called "solar time". Why a synchronous motor? A unique ability of a synchronous motor is that it will turn at a very accurate rotational speed directly related only to the frequency of the applied AC power and independent of the voltage (within it's operational limits). Synchronous motors have track records of running at very accurate speed for decades. They are also inexpensive. Other motor types need sophisticated electronic circuitry to control the motor speed and compensate for the tendency of the motor to slow down over time. Stepper motors "step" and although the steps are small they sometimes vibrate enough to be seen in the eyepiece as a slight buzzing of the image. DC Servo motors can be made very smooth and very accurate but are expensive and do not have the endurance record of synchronous motors. They seem to last just long enough to fail when replacements are no longer available. Synchronous motors do have one inconsequential disadvantage: they produce very low torque. In actual practice all are provided with internal gears that compensate for this problem and provide the desired speed, usually less than one revolution per minute. TAL motors are 1/5 RPM. They do occasionally fail to start when power is supplied. A slight tweak on the Slow Motion Knob usually solves this problem. Rates The corrector supplies the AC power to operate the motor at 60 Hz (Solar time) and at slightly different frequencies for Lunar (Moon) time and Sidereal (Star) time. This allows you to accurately track either the Sun, Moon or Stars by setting the rate via the unit controls. For example: the TAL mount worm gear will take 2 hours to make 15 turns at the solar rate, 2 hours and 4 minutes to make the same 15 turns at the lunar rate and 1 hour 59 minutes and 45 seconds to make the same number of turns at the sidereal rate. The Correctors Three units are included in this review, the JMI MotoTrak V, the APM TAL 12 Volt Quarzdriveunit (which I'll call the QuartzDrive) and the Russian Vega. The Vega is a basic easy to use corrector while the QuartzDrive adds the sophistication of continuously variable rate and the MotoTrak V has everything but the Kitchen sink. The MotoTrak is currently out of production but can be bought on the used market. Not surprisingly, the Vega is the least expensive while the MotoTrak cost the most. The Vega and QuartzDrive both eliminate the need for the large transformer that comes with the TAL scopes. They instead provide 12 VAC directly to the mount; all you need is the battery. The MotoTrak V provides 110 VAC so the transformer is needed to step its juice down to 12 V. The Vega and QuartzDrive thus are more or less dedicated to the TAL scopes. But any of the units can be adapted for use with any mount that uses a synchronous motor and slews at the solar rate when plugged in to wall power. At worse case you need a 12 V transformer. The Vega The Vega has Solar, Lunar and Sidereal pushbuttons plus arrow keys that speed up or slow down the drive for making minor corrections. I really like these buttons; they provide a very satisfying click and tactile feedback when you push them. The arrow keys shift into a different speed; faster fast or slower slow, if you hold them down for 2 seconds. To use, just attach the unit to a 12V battery via the clip leads provided to a 12 V battery plug the other cable into the mount and push the button under the symbol for the rate desired and you are in business. If you get behind just push the ">" key to catch up. If you get ahead, push the "<" key. The Vega is provided with an output cable with the matching Russian connector to mate with the connector in the motor housing. The input cable uses a standard DC power plug on one end and a pair of clip leads on the other. The use of the standard DC power plug allows use of accessory cables with cigarette lighter plugs and indicator LEDs. The Vega has one captive cable, the output. The Vega is small and very, very light. If you like simplicity and want to save a few bucks, you'll like this little box. The QuartzDrive This unit does not have pushbuttons. Instead it has a rotary switch for selecting a fixed rate and a potentiometer for controlling a variable rate. It lacks the arrow keys but makes up for this with the variable speed. This variable feature is for the more advanced observer who perhaps wants to smoothly guide the scope manually for long photographic exposures. This is a feature not even the MotoTrak has. To use it needs to be connected to a battery just like the Vega, with clip leads. It does have polarity reversal protection that prevents damage to the unit if you hook it up backwards. This is a very desirable feature. I once fried a $300 Sony radio this way. Connection to the mount is a little less easy as the unit does not come with a Russian connector. Instead the output cable splits in two with each end fitting over an individual pin in the connector inside the mount. Some force is required to push the plugs on, but it is very secure and effective. The input and output power cables are both captive to the unit and a handy "rope" loop is provided that can be used to neatly hang the unit from the mount. Next, turn the rotary switch to the desired rate and you are in business. The unit I reviewed is labeled in German but easy to figure out. The "Quarz" position is the sidereal rate, solar is "Sonne", and Lunar is "Mond". It also has "Aus" (Off) and Stop. When using the Variable rate, you must turn the knob slowly and deliberately, otherwise you'll drive the little motor nuts. The computer can change the rate much faster than the motor can respond. You might even damage the motor if you habitually turn the knob too fast. The QuartzDrive gets a little warm in use. This may turn out to be a handy feature; the unit will make a nifty hand warmer on those cold winters's nights when the stars blaze the brightest! The MotoTrak V The MotoTrak has pushbuttons for selecting the rate also, but it has 4 of them plus one labeled "?" and one labeled PEC. It has sidereal or star rate, lunar, solar and a special speed called "King rate". King is slightly faster than sidereal and is used for tracking objects near horizon when the lensing effect of the atmosphere causes the object to move faster than it does when it is higher up. It has arrow keys too, but 4 of them instead of 2. They are labeled N, S, E, and W. The N and S buttons do nothing unless your mount also has a 12 VDC declination motor (available from JMI called a "Motodec".) Note also that, with this motor, a MotoTrak and a CCD with autoguider output your mount is equipped for fully automatic tracking using a guide star. The MotoTrak has a connector socket that accepts and autoguider RJ type cable. The E and W buttons also serve as the arrow keys, providing slightly slower or faster speeds for minor adjustment. Like the Vega unit, they have two "speeds" but do not shift automatically. Instead, if you hold down both buttons for 1 second the speed changes from coarse to fine; push again and the speed again reverses from fine to coarse. The W button also serves to start the motor running. When you turn the unit ON, you must push this button to start tracking objects. The MotoTrak can also drive a focus motor! Lastly it also will power and blink a reticle eyepiece if equipped with a cable with a mini phone plug (rather than a built in battery). I have used this feature a number of nights. Now for the sticky part. The MotoTrak provides PEC or Periodic Error Correction. No worm drive is perfectly machined. There is always a small error, which shows up as a slowing down then speeding up of the drive (or vice versa) in time. Luckily the error is the same for each turn of the worm, that's why it's called "periodic". The MotoTrak "corrects" for this by watching you track an object through one turn of the worm (8 minutes for a TAL mount.) It remembers the correction you applied, even if you turn the power off, and repeats this over and over again. The hitch is that you cannot use the slow motion knob or you get out of sync. You'll notice a strange thing when you turn the MotoTrak off; it takes about 3 seconds to shut down. This is because it is slowing down the motor before power shutdown. This compensates for any tendency of the mount to "coast". TAL mount Periodic Error The TAL mounts I have tested have the error piled up at one point in the worm rather than scattered throughout the turn. You can make use of this information when making timed exposures with a camera or CCD. If you observe the error of your worm, you can mark the slow motion knob at the point when the error occurs. Then you can start a photo exposure right after that point and expose for nearly 8 minutes without correction. You can also make even longer exposure by closing your camera shutter (or covering the business end of your scope with a hat) while the mark passes by. Which corrector is for you? It depends on the features you want and the accessories you use with your scope. TAL owners are fortunate to have such a choice.