January 2014
I'm looking for a motor drive for my 6" refractor telescope. A stepper motor should give me position control, but big steppers are expensive. DC motors are cheaper, but require a complex gear box and a sensor to determine position. Is there an affordable option that provides me with the positioning benefits of a stepper? My telescope and camera back weigh about 6 lbs total.
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Telescope position control is not for the faint at heart. First of all, the controller must be able to move the telescope in Azimuth (in a circle around the scope) and elevation (vertical angle from the horizon to the point above the scope - zenith) and convert these angles to the celestial equivalents of latitude and longitude called Declination and Right Ascension, respectively.
You must start with the telescope aligned with the Earth's axis of rotation which can be accomplished by sighting Polaris (the North Star). Then the telescope position controller must be able to track the object you are interested in by countering the Earth's rotational motion.
Before tackling building a positioning controller, (not an easy feat involving electronics, mechanical systems and programming) look at Celestron (www.celestron.com), Meade (www.meade.com) or Gemini (www.bisque.com/help/theskyv6/telescope/Gemini_by_Losmandy_Instruments.htm). Both companies make telescope positioners for a variety of telescope sizes and types.
Why would you use a big stepper to drive a telescope? Surely you are not considering a direct connection to the telescope's polar axis? That would produce a very jerky motion instead of the smooth motion needed for observation or photography. Therefore, you still need gears and you really need a high ratio worm gear or a series of compound gears to get a high reduction ratio. A relatively small stepper or DC motor will then work just fine.
Amateur telescope makers have used a wide variety of drive mechanisms. I have seen worm drives made by wrapping a threaded rod around a wood disk to form the "teeth". A screw with the same thread pitch can be used to drive it. If the scope is well balanced, little torque is needed to drive it. On the other hand, second hand worm wheels can often be found in places like ebay and other resellers.
Since a telescope moves at such a slow rate, some are driven by a cord or wire or strip of metal that is wrapped around a wheel instead of a gear. The cord/wire/strip is pulled by a nut that runs on a threaded rod that is turned by the motor. Since you can track from sunset to sunrise with only a half turn of the polar axis, you don't even need the full circle on that wheel. A fast mode or a nut that can be released would allow faster resetting for another observation.
