Celestron 2-Axis DC drive/drive corrector for CG-4 mount telescopes

There are five parts to the dual axis drive. The first is a chrome and brass declination clutch assembly that attaches to one end of the mount's manual declination slow motion control shaft. The second and third parts are sidereal rate DC right ascension and declination motors. They have the appropriate mounting brackets to connect them directly to the right ascension slow motion control shaft and to the chrome and brass clutch assembly (part 1) that you have attached to the declination shaft. The fourth part is a hand control that contains the drive electronics and a built-in dual axis drive corrector. The last part is a detachable battery pack.

The right ascension motor drive works by electrically turning the mount's manual slow motion right ascension control at the precise sidereal rate needed to keep objects centered in the field of view as they move across the dome of the sky. It also allows you to speed up or slow down the drive during long exposure astrophotography to compensate for periodic right ascension errors in the motor drive train and drift due to errors in polar alignment. The declination motor drive allows you to compensate for atmospheric refraction effects and errors in polar alignment during photography. Visually, both drives let you keep objects centered while observing (such as the Moon, which would otherwise gradually drift out of the eyepiece field due to its retrograde motion across the sky). They also let you correct for drift due to polar alignment errors when observing at high powers.

The hand control has a switch that turns the drive on or off and simultaneously selects either northern or southern hemisphere operation. A second switch sets whether the drive corrections are to be made at 2x, 4x, or 8x the sidereal rate. Four pushbuttons in a diamond pattern provide the right ascension and declination drive correction functions. An indicator light in the center of the pattern indicates when the drive is active.

The hand control with its attached r. a. and dec cables weighs only 6 ounces. Four D-cell flashlight batteries (not supplied) power the drive. One set of alkaline batteries provides up to 20 hours of use (depending on the ambient temperature, with colder weather yielding a shorter useful life).

The chrome and brass clutch assembly allows you to manually override the declination motor drive at any time. This lets you fine-tune the scope's position during visual observing using the mount's manual declination slow motion control, even while the drives are operating. During astrophotography, you probably will have done a precise polar alignment, so the correction speeds of the hand control will be more than adequate to keep objects centered.

However, for visual observing, you may not choose to take the time to do a precise polar alignment. A too-casual polar alignment can occasionally lead to excessive declination drift that the normal correction speeds can have trouble keeping up with. To correct these large errors, a quick turn on the knurled chrome clutch knob temporarily disconnects the dec motor from the telescope so you can use the scope's standard manual control knob to make much greater corrections than the drive corrector can allow. Once the scope position has been manually adjusted, retightening the chrome clutch knob once again allows the motor to control the scope motion.