This low profile electric focuser for older model Celestron NexStar 5, 5i, 8, 8i, and 8i Special Edition Schmidt-Cassegrains is designed to let the focuser clear the scope base when the scope is aimed at the zenith in the standard altazimuth mode. Because of the close spacing between the scope’s rear cell and drive base when the scope is aimed at the zenith, focusing is done by means of a belt drive from a miniature motor mounted beside the focus knob, rather than a larger motor mounted directly on the knob. This method adds no depth to the focus knob, allowing it to clear the scope base. This focuser is not designed for current model Celestron NexStar 5SE, 6SE, and 8SE Schmidt-Cassegrains.
The feature image below shows the sequence used in mounting the focuser. First, the top image shows the scope with its existing focus knob removed. The motor mounting bracket has been installed around the exposed focus mechanism. The middle image shows the new focus knob installed, with two rubber O-ring drive belts stretched over the new focus knob. Two drive belts are used to increase the transmitted torque from the motor to move the heavier 8” mirror. One belt is normally used with the lighter mirror 5” scopes. The bottom image shows the completed installation with the protective cover plate mounted over the focuser’s drive belts.
The electric focus motor provides smooth, precise, vibration-free focusing without the need to touch the telescope. The rubber belt indirect drive system allows manual focusing at any time, should you want to override the electric focusing system. Mounting is easy, with no drilling or tapping needed, using supplied hardware and existing holes in the rear cell of the scope.
The focuser comes with a Pulse-Width Modulation (PWM) hand control that contains a 9-volt transistor radio battery (supplied) with a life expectancy of 6-8 months in normal use. Pushbuttons control the direction of focuser travel. The hand control has a small three-position (high / medium / low speed) slide switch that adjusts the motor’s speed for very precise fine adjustment of the focus when the directional buttons are pushed. The three slow speeds each ramp up to high speed if the directional button is held down for two seconds. This lets you make major focus changes quickly, without giving up the precision fine correction ability. Quick-stop electromagnetic braking avoids overshooting the focus point at all speeds.
The focuser comes with all necessary tools and hardware to install the focuser (a matter of about 15 minutes). You get the focus motor preinstalled in its mounting bracket, a new focus knob, the hand control (with battery), a connecting cable, three drive belts, (the two needed for the 8” scopes, plus a spare), and a protective cover plate.
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