The JMI Focus Counter is an excellent upgrade for the Celestron SCTs. It is a low-cost mechanical alternative to the electronic digital readout (DRO). Its three-digit counter display allows 10 turns of the shaft before returning to zero. For focusing repeatability, it works well with manual and motorized NGF focusers, as well as SCT (Schmidt-Cassegrain) telescopes. It is not recommended for CCD imaging due to space limitations and problems with image shifting. Also, the piggy-back option is only available at the time of purchase of the Motofocus unit and can not be added later.
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JMI Telescopes Digi-Focus Counter for Celestron SCTs Review