Motorized Focusers and Rear Cell Crayfords

Do you want to be able to focus an SCT without touching the knob and introducing shakes? The SCT accessory makers know you do and have been making "motofocus" units for years. JMI (Appendix 1) has been especially noted for its quality motorized focusers for SCTs. These battery-powered motors slip over the focus knob (sometimes, the stock focus knob is removed and replaced with one included with the motofocus) and provide remote focusing via a small hand control, or, increasingly, with a laptop computer. Does the average CAT owner need one of these $150 gadgets? The answer is probably not. They can be indispensable for imagers who just love the fine no-shake focus action and remote capability they provide.

One thing a standard motofocus unit will not do, to the surprise of some novices, is eliminate focus shift. There is no improvement at all. The image still moves in the field as focus is changed. One add-on focuser can help, the Crayford. This roller bearing focuser, originally developed for Newtonians, is now available for SCTs. It is threaded onto a scope's rear port, and an eyepiece, diagonal, or camera is inserted into it rather than into a visual back or other adapter. Since it does not provide the focus range of the SCT's normal moving-mirror system, the regular focus control has to be used initially to get in the Crayford's "range." Once that is done, the Crayford provides absolutely shift-free focus. Rear-cell Crayfords are available in both motorized and nonmotorized versions, with JMI offering a motorized model for about $450—somewhat pricey, but worth every penny for the planetary picture taker struggling with focus shift at high power.

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