Gday Ken
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The RCX design had a moving secondary for focusing.
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Not quite.
It moved the whole corrector using 3 equidistance motors. ( IIRC, the secondary was actually glued to the corrector to hold position ).
Collimation was done by tilting the whole corrector and focussing by moving the whole corrector, ie "optically", no different to a moving primary. Users have actually retrofitted the dead RCX motors with a 3rd party controller using steppers and it appears to work very well.
Andrew