Dual speed & single speed?

[farunj]

Hi all,

I would like to know what the difference is between a dual speed and a single speed focuser? Also why are dual speeds preferred for imaging?

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[Earl]

Dual speed have extra knob which allows for finer movement is it works at a different ratio.

[swamp thing]

Dunno about imaging but fast scopes benefit greatly from dual speeds. The shallower depth of field means its tricky to get good, precise focus with a single speed focuser.