The focuser is the mechanism that moves the eyepiece in and out, and it is the part of a telescope a user handles more than any other. A poor one shifts the image sideways as it moves and makes fine focus at high magnification a fight.
Rack and pinion against Crayford
A rack and pinion focuser uses a toothed rack and a gear, and cheap ones have visible backlash. A Crayford uses a smooth shaft pressing against a flat surface, which removes the backlash and can slip under a heavy load. Most instruments above the budget end now use a Crayford or a hybrid, and both are fine when well made.
Why a dual speed focuser matters at high power
At 200x the depth of focus is tiny, and a single speed focuser moves too far for the smallest turn your fingers can make. A dual speed unit adds a reduction gear, usually ten to one, so fine focus becomes a slow deliberate turn. It is the upgrade most often described as transforming an instrument.
What a focal reducer does
It shortens the effective focal length, which widens the field and speeds the focal ratio for photography. A 0.63x reducer on a long catadioptric turns a narrow planetary instrument into something that frames a whole nebula. It also demands the correct spacing to the sensor, which is where most first attempts go wrong.
Back focus, the number to check first
Every focuser has a limited range, and cameras, reducers and filter wheels all consume some of it. Adding an accessory that pushes the sensor beyond that range means the instrument simply will not come to focus. The arithmetic is published for most components and is worth doing before ordering.
Questions people ask about telescope
What does a telescope focuser do?
It moves the eyepiece or camera in and out to bring the image to a sharp focus, and its smoothness determines how easy that is at high magnification.
What is a focal reducer?
An optic that shortens the effective focal length, widening the field and speeding the focal ratio for photography, at a specific spacing from the sensor.
Is a dual speed focuser worth it?
At high magnification, yes. The ten to one fine control makes precise focus achievable where a single speed focuser overshoots.