The part that holds the eyepiece and moves it in and out to focus. Rack-and-pinion models are sturdy; friction-driven Crayford focusers are smoother with no backlash. A good focuser with a fine-focus knob helps a lot at high power and for imaging.
Related terms
- Eyepiece
The small interchangeable magnifier you look through; it sets the magnification. Its focal length is engraved on it: the shorter it is, the higher the power. The standard barrels are 1.25 inch (31.75 mm) and 2 inch (50.8 mm) wide.
- Newtonian telescope
The telescope invented by Isaac Newton in 1668: a concave primary mirror at the bottom of the tube and a small flat mirror at 45° that sends the image out to the side, near the top. With no lenses it is free of chromatic aberration and offers a lot of aperture for its simplicity. It needs regular collimation.
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