The large concave mirror at the bottom of a telescope, which collects the light and forms the image. In a Newtonian it is parabolic and sits in a cell with adjustment screws for collimation. Its diameter is the telescope’s aperture.
ToolsNewtonian calculator, Collimation
Related terms
- Secondary mirror
The small mirror that sends the light from the primary to the eyepiece. In a Newtonian it is flat, elliptical and tilted at 45° near the top of the tube; it is also called the “diagonal”. Its size sets the central obstruction and the fully illuminated field.
- 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.
- Collimation
The adjustment that lines up a telescope’s mirrors on a single axis. A Newtonian drifts a little each time it is moved and should be checked before every session, especially if it is fast (f/5 or below). A poorly collimated telescope gives soft images, even with excellent optics.
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