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.
ToolsNewtonian calculator, Collimation
Related terms
- Primary mirror
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.
- 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.
- Dobsonian
A Newtonian telescope on a simple box base that swings up and down and turns round, popularised by John Dobson in the 1960s and 1970s. You nudge it by hand to follow objects. Quick to set up and very steady, it gives a beginner the largest aperture.
- 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.
- Coma
A flaw of parabolic mirrors: away from the centre of the field, stars take the shape of a little comet, tail pointing outwards. It grows quickly as the focal ratio drops: faint at f/8, obvious at f/4. Poor collimation shifts it and makes it visible in the centre.
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