A spherical mirror or lens does not bring rays from the edge and from the centre to the same point: the image stays a little soft whatever the focus. Newtonian mirrors are therefore parabolised. With a small, slow mirror (114 mm at f/8, for example), a spherical one is still acceptable.
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.
- Star test
You point at a moderately bright star at high power and defocus it slightly on each side of focus. With good, well collimated optics you see identical concentric rings on both sides. It is the final judge of collimation and optical quality.
- 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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