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.
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.
- Central obstruction
The fraction of the aperture blocked by the secondary mirror, as a percentage: a 50 mm secondary in front of a 200 mm primary is 25%. It does not make a hole in the image but slightly lowers the contrast of fine planetary detail. Visual Newtonians aim for 20 to 25%; Schmidt-Cassegrains often exceed 30%.
- Spider
The holder of the secondary mirror, made of thin vanes (often three or four) stretched across the tube. The vanes diffract light and produce the spikes around bright stars. It carries the secondary’s adjustment screws.
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