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%.
ToolsNewtonian calculator
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.
- Airy disc
Even perfect optics do not turn a star into a point but into a tiny disc surrounded by rings, because of diffraction. Its angular radius is 1.22 λ/D: the larger the aperture, the smaller the disc. About 84% of the light falls in the central disc.
- Schmidt-Cassegrain
A compact telescope closed by a thin corrector plate, with a spherical primary and a convex secondary, usually at f/10. Versatile and portable, it often comes on a GoTo mount. Focusing moves the primary mirror, which can shift the image slightly.
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