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.
ToolsNewtonian calculator
Related terms
- Diffraction spikes
The rays of light that stick out of bright stars in a telescope with a spider. Four straight vanes give four spikes; three vanes give six. They are normal and do not point to any fault; a curved spider replaces them with a faint halo.
- 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.
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