A small marker, often a ring-shaped sticker, placed at the exact centre of a Newtonian’s primary mirror. It is the target for collimation. Because it sits in the shadow of the secondary mirror, it has no effect on the image.
ToolsCollimation
Related terms
- 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.
- 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.
- Cheshire eyepiece
An eyepiece-shaped collimation tool with no lenses: a small peephole and a 45° reflective face lit from the side. It shows a bright ring that you centre on the primary mirror’s centre spot. Simple and battery-free, it is often more reliable than a cheap laser.
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