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.
ToolsCollimation, Newtonian calculator
Related terms
- 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.
- Laser collimator
A tool that shines a laser beam into the telescope: you adjust the secondary so the dot lands on the primary’s centre spot, then the primary so the beam returns to the centre of the laser. Quick and handy in the dark. It must itself be well aligned, or it will mislead you.
- Centre spot
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.
- 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.
← The whole glossary