🔭 Telescope Collimation Check

How precisely your secondary and primary need to be aligned for the images you want, and what a star test tells you about the error you actually have. Based on the wavefront tolerance criteria from Suiter.

Choose your telescope.
How to read a star test

Defocus a bright star until you see a clean disc with diffraction rings, both inside and outside focus. A perfectly collimated scope shows identical patterns either side of focus. Coma from a tilted secondary or primary shows up as a bright crescent on one side and a dimmer one on the other — the rings bunch toward the misaligned side. Astigmatism (often a squeezed mirror clip or a bent focuser) gives an oval that rotates 90° through focus. Tilt the pattern by measuring how far off-centre the rings are, expressed here as a percentage of the defocused disc.