Collimation
Aligning a telescope's mirrors so they share one axis — essential for sharp views in reflectors.
Bumps and temperature changes nudge a reflector's mirrors out of line, making stars flare into comet shapes at high power. Adjusting the secondary and primary mirror screws, with a collimation cap, Cheshire eyepiece or laser, brings them back. Fast mirrors (f/4–f/5) need it most precisely.
At the eyepiece
Check collimation every session on a Dobsonian; it takes a minute once you've learned it.