Collimation

Collimation is the process of aligning a telescope's mirrors, most commonly in a Newtonian reflector, so that light traveling down the tube meets the eyepiece at a single precise focus point. When the primary and secondary mirrors are even slightly out of alignment, images at high magnification appear soft, distorted, or comet-shaped instead of forming tight, round points of light.

Collimation can drift from shipping, transport in a car, temperature swings, or simply time, and it is a normal part of owning a reflector rather than a sign of a defective telescope. Checking it takes only a couple of minutes with a simple collimation cap or laser collimator, and adjustment is usually done with a few screws on the back of the primary mirror cell and around the secondary mirror.

The effect of poor collimation is most obvious at high power on planets and double stars, while low-power wide views can look acceptable even when collimation is noticeably off. A common mistake is skipping collimation checks entirely because a telescope came from the factory pre-aligned, when in fact most reflectors benefit from a quick check every few observing sessions, especially after any bumps during transport.