Check it whenever the telescope has been transported, and at the start of any session where sharpness matters. A tube that lives in one place and is carried gently may hold for weeks; one that travels in a car boot may want checking the same evening.
Collimation is a five-minute routine with the right tool and pure guesswork without one, which is why so many reflectors are quietly underperforming.
What being out of true costs
Not an obvious fault. A misaligned reflector looks like a telescope of smaller aperture: soft detail, a bloated star image, planets that never quite snap into focus. Owners blame the telescope, which is why the problem persists.
When to check
After any journey. After a significant temperature change in some designs. And at the start of any session where you intend to push the magnification.
The quick test
Defocus a bright star slightly and look at the rings. Concentric means aligned; offset means not. It costs thirty seconds and tells you whether the tool needs to come out.
Testing the tool itself
A laser collimator that is not itself collimated will confidently aim the telescope wrong. Rotate it in the focuser and watch the dot: if it traces a circle rather than staying put, the tool is the problem. Look for one with adjustment screws in its own body.
The questions that keep arriving
How do I know it is out?
A bright star defocused slightly should show concentric rings. If the pattern is offset to one side, the mirrors are not aimed at each other. In focus, a misaligned telescope shows soft detail and a bloated star image.
Does a refractor need it?
Effectively no. The lens cell is fixed at the factory and holds its alignment through years of ordinary use, which is the main practical advantage of the design.
Last reviewed 17 September 2026