Collimating a Ritchey-Chrétien · Step 6 of 6

How to know you’re done (and keep it that way)

In a sentence: In focus, the four corners should look the same as each other. If they match but aren’t perfect, that’s the design: a flattener, not more screws.

The in-focus check

Focus carefully and take a 30 to 60 second frame. The stars in the four corners should look the same as each other (even if they aren’t as tight as the ones in the center). If one corner is still worse than the rest, there’s room to improve; if all of them are equally elongated, that’s the RC’s own field curvature.

Mirror spacing

If the focal length from plate solving is well off the nominal one, the mirror spacing isn’t the design spacing: on amateur RCs, every millimeter changes the focal length by about 11 to 20 mm. It’s a delicate adjustment that you almost never need: leave it for last, and only if the difference is clear.

Keeping it

A well-collimated, well-secured RC holds for months.
Check it after transporting the scope, after a knock, and if you change cameras.
Write down where the screws ended up: if something moves, you’ll know how to go back.
If nothing works, the mirror may have been mounted badly at the factory: there are optical shops that collimate RCs.