Collimating a Ritchey-Chrétien: contents
Guide · 6 steps
Collimating a Ritchey-Chrétien
From “I always have one bad corner” to a round field: what to adjust, in what order, and how to know you’re done.
The Ritchey-Chrétien (RC) has a reputation for being impossible to collimate, and the forums back that up: “I always have one bad corner” has been the most common RC thread for a decade. It isn’t bad luck. Both mirrors on an RC are adjustable, so you can get a perfect center with a scope that is still out of collimation.
The good news: there’s a method that works, and it converges in two or three rounds. Primary first, for the center; then secondary, for the corners. Here it is step by step, with what each tool is actually good for, and an assistant that reads your frames and tells you which screw to turn.
Have a Newtonian or a Schmidt-Cassegrain? This guide is about RCs; most of the “is it collimation?” step still applies to you.
Download it as a PDF To print and keep next to the scope: the 6 steps, with boxes to tick and a log sheet for every screw you turn.
What you need
- Your RC: a closed-tube 6", 8" or 10" (GSO and its rebrands: Astro-Tech, TS-Optics, Omegon, Apertura, StellaLyra…) or a truss or carbon one.
- The keys or drivers for both mirrors’ screws, and a bit of tape to label them.
- Your usual imaging camera, with no reducer or flattener.
- For the daytime part, whatever you have: a Cheshire, a well-collimated laser or a collimation scope. You don’t need to buy anything expensive.
- For the night part: a rich star field about 70° up, good tracking and an hour or two.
The 6 steps
- Step 1 Before you touch anything: is it collimation? Odd stars in the corners aren’t always the mirrors: it can be a tilted sensor, the RC’s own field curvature, guiding or a sagging focuser.
- Step 2 Why an RC is different Two hyperbolic mirrors, both adjustable: you can get a perfect center with the scope still out of collimation. That’s why the order matters.
- Step 3 Mechanics first No collimation holds with a sagging focuser, pinching screws or an imaging train that wobbles. Ten minutes of checks save whole nights.
- Step 4 By day: get it close With a Cheshire, a well-collimated laser or a collimation scope you can get an RC close in half an hour. To finish, you need stars.
- Step 5 At night: primary for the center, secondary for the corners With defocused stars and your imaging camera: the primary removes the coma in the center, the secondary balances the corners, and you repeat two or three times.
- Step 6 How to know you’re done (and keep it that way) 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.