Collimating a Ritchey-Chrétien · Step 1 of 6
Before you touch anything: is it collimation?
In a sentence: 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.
A bad corner has many parents
A lot of “my RC is out of collimation” threads turn out to be something else. Before you turn a screw, look at how the stars look and where:
| What you see | Most likely cause | What to adjust |
|---|---|---|
| All four corners equally bad, elongated toward the center or sideways | The RC’s own field curvature and astigmatism: it’s the design | Nothing in the collimation: a flattener or a smaller sensor |
| One corner or one side worse than the rest, and it goes out of focus first | A tilted sensor or focuser | The focuser, the imaging train or a tilt adapter |
| One corner “pointed” and the opposite one “flat”, with a good center | Misaligned mirrors | The secondary (step 5) |
| The center donut brighter on one side | Coma from misaligned mirrors | The primary (step 5) |
| Every star elongated the same way, center included | Guiding, tracking or wind | The mount, not the mirrors |
| Triangular stars or odd spikes | A pinched mirror: screws too tight | Back the retaining screws off a little |
What tells collimation and tilt apart: collimation changes the shape of the stars; tilt mostly changes where they come to focus.
Sensor size matters
An RC without a flattener doesn’t have a flat field. With a 4/3 camera you’ll barely notice; with APS-C the 8" RC is right at the limit (TS-Optics quotes its corrected field at about 30 mm), and with full frame the corners will always be worse. If all four corners fail equally, it isn’t collimation: no amount of turning will fix it.
Heads-upAlways test without a reducer or flattener. Otherwise you mix their backfocus with the collimation and there’s no way to tell which is which.