Collimating a Ritchey-Chrétien · Step 4 of 6
By day: get it close
In a sentence: 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.
What each tool measures
| Tool | What it does | What it doesn’t |
|---|---|---|
| Cheshire eyepiece | Centers the focuser on the secondary and gets the mirrors close | Fine-tune the primary or the secondary |
| Plain laser | Same as the Cheshire, if the laser itself is collimated and seated well | It fools you if it’s loose in the focuser or out of collimation |
| Holographic (DOE) laser | Lets you check by eye that everything is “more or less” right | Both mirrors can be off at the same time and the pattern still look fine |
| Collimation scope (Takahashi-style) | Centers the secondary’s mark and the baffle rings | Tell you whether the mark is the optical center |
| Electronic collimator (Ocal and similar) | The same as the collimation scope, on a screen | The final touch: you still need the star |
| Cassegrain laser (Hotech) | Collimates the mirrors without stars, on the bench | Repeat the same result every time: opinions are split |
No daytime tool finishes the job: they all get the scope “close”, and the star decides.
The order on the bench
- Center the focuser (or the tilt adapter) so it points at the center of the secondary.
- Adjust the secondary until its mark is centered in the collimation scope, Cheshire or laser.
- Adjust the primary until the baffle rings look concentric.
- Repeat 2 and 3 two or three times: each one moves the other a little.
- On the closed-tube 6" and 8", check the focuser after every touch of the primary.
TipIf you don’t have any of these, no problem: the night method works just as well starting from a scope that’s roughly right. It just takes a little longer.