Today, I learned a lesson in troubleshooting a collimation problem with my new dobsonian telescope. When I took the secondary mirror off to install a washer, I could not properly get it collimated with my laser collimator on reassembly. One problem with laser collimators is that it assumes that the secondary mirror is already centred under the focuser. But that is easier said than done. Until now.

Symptom: When I looked through the collimating cap after a laser collimation, it looked like the above photo. The primary mirror’s clips were off-centre. It was actually worse than what the photo shows. However, the photo provides a clue of what the problem may be: the secondary is not centred.

So below were the steps I took to collimate after disassembling the secondary:

Step 1: Above, I rough aligned the secondary under the focuser and took a photo. To the untrained eye (like mine), my secondary mirror looked centred through the 1 mm hole of my collimating cap.

Step 2: That was until I took a ruler and measured it on my screen as shown above. So I kept adjusting the secondary until both sides were equal. I pinched to zoom for more accuracy.

Step 3: I used a right-angle diagonal focuser above from my refractor so that I can adjust the primary adjustment screws and receive instant feedback without walking back and fourth. I centred the donut to the reflection of the secondary.

Step 4: I replaced the collimating cap to verify alignment and it looked good as shown above. I then followed it up with a laser collimation and it still looked good afterwards.

Out in the field, I will be star testing it for fine tuning. My laser collimator was 99% true so if you plan on doing the same procedure above, check to make sure your laser collimator is properly collimated in the first place. Credits go out to the following AstronomyForum.net members for helping me troubleshoot the problem: mlk1950, CamelHat, andrejl2, Jason_D