Now that the dobsonian telescope I was gifted has been restored I should take the time to properly collimate the primary and secondary mirrors. Collimation is the process of aligning the primary and secondary mirrors in order to bring the light of the telescope into the best possible focus. Luckily, this is not an overly complex task to perform on most telescopes especially with the help of collimation assistant tools such as a cheshire eyepiece or a laser collimator; I will be using a laser for this.
The first step is to make sure the mirrors are roughly aligned. To do this, I used a caliper to make sure that both the primary and secondary mirrors were centered in the telescope tube. Then, without an eyepiece, look through the focuser. You should be able to see the entire primary mirror through the eyepiece holder. If you do not, adjust the secondary mirror’s angle or position until you do. Then, point the telescope at a wall and install the laser collimator into the eyepiece holder and turn it on. We want the laser to start at the eyepiece, bounce of the secondary mirror, the primary mirror, back to the secondary mirror, and then to a crosshair on the laser collimator itself. Look at the wall to see if the laser is leaving the body of the telescope and not bouncing back. In my case, because I had taken everything apart and put it back together again, it was.
Using a screwdriver, adjust the screws at the bottom of the primary mirror to adjust it’s angle. Watch the laser on the wall and how it moves. You want the laser to move towards where the middle of the telescope is pointing. Eventually the laser will disappear from the wall. At this point look at the laser collimator’s crosshair and try to center the laser on that. Now the telescope is roughly aligned.
For a more precise alignment, look into the front telescope tube from, towards the primary mirror and see where the laser hits on the primary mirror. NEVER LOOK DIRECTLY AT THE LASER! Always look into the tube at an angle and observe the laser’s reflection. Only do this after you have confirmed that the laser bounced back to the collimation crosshair target. If the laser dot is not centered in the primary mirror then adjust the secondary mirror’s angle in order to slowly push the laser dot into the center. Some telescope will actually have a little circle indicator on the primary mirror that indicates the center. This particular one seems to have been built before that was standard practice so I have to estimate the middle by eye.
Once the laser is centered in the primary mirror it will no longer be perfectly centered on the laser collimator’s crosshair. Once again point the telescope at the wall and adjust the angle of the primary mirror by using the screws at the bottom to move the dot back into the center of the crosshair.
With that all done, the telescope should be collimated quite well. I guess I’ll only know for sure though once I take it outside for a night.