One of the issues that comes up when building telescopes is collimation. Once the optics have been mounted in their proper position, how then to make sure that the tilt of each surface is correct with respect to the other mirrors? Actually this is a problem even for factory made instruments because they can also get out of alignment and need correction. Because of this it is a must to know how to collimate a Newtonian either way!
Collimation can be accomplished best by placing a paper reinforcement ring (they are self adhesive) in the center of the primary mirror. This does nothing to the mirror's performance because the center of the primary is always blocked by the shadow of the secondary (or diagonal). It is important to get the ring as close to the center as possible. One way is to stretch thread across the mirror as close to the center as possible, then tape the thread to the sides of the primary. Mark the center of the thread using a black marker (careful not to mark the mirror's surface) by measuring with a wood ruler. Take a second thread and place it perpendicular to the first while lining up with the mark on the first string. Measure the center of the second string and mark it. Move the first string so it lines up with the second's mark. It may be necessary to move the second again, but with just a few iterations a very accurate center can be found. Now peel the backing off the reinforcement ring and attach it to the tip of a pointed knife. Carefully position the ring so that the hole is centered on the intersection of the strings. Then stick it into place. As always be careful not to touch the mirror's delicate coating.
The other thing you must do to get an accurate collimation is to purchase either a Cheshire eyepiece or a laser collimator. I use Astro Systems brand and have both. They are both equally accurate, but the laser is faster and can be used at night. The Cheshire has a small peep hole with a mirrored interior so that its reflection can be viewed in the primary. The Cheshire also has a cross hair on the opposite end that you look through which you use like a gun site to make sure that you are looking down the optical axis. The laser is made to fit in a 1 1/4" or 2" focuser. If it is made properly the laser is centered and parallel with the center of the focuser draw tube. A quick way to check the accuracy of both devices is to turn then in the focuser. Once the set screw has been set the position of the laser dot should return to its last position and the Cheshire cross-hairs should still be marking the same position on the primary.
I am frequently asked to help collimate telescopes at starparties and usually the Newtonians are in need of some adjustment. If the system is fairly fast (has a lower f/number than 8), then collimation is very critical and should be as close as possible. Check out my page on Collimation Problems. If you follow the procedures below you will get close enough for most applications. If you should desire to do high power planetary work then an additional step using an auto-collimating eyepiece should be used. Astro Systems also sells these, but I do not cover their use for now.
If you have a current version of Java running on your browser I have built a ray-trace simulation to help in understanding some of these issues. It will also help in determining the secondary offset. It was build using a marvelous program called Geogebra. Many thanks to Luca Cassioli for letting me know about this software. He has even converted my collimation page to Italian!
If you have a Cheshire.
If you have a Laser.
