Hobbies And Interests
Preparation
There are a few tools that are useful for achieving a successful collimation. These can be made out of common household items or bought at an astronomy supply store. A peephole can be made using a 35mm film canister with the bottom cut off and a 1 to 5 mm hole drilled in the center of the cap. A sight tube can be made from a tube of material that fits snugly over the focuser. There should be a hole in the center of about 1 to 5 mm and the length should be the inner diameter of the telescope tube times the focal length. You can make this sight tube with crosshairs of thick white thread at the inner end as well to help center the optics along the line of sight. The Cheshire eyepiece and a combination tube are other variations of the sight tube that can be useful in collimation. It is also common practice to put a small spot at the dead center of the main mirror. This can be done using a small circle of dark electrical tape cut into a circle of about 10 to 15 millimeters or a paper hole reinforcer.
Alignment
The first step in collimation requires the alignment of the secondary mirror on the axis of the focuser drawtube using the sight tube. Slide the sight tube into the focuser and view the secondary mirror through the hole in the end. If you have trouble distinguishing the edge of the secondary mirror in the reflection of the main mirror, a piece of paper or cardboard can be placed between the two mirrors. The elliptical secondary should appear round and centered in the opening of the sight tube. If it is not, either the secondary holder or the focuser can be adjusted to achieve alignment. Next the tilt of the secondary mirror needs to be adjusted to aim the axis of the focuser at the center of the primary mirror. If you placed cardboard between the mirrors earlier, remove it now. This adjustment is made easier if you have placed a center spot on the primary mirror and have crosshairs in your sight tube. If so, simply align the center spot in the middle of the crosshairs. Otherwise, with the sight tube adjusted so that you can see the entire primary mirror, adjust the tilt so that the primary mirror appears centered in the sight tube. Lastly, the tilt of the main mirror needs to be adjusted so that the 'sweet spot' is centered in the focuser. The Cheshire eyepiece is a helpful tool for this adjustment. An assistant who can loosen and tighten the screws on the primary mirror while you are looking through the eyepiece is also very helpful. This step is the one that will need to be repeated most often, as you will want to check the focus of the primary mirror before and during each viewing session. For final adjustments in the field, it is important that the telescope be allowed to equilibrate in temperature with the viewing area before adjustments are made as changes in temperature can affect the alignment.