A Collimating Checklist
If your telescope is not badly out of collimation, and you have a Cheshire, and either a sighting tube, a laser collimator, or both, here is briefly what you need to do (If you have never done a full collimation, do one first according to the full procedure). The numbering is from the full procedure on the main webpage, please refer to it if you find anything unclear here. Be sure to do things in the right order - collimate the main mirror after the secondary!
1 - Square the focuser, 2 - Center the secondary mirror in the tube, and 3 - Center the secondary mirror along the tube,
If you are sure the focuser and secondary mirrors line up, and the secondary is offset as desired, you could skip this step - these are not likely to go badly out of adjustment. If not sure, or if you have just attached the secondary holder, use any sighting tube or holographic laser to center the secondary mirror.
4 - Tilt the secondary mirror to make the extended optical axis hit the center of the main mirror .
Use the appropriate setscrews on the secondary mount. You can use a laser collimator or a crosshairs sighting tube, aiming at the main mirror center spot. You could also use a sight tube of suitable length, by centering the main mirror within the opening.
If you have made significant adjustments, go back to step 3 (and possibly step 2) and check that the adjustments still are OK, or adjust if needed. Do not skip step 5!
5 - Tilt the main mirror, to make its optical axis reflect back on itself.
If you have a mirror cell that holds the mirror very loosely (this is particularly common in Dobsonians), you may make the mirror settle by tilting the tube nearly horizontally, and then raise it vertically, before you go on.
Here you use the set screws to adjust its tilt (use 2 to adjust, and leave the 3rd).. You could use a Cheshire or a combination tube, by centering the main mirror spot in the bright spot.
If you cannot reach to adjust the collimating screws while looking into the Cheshire, find an assistant that you can ask to turn each screw while you note the effects.
You can use a laser collimator to get close, but fine tune by using a Cheshire or Barlowed laser.
6 - Check the centering of the optical axis in the telescope tube and in the focuser drawtube.
Most likely, you need not check this step. If you want to, look through the empty focuser tube and check if you can see the outer tube end reflected in the main mirror from any point within the focuser. If you cant, the centering is OK optically.
7 - Star test
You may want to check the collimation by looking at stars - use a magnification of 1-2x per mm of aperture (25-50x per inch).
Center a star in your field of view. Gently rack the focuser from one side of focus, passing the focus, going to the other side.
When you are a little defocused, you should see rings of light and a darker center, very symmetric (the images above are for an offset secondary!). If there is a marked asymmetry, try and see if the image improves if you place it off center in any direction. If so, try to tweak the main mirror to move the image towards the center of the field, in steps until it looks symmetric ( If you start with a nearly perfect collimation, it will save you a lot of tweaking). For best sensitivity, go as close to focus as you can.
8 - Check the finder and adjust if necessary
If you have tweaked the collimation, you may have shifted the optical axis slightly, and the finder may be slightly off.
9 - Enjoy!
Nils Olof Carlin