View the secondary mirror holder from the corrector plate end of the Schmidt-Cassegrain Telescope. The collimation screw nearest the ground is labeled “A”. Screws “B” and “C” are as shown in the center of the diagram above.
The eyepiece should be oriented upward opposite to the position of screw “A” as shown by “EP” in the diagram.
If an SCT is in need of collimation, a slightly out-of-focus star near the center of the field of view will appear as in one of the six eyepiece views shown above. To collimate, turn the indicated collimation screw a very small amount in the direction shown. Re-center the star and re-check the out-of-focus image of the star. Repeat as necessary using higher magnifications for best collimation.
Avoid using very bright stars (magnitude 2 or 3 is good). Always re-center after each adjustment and use only slightly out-of-focus images.
For a more detailed description of the collimation procedure for SCTs, see: http://perso.club-internet.fr/legault/collim.html
For a comprehensive article (with illustrations) about collimating a Cassegrain telescope like AAI’s 24-inch reflector, see: http://www.oldham-optical.co.uk/Cassegrain%20Setup.htm
Bob Morrow sells thumbscrews which replace the Allen-head collimation screws that come with most SCTs. He calls them Bob’s Knobs. See: http://www.bobsknobs.com