The Astro-Tech ATCOLL Newtonian reflector collimating tool is a combination 1.25" sight tube and Cheshire eyepiece. It is precisely machined of aluminum and black anodized. A soft fold-down rubber eyecup (rarely found in more expensive competitive Cheshires) shields your eye from ambient light for better contrast and protects eyeglasses from being scratched during use. It's 6" length provides higher collimating accuracy than shirter competitive Cheshires.
The Astro-Tech ATCOLL has the precisely centered sighting hole and crosshairs of a sight tube, plus the cutout in the side of the tube with the precise 45° angle matte silver reflecting surface of a Cheshire eyepiece. As it is a combination tool, it is less expensive than buying a separate sight tube and Cheshire.
For the sharpest images with a Newtonian reflector, the optics must be carefully collimated (aligned). First, the diagonal mirror must be properly centered under the focuser and aimed at the primary mirror. You cannot assume that the diagonal mirror has been properly set up at the factory, or that it did not accidentally get knocked out of alignment during transport. If the diagonal is indeed out of alignment, collimating the primary mirror becomes difficult. This is particularly true with fast focal ratio (f/4 to f/5) reflectors, as even slight collimation errors in these systems can significantly degrade the image.
Use the sight tube crosshairs of the Astro-Tech ATCOLL to confirm that your diagonal mirror is properly centered under the focuser and adjust it if needed. It then lets you adjust the tilt of the diagonal to aim it accurately at the center of the primary. Once you've set the position and orientation of the diagonal mirror with the sight tube function, use the Cheshire eyepiece function to properly align the primary mirror.
The sight tube/Cheshire tool combination generally provides accurate enough collimation for visual use and most photography. The Astro-Tech ATCOLL collimating tool is primarily designed to be used in daylight or early twilight, but the Cheshire function can also be used in complete darkness (for those times when you can't set up until late) by shining a red flashlight into the 45° reflecting cutout in the side of the tube. Instructions on how to use the Astro-Tech ATCOLL collimating tool are included.
