The Celestron Newtonian reflector collimating tool is a combination of a sight tube and a Cheshire tool in a single unit. It has the precisely centered sighting hole and crosshairs of a sight tube, plus the cutout in the side of the tube containing the very precise 45° angle reflecting surface of a Cheshire tool. As it is a combination tool, it is less expensive than buying a separate sight tube and Cheshire tool. However, it is a little more difficult to use than the two tools individually, as the 45° reflecting surface adds an extra visual element that some people find difficult to ignore when it is being used as a sight tube.
Collimating the optics of a commercial reflecting telescope by eye (as many people think is easily done) can be very frustrating. First, the diagonal mirror must be properly positioned and aligned with the primary mirror. You cannot assume that the diagonal mirror has been properly set up at the factory, or that it was not knocked out of alignment during shipping. If the diagonal is indeed out of position or alignment, collimating the primary mirror becomes difficult, even with the aid of a collimating tool such as the Cheshire. 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.
The sight tube function of the tube allows you to first set the basic mechanical alignment of the diagonal mirror in its proper location in the light path using the crosshairs. It then allows you to aim the diagonal mirror accurately at the center of the primary. Once you've set the position and orientation of the diagonal mirror with the sight tube function, the Cheshire tool function is used to properly align the primary mirror. The Cheshire function can also be used to collimate the lenses of those refractors that have an adjustable lens cell.
The sight tube/Cheshire tool combination generally provides accurate enough collimation for visual use and most photography. The Celestron 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 Celestron collimating tool are included.
