Orion’s collimating eyepiece, otherwise known as a Cheshire eyepiece, is a highly useful, often overlooked tool that can be used to quickly and easily tweak the alignment of virtually any type of telescope.
  Once you insert the collimating eyepiece into a scope, you then look into it and see the crosshair centered (hopefully) on the optics. That’s where the peephole and crosshair work together. They precisely locate your eye on the optical axis, which is where you need to be in order to “reference” proper alignment. Additionally, the reflective ring inside the device projects an annulus of light into the telescope that becomes visible in the reflections of the mirror(s). In this way, you can see the exact centerline of the system (the crosshair), center it on the optics (through the scope’s collimation screws), and then center the reflected light ring squarely on that, again by adjusting the scope’s optics.
There are a couple of tips and tricks for getting a Cheshire to work in all conditions. First, you need some kind of light source to aim into the side of the collimator to be seen inside the scope. If outside in the daylight, ambient light is usually enough. At night, I simply shine a small, LED flashlight into the device to REALLY illuminate the ring. Second, the scope’s focuser must be physically square and centered on the mechanical and optical axes of the scope.
To use the collimating eyepiece, insert it into the eyepiece tube of the scope and illuminate the angled ring (as described above). Sight down the tube and note the crosshair target. Center the FIRST MIRROR (diagonal in a Newt, secondary in a CAT) on the crosshair, and adjust its tilt until the bright ring the collimator is producing is centered in the reflection. Next, adjust each successive mirror in the scope so that the image of the ring is concentric with the optic.
That’s basically it. At about $39, it’s a real bargain.

From: | Created on: 10/16/2009 1:48:53 PM