Purpose of the Collimating Eyepiece

Aligning the mirrors seems easy enough, in theory, but in practice it can be difficult without the proper tools. A common way of collimating—the “quick and dirty” method—is to simply remove the eyepiece and look down the focuser tube to center the mirror reflections. The accuracy of this method, however, is not reliable. For one thing, there is no way to know whether you are looking straight down the focuser tube; your line of sight could be off by a degree or two. Another problem is knowing when the reflections of the mirrors are exactly centered. Just “eyeballing it” isn’t precise enough; the reflections may appear to be centered when in fact they are not.

The Collimating Eyepiece will take care of both of these problems, allowing you to achieve precise collimation without “guessing,” and thus improve your telescope’s performance. The Collimating Eyepiece is a combination of a “sight tube” and a “Cheshire eyepiece.” The sight tube’s narrow field of view and crosshairs provide a reference for centering the optical elements during the collimation process.

Using the Collimating Eyepiece

• Insert the Collimating Eyepiece directly into the 1.25" focuser. When using a 2" focuser, place the Collimating Eyepiece into a 1.25" adapter. If you use a star diagonal with your telescope, as is common for Schmidt-Cassegrain telescopes, remove it before inserting the Collimating Eyepiece. The eyepiece should be inserted to a depth such that the bottom edge of the eyepiece tube appears slightly wider than the outer edge of the secondary mirror when you look through the sight hole. Rotate the eyepiece so that the cutout side is directed toward any available external light. Make sure there is adequate light in the room or outside so that when you look through the sight hole, you can see the edges of the secondary mirror clearly. Tighten the thumbscrew on the focuser tube to secure the Collimating Eyepiece in place. Now, you are ready to proceed with collimation.
• It can be used after being inserted into the eyepiece tube. Sight down the tube and note the crosshair target. Center the First Mirroe (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.

Collimation is the process by which an instrument’s optical components are brought into precise alignment with its optical axis and mechanical axis. In other words, all the mirrors and/or lenses should be centered and angled so that light entering the telescope forms a sharp image precisely in the center of the eyepiece. If the optics are not properly aligned, stars will appear not as pinpoints as they should, but rather as flared, teardrop-shaped “comets” or “sea gulls.”