Astronomy Hacks

Build a Film Can Collimating Tool

Align your scope on the cheap.

Collimation is the process of aligning a telescope so that all of the mirrors and lenses share a common optical axis. There are numerous collimation tools available commercially, including sight tubes, Cheshire eyepieces, laser collimators, autocollimators, and so on. Two of the most popular tools are a combination sight-tube/Cheshire and a laser collimator, such as the Orion models, shown in Figure 3-11.

None of these collimating tools allows you to collimate a scope perfectly. Their purpose is to get the collimation close enough that you can do final tweaks on a defocused star to achieve perfect collimation [Hack #40]. Star-collimation allows you to adjust alignment almost perfectly, but it’s nearly impossible to star-collimate a scope unless it is already reasonably well collimated.

How close you need to get to perfect collimation before you can star-collimate depends on the focal ratio [Hack #9] of the scope. An f/5 or faster scope must be very close to perfect before it’s possible to star-collimate it. An f/8 or slower scope need only be moderately well collimated.

Figure 3-11. A sight-tube/Cheshire (left) and a laser collimator

For fast scopes, we recommend using a combination sight-tube/Cheshire to do the preliminary collimation. But for slower scopes, there’s no need to spend the $35 or so that a sight-tube/Cheshire ...