Tools for Collimation
Jack Kramer
Every owner of a Newtonian telescope should have one or more tools to help in getting the mirrors well collimated. Of course, buying a collimation tool isn't as much fun as getting a new eyepiece. It's sort of like getting a battery for your car; it lacks pizzazz. But you won't get far without it.
It's worth noting that Schmidt-Cassegrains also need periodic collimation. Astronomy author Phil Harrington pointed out that when SCTs get a bad rap for poor quality images, based on what he's seen, many of them are just badly out of collimation.
Before beginning, I should add that if you don't already have a center mark on your primary mirror, you definitely need to add one. It's impossible to effectively use any collimation device without some sort of reference mark. Most new commercially made Dobsonians come with their primary mirrors already center marked with a circular "donut".
There are five common tools:
- Collimation cap
- Sight tube
- Cheshire eyepiece
- Autocollimator
- Laser collimator
Collimation cap - This is the simplest of the tools and every owner of a reflecting telescope should have one for rough alignment and quick touch-ups. It's simply a cap with a center hole that fits into the focuser drawtube. I have an old one that began life as a short focal length eyepiece from which the lenses were removed. You can also make one from a plastic 35mm film canister with a small hole punched dead center. (It so happens that 35mm film canisters have an OD of 11/4 inches.) The collimation cap orients your eye so it's positioned in the very center of the focuser tube while adjusting the tilt of the mirrors. If you're not looking exactly down the center of your focuser tube it's impossible to judge whether the optics line up correctly. To their credit, Orion includes a collimation cap with their Newtonian telescopes. (This is also a handy device for refractor owners. Yes, refractors sometimes can be out of collimation too.)
Sight tube - A sight tube is designed for the preliminary stages of collimation. It's nothing more than a long tube with a small hole at one end that fits in the focuser, like an eyepiece. What it does, in effect, is to project the focuser downward, so you can see more easily if the secondary mirror is concentric with the focuser. Most such tubes also have cross hairs at the far end, which makes it easier to align the tilt of the secondary and get the reflection of the primary properly centered. It's a tool you won't use very often, so if you can borrow one, that would be fine.
Cheshire eyepiece - Despite its name, this isn't an eyepiece at all. It's used to adjust the primary mirror - the last major step. It's a tube with a washer-like insert tilted at 45 degrees to the focuser axis. When you illuminate the washer (there's a hole in the side of the eyepiece for this purpose) and look through the top, you see a donut-shaped reflection in addition to the usual reflections. You then adjust the screws on the primary cell until the center of the primary lines up with the center of the washer.
Autocollimator - Don't be misled by the name of this device ... there's nothing "automatic" about it. The autocollimator is a flat mirror mounted in a short tube made to fit a focuser drawtube and set accurately perpendicular to the tube's axis. Looking through its peephole, you see the primary's center spot (reflected in the secondary) and a few more reflections of the spot as a result of several reflections back and forth. When collimation is ideal, the reflections will appear stacked or coincident. But you first need to go through the steps with a sight tube and Cheshire. This tool is so critical that collimation has to be very close to begin with. It's considered the ultimate step in collimating a Newtonian. It can also be used for collimating a classical Cassegrain telescope, but not an SCT. This is the only collimation tool that I don't own, simply because I've read many times that if a good collimation is done with the other devices, there is little further to be gained by using an autocollimator. Most users report that it simply confirms that the telescope is properly collimated. I suppose it's a good backup if you have an extremely fast telescope ( Laser Collimator - It's unfortunate that many amateur astronomers rely on the laser as the last word in collimation. Lasers are definitely useful but are not the most accurate tool. Where they really shine is when used for aligning the secondary mirror to the center of the primary. It can also be used to check alignment at night when it's too dark to use other tools. Probably the biggest problem is that the laser beam actually can be made to return to its point of origin despite the fact that the optical components are still out of collimation. On-line telescope discussion forums often have a question asked about which tool to trust, since after collimation with one of the other devices, a laser sometimes shows the alignment as still being off. The answer in all cases has been to trust the other tools instead of the laser. Moreover, it's important to check periodically to make sure that the laser collimator is itself in collimation. Some laser collimators have a reticule pattern instead of a dot, making it easier to align the optical components. With the reticule laser in the focuser, the telescope is aimed at a light-colored wall and the reticule pattern will be seen projected. If the shadow of the secondary appears dead center, the scope is reasonably well collimated. The Barlowed laser method is more accurate still. Information on how to use this variation can be found at:
http://web.telia.com/~u41105032/kolli/kolli2.htm One issue is the safety of laser collimators. Most laser collimators use a class 3a laser (maximum output of 5mW). Theoretically, this can cause eye damage in less than the blink time (about 1/4 second). But according to Canadian astrophysicist Grant Gussie, ophthalmic examinations have revealed no retinal damage in several incidents where people exposed to class 3a laser radiation reported "seeing green" for extended periods after exposure. Obviously, the first safety rule is to never look directly into a laser beam. Looking at the bright spot of scattered light on the mirror surface with the reflected beam not going to your eye is perfectly safe. Summary - Before beginning any collimation, make sure the tool is seated all the way into the focuser drawtube (except for the Cheshire, which must extend part way out) and all the setscrews are tightened. Also adjust as much slop as possible out of your focuser. If you don't already have any collimation tools, you should at least get a simple collimation cap. It's the least expensive and you'll probably use it more than anything else. I've found that if a careful alignment is done with the cap, the more sophisticated tools simply confirm that the collimation is either right on or needs just a little tweak. The Cheshire eyepiece is highly accurate, and I find it very useful. If things line up properly with a Cheshire, then chances are good that the secondary is correctly positioned beneath the focuser. Orion sells an economical collimating eyepiece - a combination Cheshire/sight-tube that many have found very useful. Beyond these tools, anything additional would result in such a minor improvement to the collimation that you probably wouldn't notice it.
