Update: Since this article was first published, new collimators have come on the market. In addition, people knowledgeable in optics have pointed out that laser collimators may not always give the best indication of optical alignment. Instead, the most accurate collimation tool has proven to be the Cheshire eyepiece.
One item to hit the market in the early 1990's is the laser collimator. The prices from different suppliers range from about $90 to over $200 (though competition may spur some price cuts). With the price of a simple laser pointer now starting at $15 or even less, you might wonder what drives the high cost of the collimator. It's a matter of accuracy - the laser spot must be projected precisely in line with the axis of the collimator. The laser itself is mounted in a machined-aluminum cell. Like any piece of test equipment, it has to be trustworthy. Are they worth the cost?
First of all, let's look at what they do for you. If you have a telescope that employs a series of mirrors, such as a Newtonian or a Schmidt-Cassegrain, it's important to maintain the correct alignment of these mirrors. In other words, you want to make certain that the primary mirror focuses the light exactly at the secondary mirror, and the secondary aims the light rays directly into the center of the focuser tube. Those of us with Newtonians are constantly checking and tweaking the collimation. SCTs tend to maintain their collimation better, but they, too, need to be checked once in awhile. We've always done it by looking down a sight tube and lining up the center spot on the primary mirror so that it appears to be dead center in the secondary. Now with the laser collimator, there's no guesswork as to whether the alignment is exact. You slip the collimator into the focuser tube, then check to see whether the laser beam is centered on the primary mirror and back on itself. If all is well, the laser beam should bounce off the mirrors and be seen back over the little hole on the collimator from whence it came. This sounds like an oversimplification, but that's what it does. The collimator can also be used to properly position the focusing mount and secondary mirror.
Having borrowed a laser collimator, I checked my scope, which I had previously collimated by eye. I found my primary mirror was pretty well aligned, but I had to slightly reposition the secondary mirror. Afterwards, the images were as good as possible with my optics. I was sold on this gadget!
Several different suppliers have entered the market. The less expensive laser collimators are made to fit only 1-1/4-inch focuser tubes, while those with a higher price tag have a larger barrel that has a step milled into it so it can be inserted into either a 1-1/4-inch or 2-inch focuser. Since I use 1-1/4-inch format eyepieces about 95% of the time, the less expensive one would do just fine. On the other hand, if you had a large-format focuser with a 1-1/4-inch adapter that doesn't set squarely in the focuser or has some "slop" to it, then it would probably be to your advantage to collimate with the 2-inch focuser tube. In that case, the more expensive collimator would be preferable. (Of course, the optimal solution here would be to get a better focusing mechanism!)
The March 1999 issue of Sky & Telescope included a review of the most expensive laser collimator from LaserMax, Inc., which has since been reduced from $295 to $239. This unit is unique in that it projects a reticle pattern, instead of just a spot, and this allows you to do the easiest and most precise job of aligning all the optical components, along with the mechanical axis of the focuser. The review was quite favorable.
The S&T product review did raise one concern. In the article, they noted that the first sample unit borrowed from the supplier was found to be 10 arcminutes out of alignment. Obviously, if you have a collimator that's out of alignment, you would be constantly misaligning the optics in your scope, then wondering why the images are not as good as they should be. How do you know whether a collimator is within the stated specifications? There's a very simple test. Just place the collimator in a V-block or jig of some sort so it can be rotated about its axis. Watch the spot projected by the laser as you rotate the body of the collimator. The spot should not move. If the spot moves in a circle as the collimator is rotated, that means it's out of alignment.
Use of a laser collimator brings up a couple of other issues. First of all, if you have used a marker of some sort to place a black dot at the center of your mirror for the "old fashioned" type of collimation, you should remove it in order to use the laser. Since the laser light needs to be reflected off the dead center spot of your primary, the laser's light will be scattered and/or diminished if there's a black dot at center. The answer is to replace the black dot with a self-adhesive looseleaf paper hole reinforcer sold in office supply stores. Once the reinforcer is in place on your primary, you can use a Q-Tip moistened with alcohol to remove the black dot. This will leave an open spot for the laser's light to be reflected, plus the center point of the mirror will be obvious. (Don't worry about sticking anything onto the center of your primary mirror - the center is never used because it's in the shadow of the secondary mirror.)
Most of us have also noted that as we rack the focuser in and out, the laser point shifts just a bit on the primary mirror. This betrays a slight amount of play in the focusing mechanisms. Sometimes this can be reduced by increasing the tension on the rack and pinion or on the rollers of a Crayford; however, the drawback is that the action of the focuser will then be tighter and fine focusing will prove more difficult. If your focuser induces a large displacement of the laser point, you would certainly want to consider repairing or replacing it. A relatively small displacement seems normal. Simply note which direction of movement keeps the laser point centered, then focus accordingly when observing. For example, if the laser is centered when moving the focuser "in", then while focusing on an object in the sky, move the focus slightly out and then use in-focus travel to achieve the best image.
It's worth noting that use of a laser collimator is easier if your telescope has an open tube (truss type), rather than a solid tube. This will allow an easier and safer view of the laser spot when aligning it back onto the exit hole of the collimator. Also, being able to look from the back of the tube allows convenient access to the collimation adjustment screws.
I toyed with the idea of buying an inexpensive laser pointer and making my own collimator. But would this simple laser unit be precisely aligned in its own holder? When the local hardware store had laser pointers on sale for $9.99, it was worth a try. I set my bargain on vise jaws and rotated it. The laser spot on a wall four feet away traced out a circle almost two inches in diameter! Clearly, the necessary accuracy isn't there. Frank Suzda purchased several pointers and found that this is typical of all laser pointers. It would be possible to correct for this error when the laser is mounted, but a concern for maintaining consistent accuracy convinced me not to try making my own. On the other hand, Frank built a beautifully machined laser collimator using one of the inexpensive laser pointers. He added set screws for collimating the laser pointer barrel itself. He is also experimenting with a few different designs. Frank will eventually write an article detailing how he put together the homemade laser collimators.
The experience of using a laser collimator certainly points out the different factors that affect collimation and reminds us that maintaining perfect collimation is a constant pursuit.Published in the June 1999 issue of the NightTimes