Hack 37. 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 costs. Instead, you can make your own sight tube for $0 and a few minutes' work. All you need is an empty 35mm film can, a sharp knife, a center punch or nail, and a small drill bit (we used a 1/16" bit, but anything close to that is fine).
Coincidentally, a film can is almost exactly the same diameter as a 1.25" eyepiece, and the lid is just large enough to prevent the can from sliding down into the focuser. Ablack plastic Kodak film can with a gray plastic top is ideal. To begin, use a sharp knife or scissors to cut the bottom off the can, as shown in Figure 3-12. Work carefully, and try to avoid bending the film can out of round. Discard the bottom of the can.
The next step is to create a perfectly centered peephole in the lid, which will allow you to place your eye exactly on the optical axis of the scope. Fortunately, the lid has a small raised nub that marks its exact center. Place the film can lid upside down on a flat surface, and use a center punch, nail, or heavy needle to mark the exact center of the lid, as shown in Figure 3-13.
Once you have a center-punched dent to prevent the drill bit from "walking," use the 1/16" drill bit to cut a clean, circular hole in the center of the film can lid. Snap the lid back onto the film can, and you're done. To use your film-can collimator, simply insert it into the focuser as you would an eyepiece, put your eye to the peephole, and verify that all of the optical components appear as concentric circles in your field of view [Hack #38].
Figure 3-12. Remove the base of the film can using a sharp knife
Figure 3-13. Center punch the lid of the film can
Hack 38. Tune Your Newtonian Reflector for Maximum Performance
Align your optics to provide the best possible image quality.
Collimation is the process of aligning all mirrors and lenses in a telescope so that they share a common optical axis. Awell-collimated scope provides the best images its mirror or objective lens is capable of providing. A poorly collimated scope has significantly degraded image qualityhow degraded it is depends upon how poor the collimation is.
To understand how important proper collimation is, consider two telescopes of the same aperture and focal ratio. One scope has a typical mass-market Chinese mirror, accurate to perhaps 1/4 wavelength or about 0.80 Strehl. (Strehl ratio is a statistical measure of overall mirror quality based on interferometry testing; a perfect mirrorimpossible in the real worldhas a Strehl ratio of 1.0.) The second scope has a premium mirror, made by a master optician such as Carl Zambuto or R. F. Royce. That premium mirror may cost 5 or 10 times as much as the mass-market mirror, and it is accurate to perhaps 1/20 or 1/40 wavelength, say 0.98 Strehl. There is no comparison between these mirrors. The first is mediocre. The second is world-class.
So we set up the two scopes and point them at Jupiter or Saturn. The inexpensive scope is perfectly collimated. The premium scope is just slightly out of collimation. How do the images compare? The cheap mirror beats the premium mirror, and not just by a little bit. The cheap mirror wins hands-down. The moral here is that if you want a premium scope but can't afford one, don't despair. Learn to collimate properly instead, and the images in your inexpensive scope will be at least as good as those in a typical premium scope. (Until you run into someone with a premium scope who also knows how to collimate. Oh, well.)
Proper collimation is particularly important for telescopes with fast focal ratios [Hack #9]. An f/10 SCT, for example, provides only very slightly degraded images, even if it is only roughly collimated. Conversely, an f/6 reflector must be collimated with reasonable care to provide anything near its best image quality. Around f/5.6 to f/5, precise collimation becomes critical. Large Dobs often have focal ratios of f/4.5 down to perhaps f/4.2, and we know of one large Dob with an f/3.5 mirror. At focal ratios that fast, precise collimation is not just critical but is an ongoing concern during observing sessions. For example, owners of very fast Dobs often collimate several times during an observing session. As time passes, the temperature drop is sufficient to take the Dob out of collimation. Even changing the elevation of the Dob from, say, 30° to 60° may require tweaking the primary collimation to bring the scope back into alignment. Needless to say, owners of very fast scopes soon become accomplished experts at collimating.
Collimating a Newtonian reflector has the reputation of being a black art, but in truth there's nothing very complicated about it. The secret, if there is one, is to do things in the proper order. Most beginners make the mistake of focusing their efforts exclusively on the primary mirror. That's understandable, we suppose. After all, the primary mirror is a big, impressive chunk of glass, and the primary collimation screws are readily accessible. But you can collimate the primary mirror until you're blue in the face, and it won't help unless you've first collimated the secondary mirror. Here's the proper sequence of steps:
The first step in collimating a Newtonian reflector is to center the secondary mirror under the focuser. To begin, insert your sight tube or film can collimating tool [Hack #37] in the focuser, as shown in Figure 3-14, and peer through it to view the secondary mirror.
Figure 3-14. To begin collimating the secondary, insert the sight tube in the focuser
The secondary mirror is actually elliptical, but it is set at a 45° angle (to reflect at 90°), so from the focuser it appears to be a circle. Slide the sight tube in or out of the focuser (or use the focuser itself) to adjust the position of the sight tube until the circular edge of the secondary mirror is exactly concentric with, and just inside, the circle formed by the bottom of the sight tube, as shown in Figure 3-15.
Figure 3-15. Adjust seating depth until the edges of the secondary and sight tube are concentric
The "hall of mirrors" effect caused by the multiple reflections between the secondary and primary mirrors can be confusing when you use a sight tube. To eliminate this problem until you have the secondary mirror collimated properly, ask a helper to hold a sheet of paper between the secondary and primary mirrors, as shown in Figure 3-16. Blocking the primary mirror this way eliminates confusing multiple reflections. Depending on your scope, it may also be helpful to put a sheet of white paper on the inside of the tube, opposite the focuser, to make the edge of the secondary mirror easier to see.
Figure 3-16. Hold a sheet of paper between the secondary and primary to block multiple reflections
If the secondary mirror appears circular and is centered in the sight tube, you're ready to proceed to the next step. If the secondary mirror is offset up or down the tube, you'll need to slide the secondary mirror holder in or out until the secondary mirror is centered. Most secondary holders have a single central retaining screw, shown in Figure 3-17, that must be loosened to adjust the position of the secondary holder. (Yes, our scope is dusty; that's what happens to a working scope.)
Figure 3-17. Loosen the secondary retaining screw to adjust the position of the secondary
To move the secondary mirror closer to or farther from the primary, loosen the retaining screw and adjust the overall seating depth of the three adjustment screws visible in Figure 3-17. Drive all three screws farther in to move the secondary mirror closer to the primary, or back all three screws farther out to move the secondary toward the front of the tube and away from the primary.
The next step is to adjust the rotation and tilt of the secondary mirror so that the primary mirror appears to be centered in the reflection from the secondary. If you have a laser collimator, use it to perform this step. To do so, insert the laser collimator into the focuser and turn it on. Look down the tube to see the spot of the laser on the primary mirror. Use the three adjustment screws on the secondary holder to adjust the rotation and tilt of the secondary mirror until the laser beam strikes the primary mirror in the middle of the center spot [Hack #33], as shown in Figure 3-18.
Figure 3-18. The view looking down the tube while using a laser to collimate the secondary mirror
The primary mirror is the circular object at the center of the image, surrounded by the gray of the scope's tube, with the focuser visible as a blurred image at the upper right and the secondary holder even more blurred at the lower left. The primary mirror reflects a crescent of the upper part of the tube (at left), the ceiling of the room where this image was taken, the spider vanes that hold the secondary, the secondary mirror itself, and part of Robert's arm and hand. The two bright spots reflected at the upper right of the primary mirror are the bottom of the laser collimatorreflected in the secondary mirrorand the laser spot on the secondary mirror itself. None of that matters, but it is visually confusing. What does matter is the laser spot on the primary, which is the bright spot at the center of the primary, is centered in the notebook reinforcing ring. Having that laser spot centered in the reinforcing ring means that the secondary is collimated properly, pointing to the center of the primary mirror.
If you don't have a laser collimator, you can use the sight tube for this step. To do so, adjust the tilt and rotation of the secondary mirror (or have someone do it for you) as you view the reflections through the sight tube. When the secondary mirror is collimated properly, you will see the secondary mirror edge concentric with the edge of the sight tube, the primary mirror edge concentric with the secondary mirror edge, and the crosshairs of the sight tube centered on the center spot of the primary mirror.
Once you have the secondary properly collimated, the hard part is done. Fortunately, you probably won't have to repeat this process unless you remove the secondary mirror for cleaning, install an upgraded focuser, or take some other action that affects secondary collimation. Most secondary holders maintain collimation well. We seldom have to adjust secondary collimation from one year to the next.
The final step in collimating your scope is to collimate the primary, which consists of adjusting the primary mirror tilt until the optical axis of the primary is the same as the optical axis of the secondary and focuser. Most mirror cells use three adjustment screws (see Figure 3-19) and three locking screws. Turning an adjustment screw in or out slightly changes the tilt of the primary. Once the primary is collimated, you tighten all three locking screws to clamp the mirror cell in place. It's much easier to collimate the primary if you have a helper. One person sits or lies at the rear of the scope and tweaks the adjustment screws while the other person watches the effect from the front of the scope.
The fastest, easiest, and most precise way to collimate the primary is to use a Barlowed laser [Hack #39]. If you don't have a laser collimator, you can use a Cheshire or sight-tube/Cheshire to collimate the primary, albeit somewhat less precisely.
Figure 3-19. A primary mirror cell adjustment screw
To do so, illuminate the fine-ground 45° reflective surface of the Cheshire (visible at the center of Figure 3-14). If you are collimating after dark, point your red flashlight at the Cheshire opening. Tweak the primary adjustment screws until the center mark on the primary mirror coincides with the donut shape visible in the Cheshire.
That's all there is to collimating a Newtonian reflector. It takes a few minutes to collimate the secondary, and perhaps a minute or two to collimate the primary. Surprisingly, many Newtonian reflector owners seldom collimate their scopes, and most never collimate them. But the 5 or 10 minutes you spend to get your scope properly collimated pays off in much better image quality and more visible detail, particularly on planets and other lowcontrast objects.
You're not quite finished, though. When you finish using your sight-tube/Cheshire and laser collimator, the scope is very close to being perfectly collimated. Close, but not close enough. The final step to achieving perfect collimation is to use a defocused star to tweak the collimation to perfection [Hack #40].