Telescope Collimation What is It?
Telescope Collimation is the process by which an instruments optical components are brought into precise alignment with its optical axis and mechanical axis. In other words,telescope collimation is 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”.
Collimation of Different Telescope Types
Telescope Collimation – Refractor and Maksutov telescopes are collimated at the factory and generally should not need further adjustment. If collimation does become necessary later, it is best to have it done at the factory, since these telescopes usually possess no user-adjustable collimation settings.
Telescope collimation is more of an issue with Newtonian Reflectors and Schmidt-Cassegrain. Good telescope collimation is particularly critical for “faster” Newtonians, those with f-ratios of f/6 or lower. Once the telescope collimation is set, it will hold if care is taken in transporting and handling the optical tube. However, any sharp jolts can knock the mirrors out of alignment, as can jostling of the scope in the trunk of a car or temperature changes over a period of time. Fortunately, these telescopes are equipped with adjustment screws that permit easy telescope re-collimation.
Telescope Collimation A common way of telescope collimation is the quick method. This is to simply remove the eyepiece and look down the focuser tube to center the mirror reflections. Unfortunately this is not accurate or a reliable method. 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. You can buy a Celestron Collimating Eyepiece at Adorama for around $34.95.
Testing Telescope Collimation
Telescope Collimation – You can quickly determine whether your telescope is property collimated. Just point it at a bright star and slowly rack the image out of focus with the focusing knob. If the telescope is correctly collimated, the expanding disk should be a perfect circle. If it is unsymmetrical, the telescope is out of collimation. In reflectors and Schmidt-Cassegrain, the dark shadow cast by the secondary mirror should appear in the very center of the out-of-focus circle, like the hole in a doughnut. If the “hole” appears off-center, the telescope is out of collimation.
Collimating a Schmidt-Cassegrain
Telescope Collimation – With Schmidt-Cassegrain telescopes, collimation is best performed using a “star test” but reasonable collimation can be achieved with the Collimating Eyepiece. There is only one collimation adjustment for Schmidt-Cassegrain: the tilt of the secondary mirror. Insert the Collimating Eyepiece directly into the visual back of the telescope.
The shadow of the secondary mirror will appear as a dark circle near the middle of the field-of-view. Adjust the three Allen-head screws located in the center of the front corrector plate to center the secondary mirror on the crosshairs. Do not loosen the screws more than two turns, or the secondary mirror could fall off its mount! Likewise, do not adjust the screw in the middle of the secondary mirror cell. It holds the mirror in place.
Collimating a Newtonian Reflectors
Telescope collimation – With the telescope pointed away from the Sun, look into the front of the telescope tube. Check that the secondary (or diagonal) mirror is positioned in the center of the tube; use a ruler if necessary to measure the distance from the center of the secondary mirror holder to the inside of the tube on different sides. You may have to adjust the spider vanes or stalk. Also, check to see that the primary mirror is centered in the optical tube. A quick visual inspection usually suffices. If the primary is obviously not centered, it will need to be repositioned in its mirror cell.
Next, remove the eyepiece from the telescope. Look down into the open focuser tube. You will see the secondary mirror and mirror holder as well as reflections of the secondary mirror and its holder, the primary mirror, and your eye. It’s pretty confusing. Refer to Figure 1.
Telescope Collimation – Now insert the telescope Collimating Eyepiece into the focuser and look into the sight hole. You’ll notice that the field of view is narrower than it was when you were looking through the open focuser tube. You’ll also notice that instead of seeing a reflection of your eye, you now see a bright annulus in the reflection of the secondary mirror. The annulus is the reflection of the polished 45-degree flat of the Collimating Eyepiece.
Check whether the secondary mirror is centered underneath the focuser. Use the crosshairs at the bottom of the Collimating Eyepiece as a reference. The center of the secondary mirror should lie right at the intersection of the crosshairs. If it doesn’t, adjust the position of the mirror holder until it is centered.
For “spider-type” holders, this is usually done by turning the threaded rod that the secondary mirror mount is attached to. For single stalk-type holders, you may have to bend the stalk to center the mirror. Refer to your telescope’s manual for more specific instructions.
Telescope Collimation – Now you need to adjust the tilt of the secondary mirror so that the entire reflection of the primary mirror is precisely centered in the secondary mirror (and, thus, also on the crosshairs of the Collimating Eyepiece). Use the three telescope collimation screws on the secondary holder to adjust the tilt. The reflection of the primary mirror is centered when there is an even ring of space between it and the outer edge of the secondary mirror. Don’t worry that the reflection of the secondary mirror is off-center; you’ll fix that in the next step. If you do not see the reflection of the primary at all, the tilt of the secondary is way off, and more tinkering with the collimation screws will be necessary. If the mount is a single stalk, you will have to bend it to the correct position. The secondary mirror is now concentric, but the other rings are not concentric. Refer to figure 2.
Telescope Collimation – Now it’s time to adjust the tilt of the primary mirror. Using the three collimation bolts at the bottom of the optical tube, located behind the primary mirror, turn one at a time until the secondary mirror reflection moves into the center of the primary mirror reflection. The spot in the middle of the primary mirror should be exactly centered in the bright annulus. Refer to Figure 3.
Telescope Collimation – Now, the view through the Collimating Eyepiece should resemble the diagram below. The reflection of the primary mirror is centered in the secondary mirror, and the reflection of the secondary mirror is centered in the reflection of the primary mirror. If everything is centered on the crosshairs of the Collimating Eyepiece, the telescope is in collimation — tuned up and ready for action! The telescope is now properly collimated. All rings are concentric. Refer to Figure 4.
Telescope Collimation – Collimating your telescope’s optics will become second nature to you when you perform this task on a regular basis