Collimating Your Telescope

Initial set-up or telescope construction is not mentioned here.

Newtonian Collimation

[basic]  First “Eyeball” it. Looking in to your telescope, the mirrors should enable you to see your own eyeball in the very center, surrounded by the focuser, surrounded by the image of the primary mirror within the secondary mirror. You should also be able to see out the end of your telescope tube through all of this. If not, adjustments are needed. If you adjust the secondary mirror, hold it on its side, do not put thumb or fingerprints onto the reflective surface. These don’t clean up like a bathroom mirror. You can adjust the secondary mirror so that the primary mirror is completely visible in it. Don’t worry at this time if the side of the tube or something other than the sky is visible in the primary, we’ll get to that later. Once the primary mirror is entirely visible in the secondary mirror, then you can adjust the primary mirror so that open sky can be seen out the front end of the tube as you look in to the focuser.

[good] Laser collimation is useful and fast. You put a laser collimator into the focuser and make sure it is ON. Never look into the laser beam directly. Red lasers won’t do the eyeball damage that a green laser will, but it is similar to guns. Consider each and every one loaded and dangerous. Looking down the front end of your telescope tube, look at your secondary mirror in the reflection of the primary mirror. There should be one or two red dots on it. The first dot is from the laser hitting it directly from the focuser, and if there is a second dot, it’s a reflection of the laser as it returns from the misaligned primary mirror. If it is truly collimated, then among other things these two dots are coexistent and you’ll only see one dot. But chances are you’ll have two. Adjust either the focuser so that the dot is in the center of the secondary mirror [if you have a very fast telescope, a secondary offset is incorporated into the system and this dot will not be at the center]. Once the focuser is in the appropriate position, the dot will be in the center of the secondary mirror. Now if you adjust the secondary mirror’s direction, since it pretty much stays in the same place, the dot won’t move away from its center, but the reflection will move as you make an adjustment. Adjust it until it hits the center of the primary mirror. Many of us have the centers of these mirrors marked making this easier. Once the dot is in the center of the secondary mirror AND the center of the primary mirror, your only task is to adjust the primary mirror so that this beam retraces its path onto the secondary mirror and the laser in the focuser. As you adjust the primary, the dot will move and eventually hit the first dot on the secondary, giving you only one dot. To make sure you’re really on the nut, look at the laser coming out of the focuser and you should see that the return dot is coexistent with the output beam, and you’ll only see the one dot there too.

[better] Cheshire collimation is done with either a Cheshire eyepiece or a reflecting eyepiece cover with a hole in it. You look in to this eyepiece and see concentricity on all parts of the system with your eye in the center hole [with an offset if you have a very fast telescope].

[Ultimate] The Star Test collimation is done either with real stars or a shop false star. For most Newtonians, the shop star is too close. I use false stars at distances between 75 feet and 3000 feet, or just a real star. If you look at a star at the center of the field of view at high magnification, you should see an Airy disk surrounded by 2 or 3 diffraction rings. If you defocus one way or the other slightly, the out-of-focus disk should appear exactly the same either way. Anything else would be spherical aberration. Furthermore, the disk inside and outside of focus should be circular with the secondary shadow dead center [unless you have a very fast telescope incorporating a secondary offset]. To collimate, first use one of the above methods. That will get you close. Now grab the highest magnification eyepiece you can get as well as a Barlow if you have it and focus the star. Place it dead center in your field of view. Defocus the star a hair and you should see a Huygen’s dot near the center of the defocused star. Adjust the primary mirror until this Huygen’s dot is dead center.

SCT  and Refractor collimation

Laser collimation is a poor choice with an SCT since the reflected beam never hits the primary mirror. Star test collimation is the secret to collimating Schmidt Cassegrains and refractors. Again, use extreme power. For refractors, there is another method, but it would be a starter method and should be followed up by star testing. The starter method is to use a Cheshire eyepiece as with the Newtonian. Light shining onto the Cheshire leaves the focuser and goes to the achromat lens. Reflections from that return to the focuser where the observer can check for concentricities. A laser performs the same task but is hard to see since a refractor is usually enclosed. Once this method has been accomplished, star test the optic at extreme power.