COLLIMATING YOUR REFLECTOR:
As reflector telescopes often travel a lot or are subject to large changes in temperature, it is very important for the owner of these instruments to know how to collimate or align the optical axis to obtain sharp coma free images. This short article explains how and the tools to make it easy.
Collimation is the process of aligning the mirrors of your telescope so that the light they collect will focus at the right spot at the back of your telescope for your eyepieces to work. Collimation of newtonian telescopes is compirsed of three major steps:
Adjust focuser to 90 degrees from the optical center line.
Adjust the secondary flat so it is centered in the drawtube and the reflection of the primary is concentric with the drawtube and the secondary mirror.
Adjust the Primary mirror so that the image is delivered to the center of the eyepiece field lens.
Coma
Focuser Collimation
This must be done in two axes, longitudinal and lateral. For longitudinal take a carpenter's square and ensure the focuser is 90 degrees square to the tube, next, take a straight edge and assuming that the spider is centered in the tube, run from the center of the secondary holder to the position of the eyepiece center at the farthest focus position. Adjustments on better focusers can be done by the adjustment screws on the base, otherwise you will have to shim the focuser base.
Manual Secondary Collimation
Make a sight tube from an old 35mm film can with the bottom cut off and a 1/8" hole bored in the center of the cap and place it in the focuser. looking through the hole you should be able to see the secondary mirror centered in the focuser tube with the image of the primary mirror centered in the secondary mirror. If not, adjust the secondary until this is the case. With scope of less than f/8 focal ratio, the reflection of the primary should be offset towards the primary but the entire primary mirror should be visible including the retaining clips.
Celestron collimating cap.
Secondary adjustment screws.
Cheshire Eyepiece Collimation
This is the best tool to have and it gives the capability of very fine optical alignment. The secondary and primary alignment can be done with this tool as described above with the sight tube. the crosshairs allow you to see where the optical centerline is. You will see the crosshairs framing a light circle with a black dot in the middle which is where you are looking through. The spring loaded adjusting bolts onthe primary mirror cell are adjusted to bring the center of the primary with it's reflection of the secondary to the middle where all will appear concentric with the black dot on the very center. Remember to loosen the primary lock screws first and then adjust the actual collimating screws until you achieve your goal then lock these bolts to maintain collimation. Sometimes the locking action may change the primary alignment slightly so that re-adjustment may be necessary. Now rack the eyepiece holder in and out to make sure that alignment is unchanged, if it moves, you have to re-align the focuser and start all over again.
Primary adjustment screws
STAR TESTING:
Method: Use an eyepiece and the North Star. The easiest way to find the North Star is to look for the Big Dipper. Draw an imaginary line along the two end stars in the bowl of the Big Dipper. The first star you come to along this line is Polaris. Use your lowest power (largest number eyepiece) to acquire Polaris, centre it. Now switch to your next higher power eyepiece, while keeping the image centered. The in-focus star image will have a bright innermost point, a slightly fainter inner ring and a fainter still outer ring that is hard to see.
Perfect Test
Star testing must be carried out under conditions of good to excellent seeing in order to be able to clearly judge results. Otherwise rely on the results of your cheshire collimator.
Images generated with Aberrator Version 3
Generally I find the sight tube/cheshire/star test methods to be the simplest and best methods, laser collimation is unlikely to yield good collimation unless the laser itself is accurately centered and collimated. Otherwise the secondary alignment will be off resulting in the inability to collimate the scope at all.
Astro Baby's collimation tutorial.
Andy's shotglass collimating video.
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