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Figure 10-1 A collimation pattern (left) of a slightly out of focus
star shows concentric rings. An example of an out of collimation
system (right) is shown by non symmetric rings that flare out to one
side.
To check the collimation of your telescope you will need a light source. A bright star near the zenith is
ideal since there is a minimal amount of atmospheric distortion. Turn your telescope drive on so that you
won't have to manually track the star. If you are not using the clock drive, use Polaris. It’s position relative
to the celestial pole means that it moves very little thus eliminating the need to manually track it.
Before you begin the collimation process, be sure that your telescope is in thermal equilibrium with the
surroundings. Allow 45 minutes for the telescope to reach equilibrium if you move it between large
temperature extremes.
To verify collimation, view a star near the zenith. Use a medium to high power ocular—12mm to 6mm
focal length. It is important to center a star in the center of the field to judge collimation. Slowly cross in
and out of focus and judge the symmetry of the star. If you see a systematic skewing of the star to one side,
then re-collimation is needed.
To accomplish this, you need to locate the secondary alignment screws which are located on the secondary
mirror housing. The secondary mirror housing is mounted in the center of the corrector plate. On most
models it will be necessary to remove the cap which covers the three secondary alignment screws. The cap
just snaps off. You need to tighten the secondary collimation screw(s) that move the star across the field
toward the direction of the skewed light. These screws are located in the secondary mirror holder. Make
only a small 1/6 to 1/8 field correction and re-center the star by moving the scope before making any
improvements or before making further adjustments.
When using higher power, 6mm and above, collimation is best accomplished with the telescope in focus. In
this instance, you are observing the Airy disk (see Figure 10-1), not the shadow of the secondary housing.
This (stellar) image will appear as a bright point of light with a diffraction ring around it. When the point of
light is perfectly centered within the diffraction ring, your telescope is in collimation. Keep in mind that to
use high power, the seeing conditions must be very good.
Perfect collimation will yield a star or planetary image very symmetrical just inside and outside of focus. In
addition, perfect collimation delivers the optimal optical performance specifications that your telescope is
built to achieve.
If seeing (i.e., air steadiness) is turbulent, collimation is difficult to judge. Wait until a better night if it is
turbulent or aim to a steadier part of the sky. A steadier part of the sky is judged by steady versus twinkling
stars.
THE ADJUSTMENT SCREWS ON THE SECONDARY MIRROR ARE VERY SENSITIVE.
USUALLY A TENTH OF A TURN WILL COMPLETELY CHANGE THE COLLIMATION OF THE
TELESCOPE. DO NOT FORCE THESE SCREWS IF THEY WILL NOT TURN. IF TIGHTENING
ONE SCREW IN THE DIRECTION YOU NEED TO GO IS DIFFICULT, SIMPLY LOOSEN THE
OTHER TWO SCREWS BY EQUAL AMOUNTS TO BRING ABOUT THE SAME CHANGE. DO
NOT BE INTIMIDATED TO TOUCH UP COLLIMATION AS NEEDED TO ACHIEVE OPTIMAL
HIGH-RESOLUTION VIEWS. IT IS WORTH THE TROUBLE!!!!
