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Figure 10
A collimated 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.
Collimation is the alignment of the optical elements. In the Schmidt-Cassegrain optical system used in the
G-5, this is the alignment of the primary and secondary mirrors.
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. If your telescope has a motor drive turn it
on so that you won't have to manually track the star. Or, if you are not using the motor 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 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), 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!!!!
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