Spotting Scope Maintenance • 15
Before you begin the collimation process, be sure that your spotting scope is in
thermal equilibrium with the surroundings. Allow 45 minutes for the spotting scope
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 recollimation is needed.
To accomplish this, you need to adjust the secondary collimation screw(s) that
move the star across the field toward the direction of the skewed light (see
figure 3-1). Make only small corrections, approximately 1/6 to 1/8 of the field.
Recenter the star by moving the spotting scope before making further adjustments.
When using higher power, 6mm and above, collimation is best accomplished
with the spotting scope in focus. In this instance, you are observing the Airy disk
(see figure 3-2), not the shadow of the secondary housing. This (stellar) image
appears 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 spotting scope is
in collimation. Keep in mind that to use high power, the seeing conditions must be
very good.
Perfect collimation yields a star or planetary image very symmetrical just
inside and outside of focus. Also, perfect collimation delivers the optimal
optical performance specifications that your spotting scope 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.
NOTE: THE ADJUSTMENT SCREWS ON THE SECONDARY MIRROR ARE
VERY SENSITIVE. USUALLY A TENTH OF A TURN WILL COM-
PLETELY CHANGE THE COLLIMATION OF THE SPOTTING SCOPE.
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 BY TOUCHING UP COLLIMATION AS NEEDED
TO ACHIEVE OPTIMAL HIGH-RESOLUTION VIEWS. IT IS WORTH
THE TROUBLE!!!!
Figure 3-1
Left: With an out-of-focus
star image at the center of
the field, the secondary
mirror shadow is off center
indicating the spotting scope is
out of collimation Right: The
out-of-focus star image
showing good collimation.
Figure 3-2
In focus images show the
C5 in collimation (left) and
out of collimation (right).
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