6
Or, if you do not want to power up your telescope, you can use
Polaris. Its position relative to the celestial pole 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.
Use a medium to high power ocular, something in the range of
12mm to 6mm focal length. It is important to position a star in
the center of the field of view 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 recol-
limation is needed.
To accomplish this, you will need to adjust the secondary mir-
ror collimation screw(s) located on the secondary mirror holder
(see Figure 6). To make collimation a simple procedure, follow
these easy steps:
1. While looking through a medium- to high-power eyepiece,
center a bright star in the field of view, then de-focus it until
a ring pattern with a dark shadow appears (see Figure 7).
Notice in which direction the central shadow is skewed.
2. Place your finger along the edge of the front cell of the tele-
scope (be careful not to touch the corrector plate), pointing
toward the collimation screws. The shadow of your finger
should be visible when looking into the eyepiece. Rotate
your finger around the tube edge until its shadow is seen
closest to the narrowest portion of the rings (i.e., the same
direction in which the central shadow is skewed).
3. Locate the collimation screw closest to where your finger is
positioned. This will be the collimation screw you will need
to adjust first. If your finger is positioned exactly between
two of the collimation screws, then you will need to adjust
the screw opposite where your finger is located.
4. Use the hand control buttons to move the de-focused star
image to the edge of the field of view, in the same direction
that the central obstruction of the star image is skewed.
5. While looking through the eyepiece, use an Allen wrench
to turn the collimation screw you located in steps 2 and 3.
Usually a tenth of a turn is enough to notice a change in
collimation. If the star image moves out of the field of view
in the direction that the central shadow is skewed, then you
are turning the collimation screw the wrong way. Turn the
screw in the opposite direction, so that the star image is
moving toward the center of the field of view.
Warning: The secondary mirror should never be removed.
Adjustments to collimation can easily be made by turning
the screws on the secondary mirror mount without ever
having to remove the secondary mirror.
6. If while turning you notice that the screws get very loose,
then simply tighten the other two screws by the same
amount. Conversely, if a collimation screw gets too tight,
then loosen the other two screws by the same amount.
7. Once the star image is in the center of the field of view,
check to see if the rings are concentric. If the central
obstruction is still skewed in the same direction, then con-
tinue turning the screw(s) in the same direction. If you find
that the ring pattern is skewed in a different direction, then
simply repeat steps 2 through 6 as described above for the
new direction.
Perfect collimation will yield a ring pattern that’s symmetrical
just inside and outside of focus (Figure 8). Well-collimated
optics deliver the optimal performance specifications that your
telescope is built to achieve.
If “seeing” (i.e., air steadiness) is turbulent, collimation is dif-
ficult to judge. If stars are twinkling, that’s a tipoff that the see-
ing is bad. In that case you might want to wait for a night with
better seeing (i.e., when stars aren’t twinkling) to perform the
collimation procedure, or you could try pointing the telescope
to a part of the sky that appears to have steadier air.
Note: Laser collimators will not work with Schmidt-Cassegrains
and should not be used in collimating the optics.
Figure 7.
Even though the star pattern appears the same on both sides of focus, it is asymmetric. The dark
obstruction is skewed off to the left side of the diffraction pattern indicating poor collimation.
Figure 8.
In a well-collimated
telescope, a defocused star image
should appear symmetrical, with the
dark central obstruction centered in
the star’s ring pattern.
Summary of Contents for Telescope
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