Celestron PowerSeeker 21008 Instruction Manual page 23

With Polaris or a bright star centered within the field of view, focus with either the standard ocular or your highest power
ocular, i.e. the shortest focal length in mm, such as a 6mm or 4mm. Another option is to use a longer focal length ocular
with a Barlow lens. When a star is in focus it should look like a sharp pinpoint of light. If, when focusing on the star, it is
irregular in shape or appears to have a flare of light at its edge, this means your mirrors aren't in alignment. If you notice
the appearance of a flare of light from the star that remains stable in location, just as you go in and out of exact focus, then
re-collimation will help sharpen the image.
When satisfied with the collimation, tighten the small locking screws
Even though the star pattern appears the same on both sides of focus, they are asymmetric. The
dark obstruction is skewed off to the left side of the diffraction pattern indicating poor
collimation.
Take note of the direction the light appears to flare. For example, if it appears to flare toward the three o'clock position in
the field of view, then you must move whichever screw or combination of collimation screws necessary to move the star's
image toward the direction of the flaring. In this example, you would want to move the image of the star in your eyepiece,
by adjusting the collimation screws, toward the three o'clock position in the field of view. It may only be necessary to
adjust a screw enough to move the star's image from the center of the field of view to about halfway, or less, toward the
field's edge (when using a high power ocular).
Collimation adjustments are best made while viewing the star's position in the field of view
and turning the adjustment screws simultaneously. This way, you can see exactly which way
the movement occurs. It may be helpful to have two people working together: one viewing
and instructing which screws to turn and by how much, and the other performing the
adjustments.
IMPORTANT: After making the first, or each adjustment, it is necessary to re-aim the
telescope tube to re-center the star again in the center of the field of view. The star image can
then be judged for symmetry by going just inside and outside of exact focus and noting the
star's pattern. Improvement should be seen if the proper adjustments are made. Since three
screws are present, it may be necessary to move at least two to achieve the necessary mirror
movement.
Figure 7-2
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Figure 7-3
A collimated telescope
should appear as a
symmetrical ring pattern
similar to the diffraction
disk seen here.