Celestron AstroMaster 130 EQ User Manual page 29

It is best to use the optional collimation tool or collimating eyepiece. Look into the focuser and notice if
the secondary reflection has moved closer to the center of the primary mirror.
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.