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CHECKING COLLIMATION –
HOW DO I KNOW IF MY OPTICS
NEED ADJUSTMENT?
Checking optical collimation is quick and easy. We recommend
taking a few moments to check collimation before each observing
session. To do this, you'll need to examine a star image through
the telescope's eyepiece.
1. To begin, set up your telescope as you normally would for an
observing session. Connect a higher power eyepiece (ideally,
10mm focal length or less). Before you begin, be sure that your
telescope is in thermal equilibrium with its surroundings. Allow
45 minutes for the telescope to reach equilibrium if you move it
between large temperature extremes, like from a heated car or
house into the cold night air.
2. Choose a star at least 20 degrees above the horizon to
minimize the effect of atmospheric seeing conditions and slew
the telescope to it. Center the star in the field of view.
3 Defocus the telescope by about one turn of the focus knob
until the star image appears doughnut shaped. There will be
brighter rings of light surrounded by a dark hole (i.e. the shadow
from the secondary obstruction) in the center (see Figure 2).
If the telescope is collimated, the hole should appear centered
relative to the rings. If the hole is not centered, then adjustment
to the collimation is needed (see Figure 3).
4. Next, check the other side of focus by reversing direction of
the focus knob so that the centered star is focused and then
defocused again. The hole should still be centered in the rings.
If it isn't, some collimation adjustment is needed.
If you are unsure if the telescope needs adjustment or not, the
collimation is likely OK. In this case, it's best not to make any
adjustments, as doing so can easily make the collimation worse,
not better.
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SCT & EDGEHD COLLIMATION GUIDE
Out of focus star with proper collimation
Fig. 2
Out of focus star with collimation needed
Fig. 3
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