[Starkler (Geoff)]

Heres an exerpt from my upcomming how-to you might find useful.
You may not have to do anything. Check first.


Star testing for collimation and astigmatism

We can check for correct collimation and detect astigmatism in our scopes optics by performing a “star test”.
Make sure you put your scope outdoors with covers off for at least 60 minutes before starting because to successfully star test requires a properly cooled scope and also steady atmospheric conditions.

Take your highest power eyepiece (shortest focal length) eyepiece, find a nice medium bright star and focus in the center of the field of view.
Now the adjust the focuser outwards slowly until the star defocuses and you see a series of rings like in fig 1 below.

(insert startest.gif)

If you see nice concentric round rings like in A. your scope is well collimated (assuming your secondary mirror is correctly placed).
Most likely it will look more like B, indicating some tweaking is required. If it looks like C, your scope is way out of alignment!

If you are getting nice round circles all is well. But what if the rings are not perfect round circles?

Often the outermost ring will appear a bit “hairy” and waver about a bit. This is due to either atmospheric conditions, or a scope that hasn’t cooled fully down to ambient temperature.
If the image is reasonably steady, take note of the general shape and rack the focuser in and out on both sides of focus.

Does the image look a bit like a three/four leaf clover or triangular/squarish?
This usually indicates astigmatism in the primary mirror caused by the three/four rubber holding clips being clamped down too hard.

Does the image appear oval or some other shape and shift plane by 90 degrees either side of focus?
This could indicate a pinched secondary mirror caused by its plastic holder being too tight. Try moving your head to rotate your eye relative to the eyepiece. Does the plane of the image follow the movement of your eye or stay fixed in relation to the scope?
If it moves with your eye I’m sorry to inform you that’s where the astigmatism lies. If its in the scope there’s still hope ! J