ATM Cheap Collimation Tricks

["Bell, Andrew" <AMB2@pge.com>]

I'll admit it -- I like nice crisp high-contrast images, but I hate the
really hard parts of collimating!  I like the summary that Dwight Elvey
posted last night, more or less as follows:

        1. Focuser tilt: this causes vignetting if not compensated for by
the secondary position. Significant side tilt can cause some clipping of
incoming light. This is difficult to see, even in diffraction images unless
it is severe or it puts the beam into tube currents. The primary effect is
centering of the 100% illuminated area.

        2. Secondary tilt, centering, offset: these all cause vignetting.
The only effect on the diffraction image is related to vignetting. See the
recent S&T article that shows excellent pictures of diffraction image
clipping or vignetting.

        3. Primary tilt: axis of the primary needs to be centered in the
eyepiece. Result of this not being done will cause poor contrast on high
power and noticeable unevenness of diffraction rings. Don't confuse these
effects with vignetting or input beam clipping. The effects are different.

        4. Other problems: tube currents, bad mirror cells, warped primary
and warped secondary can cause various distortions that cannot be corrected
by normal alignment.

With my set-up, both focuser and secondary tilt are difficult to adjust, and
adjusting the secondary placement is almost impossible.  Part of what I am
reading into Dwight's summary is that adjusting the primary tilt (which is
relatively easy) is by far the most critical aspect of collimating, and can
even be used to compensate for errors in the first two steps.  If my
secondary isn't tilted at exactly 45 degrees and/or my focuser might be a
little bit off from perpendicular, it still seems like I should be able to
adjust the primary until the axis of the primary is pointed directly at the
center of my eyepiece, right?

        I have been assuming that the way to get the primary tilt right is
just to make sure the shadow of the secondary is centered right in the
middle of an out of focus star image.  Is it really that easy [for visual
use only - and recognizing that even with the center of the optical axis
pointed directly at the center of the eyepiece, the plane of the eyepiece
might not be exactly perpendicular to the optical axis], or am I missing
something?

- Andrew Bell