Hello again,

Just a quick note on artificial stars. It is a bit of misconception that
one needs a really tiny pinhole....especially if you are planning only to
use the star for collimation (and not for the more demanding requirements
of testing for spherical aberration, etc).

A typical pin-prick into aluminum foil that gives you a 100micron (0.1mm)
hole is fine for collimation use. In fact, smaller holes become very dim
and are hard to use in daylight.

Also, you can use the artificial star at a fairly close range (ie 40 feet)
to do the collimation....since you are just interested in the centering of
the optical axis....and not worried about how your diffraction pattern may
or may not show spherical aberration from the unusual closeness of the
star.

To really test your scope, you need to move the artificial star at least
several hundred feet away. If you want it to be smaller than your
"Airy" disk, use a rule of thumb that 1 arc second is equivalent to about
a 1 million to 1 ratio of star diameter to star distance.

So if you want a 0.5 arc second test star.....put your 100 micron pinhole
at about 700 feet distance. Such a test star is the best (and in the
real world....the only practical) way to get a rock-stable view of your
diffraction pattern. The sky will never be stable enough....even on the
best night...to allow you to photograph (or video...even better) your
diffraction pattern as a function of focal travel. But with the
artificial star, you can get a repeatable photo-record of your performance
to compare with later scope tunings .....or after it returns from a Mead
service trip!

Tom Whicker

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