In a message dated 06/01/1999 9:30:26 AM Eastern Daylight Time,

<< Chris,
Seeing that the moon is only 250,000 miles distance and the star
was at least 2 light years away and a point source of light, I certainly
hope you had to re-focus a bit. No worries mate!

john
Willard Observatory >>

I hope John was joking, rather than being wrong.

There is only one valid reason for a SLIGHT refocusing between VISUALLY
viewing the moon and a star, and that is because your iris closes down for
the moon, and it is known that different parts of some people's eyes reach
different focus. Not a good sign for what you see when the eye opens up to a
large exit pupil.

This does not apply to CCD or film photography where the plane of infinity
focus is the plane of infinity focus. And certainly, if you're really
cranking the focus between celestial objects, there is something terribly
wrong.

To give you an idea of how small the focus difference is between 250,000
miles and infinity, a scope focused on a star will lower its resolving power
on the moon (or any other object for that matter) by the diameter of the
scope's aperture. For instance, if you could resolve a feature 5000 feet in
size on the moon if focused at 250,000 miles, you could resolve a feature
5000 feet 10 inches in size if focused at true infinity in a 10 inch scope.
In reality, you can't tell the difference between those two cases whatsoever
due to the fact that 10 inches at 250,000 miles is an incredibly tiny angle,
under 0.0002 arc seconds.

John

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