Re: ATM Re: Vignet in focuser tube

[Mel Bartels <mbartels@efn.org>]

The issue here is one of judgement: when running software that gives
numbers, how should the results be judged?

If focuser base vignetting is indicated, then how is this to be judged?

In this case, the numbers make it appear worse than it really is.

I like to think of vignetting as a friend, not an enemy.  The focuser base
acts as a baffle.  Baffling is more important that vignetting here.  But
only experience of building many different newtonians will make this
clear.

This is what I recommend: take a piece of paper and a pencil, or, a CAD
program, and draw the on axis rays from edge of mirror to focus point at
center of field.  If the focuser base is outside of this cone, you are
fine.  Draw more rays from edge of mirror to a focus point, but at edge of
field stop of your low power eyepiece.  Now estimage by drawing
overlapping circles the percentage of vignetting.  Chances are that it
will be very small.

Mel Bartels