Re: [APML] needed- an excellent sense of direction

["Bobby Middleton" <bobm@koyote.com>]

----- Original Message -----
From: <Aplanatic@aol.com>
>
> Thinking out loud (or the email equivalent):  I suspect that a combination
of
> things may be going on here, the most likely of which is a bit of flexure
> combined with a bit of coma.  I'm not really sure yet.

There maybe is some slight elongation across the frame, but it's definately
worse in one corner. Your thoughts on it being worse in the corner is due to
coma?

>
> Yes, that's another big problem with collimators if they don't fit
perfectly
> in the drawtube.  Let's do a quick analysis.  Your Newtonian will start to
> show focus problems when the corner of a 35 mm film frame is about 3 mils
> out-of-focus, which turns out to be about 0.004 radians.  Your focal
length
> is, what?, about 90 inches?

87 inches.

 So, if the spot moves less than 0.25 inches or
> so at the primary when you diddle the screws, then I don't think that we
can
> blame it on the undersized barrel.  But if it moves this much or more then
it
> could indeed be the problem.

It won't usually move that much but can get close to that much movement.
1/8" is a typical move.

>
> Why are you using three screws?

Because I drilled a third one in there thinking 3 was better that two <g>.
What will the 3rd screw do, open up more potential for flexure?
Bobby






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