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

[Aplanatic@aol.com]

Hi Bobby,

> 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?

Suppose that there is a small amount of coma in your system.  Coma produces radially-elongated stars.  If there is a also a small amount of drift during the exposure then in the direction of drift the elongation will add with the coma component.  It the orthogonal direction the drift+coma will tend to produce rounder stars.

Regarding the collimator:

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

That shouldn't be enough to seriously affect the alignment of the focal plane.

> What will the 3rd screw do, open up more 
> potential for flexure?

Yes.  With three screws the barrel is not in good contact with the side of the drawtube.  This may, in some cases, allow the lower end of the barrel to have a small amount of slop.  By using two screws, the barrel is pushed firmly up against the side of the drawtube and the lower end of the barrel will be less likely to move.  As I said previously, it's not a perfect solution, but works most of the time.  I have developed the habit of always testing the drawtube-barrel interface by applying torque to the barrel to see if I can feel any relative motion.  If you can feel anything at all, it's too much.

Dave Rowe


--  APML Archives at <http://astro.umsystem.edu/apml/>  ---
             Unsubscribe at <majordomo@seds.org>