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

[Aplanatic@aol.com]

Bobby,

>  But I've got something besides flexure Dave. The long stars are only at
>  opposite corners of the image. If I had flexure it would be across the 
whole
>  frame, right?

Yes, flexure would show up across the whole frame.  I took a fairly long look 
at:

> http://www.koyote.com/users/bobm/ngc281.htm

in PS and I am not so sure about your two-corner-only observation.  I see a 
bit of elongation in the center of the frame as well.  Isn't there a program 
that the CCD folks use that will give us a quantitative evaluation of the 
star's roundness in all four corners and the center?

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.

>  I've done the laser rotation thing. It looks good. But here's a nasty quirk
>  I found this afternoon. The laser spot will move IF I tighten down the 3
>  thumbscrews in different order. I guess that means too much play in the
>  focuser tube which allows the collimator to slightly shift. Maybe i could
>  use something to increase the diameter of the collimator like several
>  adhesive labels or tape around it to keep away any play.

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?  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.

Why are you using three screws?  You should only be using two screws to lock 
down the collimator in the drawtube and they should be 90 to 120 degrees 
apart.  Three screws is, I believe, an unstable situation.  Using the two 
screws you want to push the barrel hard against the side of the drawtube.  
Even this is not the correct kinetic solution, but it's the best we've got 
for this mechanical arrangement.

Dave Rowe


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