Re: ATM "collimating" diagonals

["Nils Olof Carlin" <nilsolof.carlin@telia.com>]

Frank,

>A few days ago, someone posted a message regarding a wandering laser
>collimator beam when racking his JMI focuser in and out. 

Al Klayton - come to think of it, one out-of-round bearing (or some kind of dirt on it or 
the tube) would likely produce what you describe. It seems likely to me that normal 
manufacturing tolerances would not produce so much "wandering" as 3/8" at the mirro
r.

>Inserting the Vernonscope star diagonal (at 12:00 position) collimation was
>still fine.  However, rotating the star diagonal 90 degrees clockwise (as I
>often do to view comfortably) resulted in a fairly dramatic shift of the
>laser beam off the return path.  I repeated this exercise with the standard
>Meade star diagonal (JMI motofocus off the setup), and the results were
>similar ..

>How important or degrading to the image are these apparent misalignments??


My best guess is that the error of angulation here, as in the Newt, is not critical to 
the image as long as the optical axis is centered well enough. If the defocused star test 
shows concentric rings, it should be OK. 

I believe if the beam returns to its origin, it is because it hits the secondary at the 
point whose normal points back to the origin regardless of the collimation of the main 
mirror. What you could do is check that the beam exits the star diagonal precisely at its 
center (or else shim the mirror). Only when it does, check that the beam does not wander 
if you rotate the diagonal/collimator.

These are untested ideas...

Nils Olof