Re: SEC: UNCLASSIFIED RE: ATM Collimation Question

[Dwight Elvey <elvey@hal.com>]

afmartyn@raaf.defence.gov.au wrote:
>  So as far as I'm concerned, the star test 
> alignment only works for non-offset diagonals.

Hi
 I think there is a miss understanding about offset.
First, if you have a reasonable area of 100 % illumination
you'll not notice much effect of having the offset
off. This is because the offset really only effects the
vignetting of the light. I also hear people mentioning
doing shadow centering as a star alignment. This is
only a course alignment. The only alignment that counts
is at focus. If you have your telescope set without
offset, you will normally be able to get good center
alignment using shadow centering or focused centering.
There is a problem for fast telescopes or telescopes
with smaller secondaries. In order to not vignette the
center of the field, you'll need to have some offset.
It is a simple ray trace problem.
 You of course can't use the secondary shadow as centered
and also have an offset secondary. I don't think we
should confuse the effects of out of focus images with
a properly collimated telescope. Having a offset secondary
will off center the out of focus shadow. This does
not mean the telescope is out of collimation.
Dwight