ATM Re: Collimation

[Steve Bell <sb635@delphi.com>]

Gary Myers wrote:
> 
> >> Steve Bell wrote :
> >>It may be that the only condition where
> >>it should look centered, to the human eye, is when the secondary is at
> >>45 deg.
> 
> Not true! It can look centered if the secondary is placed at the
> tubaxial offset (down) and NOT the focuser offset (away). The problem is
> when this condition happens that the center line now shifts down towards
> the primary by the amount of the offset! Also for a closed system
> (cheshire setup) the primary will have to be tilted!
> 

It's unfortunate that we only have words to communicate with on these
lists. I'd love to be able to draw some sketches for this thread. 

I completely agree - when implementing the offset, if you move the
secondary along the mechanical axis of the tube by using the nuts at
different places along the threaded rod, you can get it to look centered
when looking through a peep-hole cap in the focuser. Then you have one
part of the offset done. And at the same time the secondary should still
ideally be 45 degrees tilted to the tube's mechanical axis, given that
the tube's mechanical axis and the primary's optical axis are
coincident. So, your words don't tell me why you said "Not true!"

My statement is based on this. If the diagonal is *not* set at 45 deg,
then obvioulsy the "close to the eye" edge of the ellipse and the "far
away from the eye" edge are *not* the same distances as if were set at
45 degrees. For example, the extreme case is when it's tilted 0 degrees
(not actually possible with most secondary holders, but visualize it),
so that both edges are the same distance from the eye, and the secondary
looks (to the brain) exactly like what it is - an ellipse. Then, this
visual process of centering will center the actual physical center of
the secondary (ie, where its semi-major and minor axes intersect)
directly under the focuser, not any offset location, because then all
edges of the secondary are the same distance from the eye. Obviously
then, what the brain sees as "centered," and what dot that corresponds
to on the surface of the secondary is theoretically related to the
actual tilt of the secondary. This being true, my intuition tells me
that the only angle that will give the correct "along the OTA's axis"
offset is a 45 degree one, if you use this "looking through a peep-hole
to center" technique for doing that part of the offset.

-- 
Clear skies,
Steve Bell
email: sb635@delphi.com
Astrophoto page: http://people.delphi.com/sb635