Re: [ATM] squaring truss tubes and collimation

[Mark Holm <mdholm@telerama.com>]

Jay Kirkland wrote:
>That has to effect performance.

Not much really.  A bit of vignetting on one side of the field and a
very slight increase in diffraction.  You would be hard pressed to notice.



>(what IS the plural of axis??)

axes  pronounced axees  and yes I know there is another English word
with the same spelling pronounced a different way.  (English spelling is
a mess.  The last people to try to clean it up in the USA were labeled
socialists or anarchists or something else unsavory and accused of
trying to fluoridate drinking water or some other dastardly un American
acts.)


> My point is,  on truss tube scopes, it's easy to be way off and not even know it.

It is a good point.  Even if the optical effect is small, one would like
to be a little more lined up.  Of course, the rotating top end thing is
a whole different animal.

>     I'm still not sure exactly what's worrying me about the squareness 
> of the focuser to the optical axis.
...
> that the beam doesn't 
> travel all the way thru the optical axis. But then it doesn't have to, 
> does it?
...
>     So the focuser issue is not a valid one, unless the focuser is out 
> of square with the secondary axis.

If the laser beam starts from the center of the eyepiece field of view
(presumably at the center of the focuser tube) reflects off both mirrors
and then makes it's way back to where it started, then it is by
definition traveling on the optical axis.  The only other considerations
are:

1. That it actually hit the optical center of the mirror (otherwise you
will get some nasty astig/coma from being off the center of the parabola.

2. That the focuser motion be parallel to the optical axis.  If you
never adjust your focuser, then this isn't an issue, but most of us do :)

3. As you have already observed, there are some secondary consequences
of not having the optical axis lined up with the mechanical structure of
the tube, but those are primarily mechanical.  You have to be really
badly off before the optical consequences are bad.

> THAT'S when apo vs achromat-type 
> focus problems would become real.

Not sure what you mean by this.  I don't think apo vs achromat type
issues come into play here at all.

>     Which I guess just leave me with my original thought, that, 
> especially for big scopes, or weird scopes, the directions in 
> Kriege/Berry aren't sufficient to ensure that the mechanical centre of 
> the spider cage is co-incident or even reasonably close, with the 
> mechanical centre of the mirror box.

I think you are right.  A nice web page discussing it and various ways
to deal with it would be a good contribution.  In fact, if you would
write it up well, with good diagrams and pictures, I'll bet Gary Seronik
at S&T would consider publishing it.  (He might not want it if it was
already published on a web page.)

Mark Holm
mdholm@telerama.com

_______________________________________________
ATM mailing list http://www.atmlist.net/