[ATM] squaring truss tubes and collimation

[Jay Kirkland <jaykirk2@compusmart.ab.ca>]

    Sorry about not using the correct terminology. Yes, I understand 
that the optical axis of the telescope and the mechanical axis of the 
telescope are not the same thing, probably won't be the same thing, 
because of offset at the very least, even with a closed tube scope made 
to exact tolerances. And I know that they only need to be parallel if 
you're using digital setting circles, etc. But I managed to barlow 
collimate my scope as good as it could be done using that method with 
the mechanical axis of the spider ring  a little more than 2" lower than 
the mechanical axis of the mirror box. Depending on whose equation you 
use, the minimum tube diameter  for a 20" f5.7 is between 21.15 and 
21.6", with my actual ID  being 21.5". A 2" sag takes 1" off that 
radius, meaning I had my tube obstructing the light path by 3/4-1" on 
one side. And because of the size of the scope, there was nothing to 
hint at this until I wondered why I had to tilt my secondary assembly to 
make the collimation work. That has to effect performance. And there was 
no way of determining the extent of this off-centeredness until I 
mounted a laser on top of the scope. On smaller scopes, you could do it 
just as easily with a sight tube mounted square and center on the end of 
your scope. Assuming the end of your scope is square, which is easy 
enough to measure.
    And I had already assembled the thing on a jig, etc, to ensure that 
the truss tubes were the same length, in the proper places, etc. 
Obviously that wasn't enough.
     So, yes, I agree that the optical and mechanical axise (what IS the 
plural of axis??) don't have to the the same or even parallel for the 
scope to be properly collimated. My point is,  on truss tube scopes, 
it's easy to be way off and not even know it. And again, possibly not a 
big issue unless you need the axis to be near the same. In which case, 
it might actually be easier in the long run to build the scope with a 
non-adjustable spider with the off-set built in and the mirror fixed at 
45 degrees. The only adjustment you'd need would be rotational.
    I'm still not sure exactly what's worrying me about the squareness 
of the focuser to the optical axis. If it's axis is centered on the 
secondary, and the secondary is tilted at 40 degrees to that axis, 
rather than 45 degrees, the light still has to travel down exactly the 
same path from the eye/laser/crosshair to and from the primary. But 
there's something there that still nags at me - perhaps the fact that a 
crosshair is not all that sensitive (compare the results of crosshair vs 
laser vs barlowed laser collimating for sensitivity) and that the laser 
only illuminates the base of the laser itself, that the beam doesn't 
travel all the way thru the optical axis. But then it doesn't have to, 
does it? If it travels part of the way, back to its source, it has to be 
following the exact same path, doesn't it? it's not going to be crossing 
over on itself anywhere. No, it can't.
    So the focuser issue is not a valid one, unless the focuser is out 
of square with the secondary axis. THAT'S when apo vs achromat-type 
focus problems would become real.
    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.

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