[ATM] squaring truss tubes and collimation

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

    "As long as the focuser operates along the secondary optical axis"
    Exactly.
    What if you've squared the focuser to the secondary cage/ring? If 
you've made sure that the focuser is aimed at a point 180 degress 
opposite the focuser centre, and exactly the same the same distance 
from, on a scope with a full spider cage, the back ring of the spider 
cage? The very definition of a squared focuser? If the spider cage isn't 
physically centered on the primary opitcal axis, then the secondary 
optical axis won't meet the eyepiece at 90 degrees. It will still be 
very easy to make the secondary axis meet the centre of the eyepiece, 
you could do that with the eyepiece tilted away at 45 degrees, even. 
With the secondary cage out of alignment with the primary axis, you 
could have everything perfectly centered, but the secondary axis might 
be, I don't know, 5 degrees? out of square when it reaches the eyepiece. 
I don't know much... well, about anything, but my understanding of light 
waves is that they are very small. The point of collimating is to get 
the light waves all to focus at the same place. On an achromat, the 
various wavelengths of light don't meet, so you get chromatic abberation 
or something. So they invented apos and use exotic glasses. Wouldn't 
this be similar? It's not various wavelengths, tho, it's that 
wavelengths coming from one seide of the secondary are reaching the 
eyepiece a few peaks and troughs before wavelengths from the other side 
of the secondary are reaching the other side of the eyepiece.
    I don't know how much effect this might have on what you're seeing. 
Maybe a worst case scenario would be that it would be like looking thru 
an obstructed achromat. But if so much fuss and bother has been made 
over apos, wouldn't a reflector user want to kick it up a notch and be 
looking thru an obstructed apo?
    Maybe that's why so many reflectors seem to be underperformers 
compared with other reflectors with the same specs or with refractors. I 
mean, I barlow laser collimate all my scopes, and I would have thought 
they were dead on. But surely having a perfectly square focuser aimed at 
a  perfectly centered, properly offset secondary in a spider cage that's 
1/2 - 2" off the primary axis is going to have some effect on the scope.  
    Someone on this list once wrote in to say that if the scope is 
properly built, and the secondary cell is precisely built, the secondary 
wouldn't need any provisions for adjustment at all, that secondary 
adjustment was only something you needed to compensate for sloppy 
workmanship. Call me sloppy. But I see the point. In theory.
    I'll bet that if you test a lot of seemingly square scopes out there 
with a laser running thru the centre of the secondary cage, you'll find 
a lot of them aren't nearly as close to centered as you thought. I'd 
love to know if fixing that results in noticable performance 
differences.  Perhaps that contributes to reflectors reputations as poor 
performers, even when the mirrors are of good quality and everything 
seems collimated and should be working perfectly.
    Another plus to testing the centeredness of the secondary with a 
laser is that you can, as you raise and lower the scope in altitude, 
check for any sag of the secondary cage. I can report that with a 
lightweight spider ring and the monster 31mm Nagler, there is no sign of 
increased sagging  thru altitude using  9' long 1 1/4" aluminum tubes. 
Controlling vibration was another post.
    As for vignetting, would the effects really be neglible?  Or would 
the effects be more like using curved spider vanes, "diffraction 
spreaders" as they've been called?
   
   

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