Re: [ATM] trilateral scope problem

[Dave Smith <smithersscope@yahoo.com>]

--- Mike Lockwood <melockwo@uiuc.edu> wrote:

> Jan van Gastel wrote:
> > The problem now, is that the scope doesn't hold
> collimation because of
> > bending. 

> My main bit of advice is to be sure that the
> collimation shift is due 
> to flexing tubes and not something else.  Lots of
> other things can 
> cause the drift, especially slop or flex in a mirror
> cell, slop at the 
> pole attachment points, or a flexing secondary cage.
>  (I'm sure you're 
> familiar with those problems, but I want to point
> them out to others.)

I'd like to make one addition to Mike's excellent
points. In my first iteration of the 16" tridob I
discovered a similar situation as you. The largest
contributor turned out to be the mirror box wall
flexing. How stiff is your mirror box front wall?

On another note, your "front facing" tubes (based on
the image in the link) will be under tension as the
scope moves to the horizon while the back trusses will
be under compression (if I'm not mistaken). I noticed
that your "triangle" on the front face is weakened by
a large gap between the truss clamps that straddle the
center fin. Is there a mechanical reason for the large
gap? Does the gap contribute to flexure of the tubes?
IIRC, Mel runs 0.75" thin wall tubes on his 20"
without issues. If the trusses are designed and
executed well, your tube diameter should be fine.
Makes me think the flexure is elsewhere.
Keep us posted on the issue...I'm very interested in
your results. (I just fired up the tablesaw this week
to begin my rebuild and motorization. I'll be driving
altitude tangentially from the centerfin. Should be
interesting--might be a disaster! ;o)   )

Dave

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