For my 10" newt (still under construction), I am making a spider that only has two degrees of adjustment - rotation and tilt. I will mechanically position it well enough to not need the others. Also, for the tilt adjustment I am going to take a different approach from the typical ball pivot used (in Novaks, etc). I am going to machine the secondary mount so that a solid flexure is used (picture an I-beam, but with the vertical portion of the I being thin enough to easily bend). The mount is then flexed by two push/push screws (or you could use push/pull on the same side of the flexure).
Here is a poor ascii side view:
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I_________________I
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flexure
screw screw
> I am dismayed when I rotate commonly available focusers on the market only
> to discover that the 'aim' point on the diagonal circumscribes a circle,
> instead of staying steady. Imagine adding a rotating upper section, or an
> image rotator for CCD imaging on an altaz to that! What a nightmare! It
> was a lot of years before I stopped assuming (as many others must continue
> to do now) that attaching a focuser meant that the focuser was automatically
> aimed at the diagonal center. A lot of commercial scopes that I see don't
> have the focuser aimed properly. So, this is an area that I think needs
> more attention from discerning amateurs.
>
> One of my first homemade focusers for a homebuilt cold camera had tilt
> adjustments built into it. I could never get the film focused from side to
> side. I eventually gave up and did my best to build 'model II'
> perpendicular to start with. It worked much better. I also remember a 20"
> f/5, when big dobs just were starting to become popular, that had a
> homebuilt focuser so misaligned that stars on either side were obviously out
> of focus, even when the center of the field was ok (well, sort of ok). I
> had already seen that effect with my camera, so that was an easy diagnosis.
>
> That's one good thing about imaging (CCD, photography). It really points up
> the weaknesses in scope design.
>
> By the way, for you purists out there, try to calculate the droop in the
> laser collimation beam due to gravity between horizontal and vertical tube
> positions. You know, "Wow, your scope looks nicely collimated, but have you
> accounted for...?"
That is a neat attribute of using a laser - you can watch the beam/spot move around as you move or flex things. It can also be used to detect loose components by tapping on them and watching the effect on the spot. I once caught a mis-adjusted mirror sling on my dob this way (the mirror was resting on the two 'safety supports' instead of the sling, this resulted in collimation changing from low altitude to high altitude).
> Mel Bartels
>
Mind if I call you Bob? That would be less confusing... (or Bruce, for any Monty Python fans).
Bob L