Re: [ATM] Compensating sag of the truss frame by flexibility in themirror cell

["Robert Houdart" <robert.houdart@cruxis.be>]

Hi Ross,

> I do like your idea very much, but I don't think most
> of us regularly collimate our Newtonians to an axial
> error within 0.3 mm (am I wrong about that?).  Would
> there be much point in compensating for a sag much
> smaller than the likely collimation error?

I agree that the error is quite small (probably below practical
importance), but it's more the concept I find interesting.

Note that if I would turn the mirror cell upside down, the two effects
would be added and the total error would be about 0.6 mm.
Taking into account uncertainties in material properties and dimensional
tolerances, the real error might very well be close to a mm.

It's better to be aware of the two effects, and try to make sure they
compensate each other. 

> Instead of fiddling with the cell structure itself,
> could you get the same effect by controlling the
> springiness of the sling (for those who still use
> them) or the +/- 45 degree edge supports?  That way,
> instead of tilting the mirror you let it sag parallel
> to the UTA (as in the original Serrurier
> double-truss).

That's possible too, but then the mirror would move with respect to its
support pads, which will create friction and unbalance in the supporting
triangles. Something to avoid!

Kind regards,
Robert

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