Re: ATM Filling truss tubes

[Mel Bartels <mbartels@efn.org>]

Vibration in telescopes:

    The way I view it is that the telescope has a characteristic resonance and
the associated harmonics.  This overall resonance is the product of the
individual resonances and harmonics of the various telescope parts and how
tightly coupled these parts are too each other.

    All truss tubes will vibrate.  Whether this vibration is transmitted into
other parts of the telescope, and reflected back into the truss tubes, and
whether vibrations make it to the eyepiece all depends on many factors including
truss tube geometry and materials.

    Sometimes a tightly coupled metal joint will transmit worse vibrations than
a losely coupled joint with a leather or sorbothane insulator sheet.

    It is not necessary to fill the truss tubes unless it can be demonstrated
that the truss tube resonance is contributing to vibration at the eyepiece.
Filled truss tubes will vibrate, but at a much higher frequency than unfilled
tubes.  Also filled tubes will probably dampen faster.

    Historically truss tubes have been analyzed for deflection, but this is not
the complete story until resonances and harmonics are taken into account.

--
Clear skies, Mel Bartels
http://www.efn.org/~mbartels