Re: [ATM] Designing a simple fork with desirableflexurecharacteristics

[<atm@hudler.org>]

Tom,

Your guide tolerances are dictated by the instrument, what is field of view
for the photometer to operate in?

About your previous fork design; it is a good start however; you're only
modeling one aspect of a more complicated fork. Statically a fork is dual
cantilever that's grounded in the middle. Try modeling a fork as a unit and
see what happens in the fork base. At the meridian the downward moment
translate into twisting at the base, and at other angles it can be a
combination of twisting and bending.

I don't have any experience with grape (I use Autodesk Inventor 10 with
ANSYS FEA) so I don't know how well it can model the entire fork with a
simulated OTA load. Same for Mel Bartels software, I have written my own
based on various professional scopes. It runs on Microsoft Windows
2K/XP/2K3/Vista and uses TPOINT pro for mount corrections. 

After some more thinking about this setup, I would 86 the equatorial and go
alt/az. This design virtually eliminates all the twisting and drooping due
to the variations pointing angles. If the only purpose of this scope is
photometry then field rotation isn't an issue.

Another advantage to alt/az is the bearing requirements are reduced because
you don't have to handle thrust from tensioning the fork. Tension is another
weird source of errors in an equatorial, as forces translating through the
forks into the OTA mount and base can look a little chaotic at various
orientations. 

Personally, I'll never build another equatorial mount again. Actually my
next one's going to be on a hexapod. 

How good is good enough, 1/4 of the photometer's field of view, for all sky
pointing is a good start.

Jack


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