Re: [ATM] Designing a simple fork with desirable flexurecharacteristics

[<atm@hudler.org>]

Forgive me if this seems a little blunt here. 

	Having built scopes weighing 1000's of pounds, I think you're overly
complicating this. While it's very good that you're using FEA to determine
flexure of various materials; don't forget about those miles of atmosphere
you have to sample through (see Franks quote below).
	It would be far quicker and fewer gray hairs to add a guide scope
and stick an autoguider on it. 
	Attempting to model the flexure of a telescope and compensate for
stiffness at all angles, is at best an intellectual or worse futile
exercise. 

Once you build this mount you will discover things such as; 
	+ There is no such thing as linear flexure in a telescope.
	+ Welds on one joint don't have the same penetration; thus
asymmetry.
	+ Welds of different filler. (TIG everything)
	+ Steel plate or tubing from different lots, forges, or companies.
	+ Bearings act differently at different force moments, temperatures.
	+ CTE, CTE, CTE, CTE... did I mention CTE issues? Everything is CTE.
	+ What about the OTA.
	+ Refraction is a bitch!
	+ There's no such thing as a nearly perfect scope. :)

	That's why we don't sweat the small stuff and built it as best we
can. Then correct the rest in software. I don't know what Patrick's license
price is but if you want to get even close to nirvana (without autoguider);
get the pro version of TPOINT or roll your own polynomial surface fitter (I
prefer the former, there's better support :-), and been there done that on
the latter).

	By all means don't stop what you are doing, I think in the end of
your journey you will have learned a great deal and reluctantly decide that
almost unanswerable question; how good is "good enough".
	Either that or you've bankrupted yourself buying professional FEA
software :).

Jack



To liberally quote Frank;

How good is "good enough"?

Even the "best" telescopes will need some form of guiding or correcting the
telescope tracking using optical feedback from a star. If the telescope
tracks perfectly, tracking corrections will still be needed to correct for
small refraction effects (seeing for example).


PS:
Take a 4 foot piece of square tubing of any size or quality, clamp it firmly
at one end and place the other end over table (preferably granite flat).
Set up 2 dial indicators (.0001) measuring on the bottom left side and
bottom right (bottom or top doesn't matter).
Take a cigarette lighter and heat any side of tube. Watch the indicators.
Try heating it at various positions. 
Now move the indicators to the vertical axis.
If I'm lucky you'll find the results interesting.


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