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

["Tom Krajci" <tom_krajci@tularosa.net>]

>From: <atm@hudler.org>

>	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.

I didn't state it in my original post, but one of the goals I'd like to meet
is non-reliance on autoguiders.

I take time-series photometric data just about every clear night...even with
scattered clouds.  Clouds can make autoguiders go stupid.  If a have a good
drive, well-behaved flexure characteristics, good pointing correction
model...I would like to avoid use of autoguiders.

>	Attempting to model the flexure of a telescope and compensate for
>stiffness at all angles, is at best an intellectual or worse futile
>exercise.

Understood.  I am learning some things by doing this analysis.  And once
this is built I'll need to take empirical data to model pointing errors.

>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. :)

What comments do you have on my previous analysis of truss designs and fork
arms at:
http://overton2.tamu.edu/aset/krajci/fork-analysis/fork-analysis.htm
http://overton2.tamu.edu/aset/krajci/truss.htm
http://overton2.tamu.edu/aset/krajci/truss-analysis/truss-analysis.htm

>That's why we don't sweat the small stuff and built it as best we
can.

How do I build 'best' it if I can't look at/analyze a model of it first?
Does build it best mean I should simply double the thickness of all
components?

>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).

I intend to use Mel Bartels' software.
http://www.bbastrodesigns.com/cot/cot.html
http://www.bbastrodesigns.com/

Back around 1998/1999, with a home made, plywood dob, and home made molded
worm gears, using Mel's stepper software on a laptop...after
measuring/modeling my pointing errors...I was getting all-sky RMS pointing
errors of around 2 arc minutes.  When chasing faint galaxies I didn't have
to remove the high power eyepiece from the focuser...command a slew...the
galaxy was in the field of view.
http://overton2.tamu.edu/aset/krajci/molded-worm/molded-worm.htm
I hope that a metal mount/drive will give me better performance.  How much
better?  That's the $64 question.

Tom Krajci
Cloudcroft, New Mexico
http://overton2.tamu.edu/aset/krajci/



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