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

[Polaraligned <polaraligned@optonline.net>]

----- Original Message ----- 
From: "Tom Krajci" <tom_krajci@tularosa.net>



>I plan to change my German equatorial mount to a fork.  It will be used for
> time-series photometry runs that can last all night.  Accurate pointing 
> and
> polar alignment are highly desirable.  I want the scope/mount to flex in
> small, but predictable ways so that I do not need a complex software
> solution to improve pointing.

Louis Boyd (Fairborn Observatory), his scopes hold a couple of
arcseconds for integration times of typically 30 sec with no guiding.
Centering is done to the arcsecond..  This would make for crummy
photos, but ok for photometry.

Probably going to need T-point or similar software to get arc
second pointing.   It is relatively inexpensive though.
Take a look at what Observatory Scope uses for their drive
setup:  http://www.observatoryscope.com/electronics/electronics.html
This is probably similar to what I am going to use.





>
> Design constraints:
> - Steel is the material of choice
> - Square tubing, ? inch wall thickness will be used.  Available
> sizes/choices are 2, 2 ?, and 3 inches wide.
> - If the crossbar needs to be made from a thicker section of square tubing
> than the fork arms, that is not a problem
> - If crossbar length, or fork arm length need to be changed slightly to
> achieve the desired stiffness equality at various orientations, that is
> acceptable
> - Fork length is 22 inches (from crossbar axis to declination axis)
> - Fork width is 18 inches (from one fork arm axis to the fork arm other
> axis)
> - Telescope/load will be 50 pounds


Whatever you calculate, I would go with a bigger size.  I prefer to
be overdesigned than have a large time/money investment and
be disappointed.
How about fork arms with a rectangular profile?  I think they will
be overall stronger considering the primary area of the sky in which you
will be collecting photons.  Larger arms allow you to use lighter tubing.

How about drawing up arms in a cad program and having sheet steel
CNC cut out to the shape.  Their is a guy on E-bay who CNC cuts steel
at a per sq in cost after you e-mail  him a DXF file.  You need 2 arms cut
and sheet welded between them to hold them seperated a specific distance.
This creates the "box" of the arms.  You could get a profile this way that 
is
stronger than just welding box shapes together.  Just an idea.
I would be more than happy to do the welding/machining if you are ever
in the NJ area.  Or you can send me a part.
What OTA are you going to use on this mount?


Scott








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