Brendan,
I just calculated the deflection of a 50kg load on a round 200mm dia 6mm wall thickness steel pipe (not high tensile or Square section)
A 50kg lateral load deflects the pier 11.5 arc secs
Whereas a 3.5kg load (more typical of the sorts of loads for cable drag plus eccentric loads due to camera rotation and out of balance due meridian flips and maybe a bit of light breeze etc) deflects just under 1 arc sec which is more in the realm of where you want to be
Preferably well below your image scale so its lost in the other noise of your system.
Some of the deflection is dynamic - eg breeze, knocks, vibration of fans and motors etc some is more static in nature - eg a complex rig with guide scope and camera with filter wheel can be difficult if not impossible to balance perfectly on both sides of the meridian especially with the rotation of say a 8 x 50mm filter wheel and camera and an OAG etc
So stability will affect pointing and it will affect the quality of the image - small vibrations and movement simply translate into blurred detail.
I think the trick is to cost effectively ensure that the pier becomes the least cause of unwanted noise in your system - so bigger (using that philosophy) is better, especially since scrap steel pipe only costs $100/m or less at the local junk yard
Its very cheap insurance !
To the OPs orginal thread - Steel is usually easier to work with and modify and can be potentially cheaper than concrete.
But you need to be able to weld it !
Solid concrete for the same OD is going to be more rigid, but there isnt much in it.
Rally
Quote:
Originally Posted by bmitchell82
. . .
Just out of pure interest a little while back i did do some calculations on a 200x5 SHS at approximately 900mm in height it required 50kgs of horizontal force at the top of the section to produce approximately 5 micron of movement. Now i don't know about you but that is a bucket load of force for something that tracks with the earth and isn't flying around all over the shop. You are more likely to get larger errors within your mount.
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