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Brendan,
It depends on what you want to do ?
For Visual - almost irrelevant
For Planetary imaging with a webcam - almost irrelevant
For Deep Sky imaging - it needs to be very rigid
Portable or Permanent ?
Is your pier going to be protected from the wind ?
How high does it need to be from the ground
What is the focal length or more importantly the image scale
Think of it as a big lever, anything acting on the top of it like wind, or any sideways load (shifting weight) etc will cause the pier to move by microns.
Typical notional sideways load for design is deemed to be around 2-3kgs
The higher it is (the bigger the lever) the more rigid it needs to be.
You are trying to ensure the pier doesn't move more than a pixel's worth in the final image !
You are also trying to ensure that the pier doesn't suffer from vibration and resonances - not at audible levels - the amplitude is too low to affect anything much, but at very low frequencies.
The way around that is to make the pier very strong and much more massive than ordinary structural engineering design would call for.
So the bigger the diameter the better !
Wall thickness is important but not as important as diameter, but 3mm is bit thin IMO.
That means as big in diameter and wall thickness as you can afford or manhandle - I would regard a good pier as one where it takes two men to manhandle it off the trailer into position !
Paul's suggestion is on the money - or alternatively use solid concrete - its not as rigid as steel but you can make it solid, but it may be cheaper or easier for you to manage or to construct.
The footing is important too.
If the pier height is quite high because the Obs is raised then you can make a larger pier base of concrete and bolt a short steel pier to that rather than make a very large, very heavy steel pier.
Rally
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