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Brendan,
I appreciate your concerns about people overspending/overengineering a pier, but in my opinion a good approach would be to reinforce that the rigidity of the pier in a high end astrophotography setup or tracking rig where pointing accuracy (especially at long focal length with a small but sensitive camera) is more important than you dismiss.
For Visual it just doesnt matter at all and much the same for Planetary where best frame selection and high shutter speeds will also make it much less important.
Large astro cameras are often the biggest balance culprits where no matter what you do there will always be an out of balance or substantial change in balance situation at some point in the sky or camera rotation angle.
Its not the strength of the pier that is important its the rigidity.
Problem is, to get rigidity the cheapest and easiest way you get excessive strength as a byproduct.
If in fact that is the sort of requirement that you have to start with ?
Since many dont need it and may never need it, it is therefore not essential but its a 'horses for courses' requirement.
If the mount is not up to the grade then the mount is going to be the weakest link, but if the mount is good with PEC, Tpoint modelling and say Protrack or similar in operation then the pier and other fixtures and adapters will start to become the next weakest link producing noise and errors.
If one is never going above say an EQ6 for example then one could argue why bother and I dont disagree, but the cost difference is only pennies and you never know what you might decide to do in the future.
To a degree Tpoint can help model this out - but Tpoint does not understand or recognise camera rotation in any way whatsoever, so it cannot correct for this type of changing deflection.
I just did the calcs for a 200mm OD x 900mm concrete pier (unreinforced) and the deflection was 1.2 arc secs compared to just under 1 arc secs for the steel one.
But I cant imagine anyone doing one without at least some reo inside (reo is dirt cheap) !
I am also guessing that 32mpa concrete used in your example isnt something the average Joe is going to do at home with a wheelbarrow and shovel !
But certainly concrete is a very viable option but I am still not sure that its as cheap as salvaged steel pipe (after formwork, reo, mount adapter bolts, conduit for cable etc) and I would think much more work, although labour is free and concrete is still needed for the footing in any case, but on a different day !
I bought some pipe with 12.7mm wall thickness around 320mm OD (12.75"= 12" nominal bore) and it cost $100 per metre (smaller diameter with lesser wall thickness was much cheaper)
As far as steel pipe availability goes, either recycled gas pipe or structural salvage pipe is readily available in most capital cities in wall thicknesses from 4mm up to 16+mm and diameters from 5" to 14" nominal bore.
A large number of piers used within wooden floored observatories are longer than 900mm so wall thickness becomes more important as the height increases - ie deflection is proportional to height squared
I have calculated a few and some that I have the figures for were 1220, 1900mm and 2700mm !
Cheers
Rally
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