Quote:
Originally Posted by Peter Ward
Leon, for sure there often very good reasons why you need to go down that path.
When you run the math, the bending moment of a 1/2" bolt compared to a 6 to 10" pier is not insignificant, and the frequency of the system is set by it's weakest link (comparatively thin bolt), hence my tip  |
Peter makes a valid point of course. But to understand all the effects introduced by suspending the pier on levelling bolts, you've also got to consider (among many other things) the distance between the pad and the pier and the distribution diameter of the bolts. While contructing my new obs, I pondered this point. I wondered whether I should:
a) plonk the pier on the pad (no levelling but using a suitable filler material to properly couple the two),
b) suspend it on the bolts leaving an air gap (allows for re-levelling)
c) suspend on bolts but fill the gap to couple the pier to the pad (initial levelling only).
Here was my thought process:- I'm trying to make sure the resonant frequency of the system (if there is one) is high and damping is maximised
- I have a 10" diam pier that would be suspended on three 3/4" (= pretty thick) stainless bolts on a 1" thick aluminium flange approximately 20mm above a 670mm tall concrete column 400mm in diameter. The distribution diameter of the bolts is 325mm. I.e. a pretty chunky setup for my equipment. The rigidity and resonant frequency will, almost by definition, be high for all three options given these parameters.
- I have long since forgotten how to calculate resonant frequencies and bending moments for such a system let alone the contribution of the bolts!!!
Anyway, I'm an empirical kind of guy! Maybe Peter can do the maths for me!!
- I want to be able to level. If I adjust azimuth, I don't want to affect altitude & vice versa. Remember, even though it's a permanent setup, tweaking of polar alignment is a requirement over time (precession and maybe minor pier settling). Anyway, I'm a perpendicular kind of guy.
- It's unlikely anyone will be jack-hammering anywhere near me while I'm imaging
- There's a couple of hundred meters between me and the nearest road (with very light traffic)
- I'm in a domed enclosure so wind is not much of a problem. And, more importantly, if the wind is strong enough to affect me, I’ll be inside sipping (polite talk for guzzling) wine.
- I know I can couple the pier to the pad later if I really need to (but if I do, re-levelling will be a bugger of a job).
So, my conclusion was simple - stop pondering and suspend the b*&#$y thing on the levelling bolts and see if it works!! 
My only tip if you go this way and you have a choice? Use no more than 3 heavier gauge bolts. The more bolts, the harder it is to level.
Result? So far so good. Simple observation through the eyepiece after a kick-the-pier test shows very fast damping and the system doesn't appear to ring. I'd describe it as rock solid. The acid test is when I start imaging again - if the clouds ever go away! Realistically though, I expect absolutely no problem.
Cheers, Marcus