size of pier post

[bmitchell82 (Brendan)]

Can anybody advise on the best size for a steel pier post? im guessing somewhere between 150 200 mm with 3mm walls.

[Paul Haese]

200mm ID and 12mm walls. 1500mm high. With that you are sure to have a very rigid pier.

[Dennis]

And don’t forget suitably sized gussets, as these appear to reduce vibration from what I have read.

Cheers

Dennis

[CoolhandJo (Paul)]

The base and top plates need to be at least 10mm if not 20mm thick - depending on the size of the scope/mount. attached picture of mine.

[rally]

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

[bmitchell82 (Brendan)]

Thanks for all the replys!

Well As cost and construction isn't going to be too much of a issue (thanks protyping lab). Im running my 10" which is at f4.7 at most i will barlow this setup which will run it to f9.4 with a focal length of 2.6m. The pier will be enclosed by the ROR that im about to construct, (see my other post green light for ROR obs) the walls of the obs will be approximately 1.5m off the ground.

Rally, I am aware of the leaver, but when using a Canon 40d, i do not belive that a micron difference is going to affect such a scope maby 30 or 40 micron might have a slight effect but with the resolution captured by the 40d its not SBIG quality and the SBIG is a long way away!

As to dampen the internal external vibrations sand will be added and compacted within the pier.

CoolhandJo, The idea of just using a square tube is most appeling as it is easy to obtain a sizeable piece at a resonable cost. what are the dimensions of your post?

[Paul Haese]

Rally, I got that suggestion from you mate, my pier is so solid it does not ring.

The pier is really important Brendan, go as big as you can afford, this is almost as important as the mount; it will be there for years to come and if you ever upgrade to a bigger mount you will be kicking yourself if you have to rip out the old pier. Go with what we suggested, it is worth the doe. You have seen my Clayton images and that is primarily because the mount and pier are very solid.

Sand I found to be a complete waste of time. I used foundary sand in my Blackwood pier and while it stopped the ringing it has little if any effect on the vibration.

[bmitchell82 (Brendan)]

mmm,

The only thing is that i will most likely use the pier for 3 - 4 years max (and im in the middle of perth so it will be narrow band imgaing). the pier will be at floor height though i will be cutting the concrete and digging a 600 by 400 x 400 hole to drop the foundations into which will be seperate from the shed floor.

so as for size like paul suggests if i had a 200mm Square tube with say 10-12mm walls this should be 100% for what i need. Wind should not be a factor here as the obs will protect it a reasonable amount and if its blowing its A55 out your not going to get good images anyhow. Im expecting the pier height to be approximately 1.2m tall and I know my tube/accessories/guidescope is approximately 26kgs + the mount head (eq6)

I know i wont be upgrading my telescope for now because of the fact im a university student and i purchased the bulk of the equipment when i was working fulltime... plus the minister of war and finance wouldn't approve such purchases without me having a full time job again and that will occure when i move out of the house i am in at the moment hence ill build a brand new obs with all the fandangled bits!

[CoolhandJo (Paul)]

Brendan,
My pier has 12mm thick walls, about 1.1m high, 200 x 200, and base plates at 20mm. Very heavy! I had it made, but you can get an idea of pre-fab design from bintel's site under "mounts".

I got extra height by building an above ground footing at about 800mm high and set bolts into the blockwork. Simply sat the pier on it.