I am in the initial stages of planning a permanent pier for my 8" Meade LX-5 - which will be followed by a permanent observatory around it as/when finances allow.
My plans were originally to pour a large concrete base, onto which I'd bolt a steel pier which is as yet to be fabricated. But I've now started to wonder if it might be easier simply to pour the entire pier from concrete?
I would appreciate the opinions of the experts amongst you - which way should I go? Is there anything intrinsically better about a steel pier over a concrete one, or vice versa?
I should mention that I live about 25km from the nearest city lights on a small block of land (so have a pretty dark sky), and we're not planning on any moves from here for at least the next 15-20 years.
I did the calcs some time ago (here on IIS somewhere...) but the basic results are that Young's modulus for concrete is about 10x less than steel. This mean that for a given diameter steel is 10x stiffer, which means higher vibration frequency and lower amplitude. Packing a hollow steel pier with sand will help to dampen vibrations, but will add mass and lower the vibration frequency.
Having said all that, honestly, I doubt it matters much. Most piers are so over engineered that the differences are simply too small to detect (which is what we want). If you want to design a pier that is the lightest possible that will give good performance, then steel is the way to go, otherwise... it won't matter. Whatever you prefer is the correct answer.
Thanks for the comments guys. Concrete sounds like the way to go - certainly easier and cheaper to build than the steel option. And if I can get equal performance from a properly constructed concrete pier, that's the way I'll go.
This is very high pier... in first post
There was similar pier on Mt Stromlo, for their 12" (I think) Celestron.. The pier itself was really high, like 3-4 metres if not higher (because the telescope was above restaurant in the visitor's area).
If you are going to use concrete, it's worth the time to read this first
James
Thanks for the link James - have read that all the way from start to finish. What a saga! I'm glad that my pier will not need to be anywhere near the height of that one . But it did provide some insights into possible pitfalls of a concrete pier which I'll keep in mind as I develop my plans.
Come in Spinna
I went with 8" steel buried 4ft in 3mt of concrete
with the original 8" LX5
now it rides a GPS12 with no problems
Looks good mate - thanks for posting the pic. Oh to be able to afford a GPS12 ;-) Definitely on my wishlist, but someway off unfortunately. But whatever I build will be enough to withstand a much bigger 'scope.
Dont for get to sink long bolts in for attaching the mount, and a PVC tube poking out at a right angle at the top and bottom under the floor for cables-through-the-pier, neat and nothing to trip on ;-)
Another excellent option is 100-150mm diameter cast iron - heavy, but its crystalline structure has excellent dampening properties. These can be had as recycled material and often have fitted flanges for attaching to the mount base or a concrete foundation. The old Meade users website forum used to have some technical excellent analyses of the pros and cons of differnt permanent pier materials.
I have an old cast iron stage light base adapted for a GEM base - it is an outstanding low vibration base. Almost as good as the large concrete and steel pier in my observatory.
guy
If you are going to use concrete, it's worth the time to read this first
James
That bloke needs a star named after him if only for his doggedness.
I was sick of that pier just reading about the dramas.
I must admit tho, I dont understand why they didnt just add water down the tube and get the vibrator busy once they realized that the mix was too dry. I also dont know how you can pour THAT many mixes down the tube and not look at the state of the mix before doing so. Maybe Ive missed something.
Anyway, these questions are not what the threads about, thanks for including the link, it was an interesting read
With a steel pier it is all about wall thickness. One of my piers has a wall thickness of 13mm and as a result it is extremely rigid. If you knock it there is no vibration whatsoever visible in the scope. It does not ring at all. It is bolted to a 2.1 ton block of concrete. My preference is for steel over concrete.
Pouring concrete in a tube can be difficult. Remember to tap the side of the tube with a rubber mallet lots while pouring the concrete if you go down that path. This will remove air bubbles. Better still hire a vibrator to vibrate the concrete. It will give you the best chance of doing a good job with concrete.
I suppose it depends a bit on what you intend mounting on the pier along with the length of said pier.
Prior to building the pier for my observatory I was fortunate to be visiting Coonabarabran when the Sky Watch observatory was in its construction phase. The dome was on the ground and the pier was in place. The pier was steel pipe approx 8" dia and was filled with sand. The concrete floor had not been poured and there was a lot of rubble still laying around from the earth works. I picked up a rock and tapped the pier and I was quite surprised at how much it resonated.
Then and there I decided that my pier, which was of similar dia and length, would be filled with concrete.
My pier is 8" steel pipe with a 8" pipe flange welded on top. It protrudes 2.3 m above the concrete ground floor of my observatory. It is set in its own separate plug of concrete that is 580mm x 580mm and is 1 metre deep. This plug of concrete is separated from the concrete pour of the ground floor by 25mm thick strips of polystyrene. I have 6 lengths of twisted reo running the full length of the pier embedded within the concrete. I had no trouble filling the pier with concrete, obviously due to the dia of the open end.
My pier does not resonate at all, you could hit it with a sledge hammer and it would not resonate.
I do a considerable amount of planetary imaging at very high power, I use a manual filter wheel which requires me to physically rotate each filter into position, there is absolutely no delay to wait for the image to stabilize. As quickly as I can rotate the wheel and sit back down at my laptop I can start the next capture.
The load on my pier is considerable, a 16" F4.5 Newt on a very heavy GEM, I think it is about 150kg with scope and mount.
I have attached an image of the pier from the ground floor, you can see the polystyrene strips that isolate the pier's plug of concrete from the main floor.
My observatory, has been in its current configuration for 15 yrs and I have never had a problem with vibration ever.
get two 8" pipes one with a wall thickness of 13mm and one with a wall thickness of 4mm. The 4mm one will be no good for imaging. as it will vibrate. Of course it is the wall thickness. Just do the calcs and you will see what is more vital.
Just do the calcs and you will see what is more vital.
I have already - that's why I attached the link that contains the calculations that clearly show diameter is more important that thickness. Sure it can't be paper thin, but if you are paying per KG of steel, go for more diameter if you can.
I can tell you that an 8" pier at 1300 high takes two burly men (100-125kg each) to slide and move the pier with a wall thickness of 13mm. In fact it weighs in at 150kg. A 12 inch pipe with 7mm is going to be a bit heavier still and I cannot see the purpose of doing this. The bottom line is wall thickness will play more of a role once you are over 7" in diameter. Deflection aside the pipe will resonate more with a thinner wall thickness.
). If you want to design a pier that is the lightest possible that will give good performance, then steel is the way to go, otherwise... it won't matter
. Whatever you prefer is the correct answer.
. But it did provide some insights into possible pitfalls of a concrete pier which I'll keep in mind as I develop my plans.
