Pier advice

[Barfbagger]

I have a large Newtonian with a NEQ6 tripod mount and am 99% certain of building an observatory/shed this summer. I would like to install a pier of some kind and would be grateful for advice of what sort and what type of fixing/top-plate to install bearing in mind that I might want to upgrade to something bigger like a large catadioptric should my investments pay off.

[DOBBY]

check out the 'home-made obsy' posts on the forum.. best advice is... however deep ya hole is for the pier....make it deeper it needs to be rock-solid & not in contact with the shed / obsy floor

[SteveA]

I opted for the concrete filled tube and a pier head adapter on top.

I went for this option principly as I assumed it would be cheaper ... which of course it could have been! But with most things I do I had severe "scope creep" and I ended up spending almost as much as buying a full pier solution.

I used a pier adapter and pier top from Epsilon Telescopes for mounting an EQ6-Pro (not quite sure how much I paid now)

Epsilon Telescopes

A 2 m stainless steel tube (sourced from ebay, at about £180)

Including the concrete, threaded bolts etc... I must have spent close to £400....

If I were doing it again...I would buy a solution like that from Altair astro;

Altair Skyshed Observatory Pier 8" OD :: Telescope Mount Accessories :: Mounts & Tripods :: Altair Astro

You still have to do some hole digging and concrete work...but at the end of the day its not a lot more expensive and when you move house....you take it with you..

[Bizibilder]

Get your local "Steel Fabricator" to make a bespoke pier for you. It will almost certainly be cheaper than the commercial ones! Most of these folk have offcuts of pipe and flat sheet and will happily make one if you explain what it is for - much more interesting than another fire escape!

[ollypenrice]

I think the 'ten tons of concrete buried eight metres into the ground' is a bit of widely accepted and oft-repeated nonsense, I'm afraid. Yves and I stand a 14 inch ODK on a proprietory pier standing on a few inches of concrete on which the observatory also stands. Nobody has looked at our pictures and said, 'Pity you didn't have a better pier.' It is one of those things that has taken on an internet life of its own. Build a good pier, yes, but tons of concrete and metres of depth? Forget it. Also, don't get hung up on leveling the mount. Lots of people build massive piers which end in flimsy levelling adjusters and the mount doesn't have to be even slightly level to be perfectly polar aligned! The Universe has no interest in where your tripod legs are pointing.

If you are starting out on an observatory build I would give serious thought to buying a hobby MIG welder. I bought one eight years ago and have built three full roll offs with it, plus a rolling roof, an assortement of piers, observing chairs, computer tables...etc etc. Worth every penny, and you can learn to use them in an hour.

Olly

[laser_jock99]

. Also, don't get hung up on leveling the mount. Lots of people build massive piers which end in flimsy levelling adjusters and the mount doesn't have to be even slightly level to be perfectly polar aligned! The Universe has no interest in where your tripod legs are pointing.

Olly

Well said- many an otherwise solid pier & mount is comprimised by ending in three flimsy 'leveling' bolts which are not even needed anyway! All the adjustment required for polar alighnment is already built into the mount. t's better to have a more solid connection between pier and mount.

[Rustysplit]

Olly, Have you got any pics of your piers? What sort of depth of concrete are we talking?

Contemplating a permanent pier in the garden, but I think a huge cube of concrete is not gonna go down to well with my lovely wife, or me if I'm honest.

[laser_jock99]

You could make yor pier semi-permanent by just concreting in some mounting bolts. Then get your local engineering/welding firm to fab up a flanged pier. That way you could at least just un-bolt the pier if you move house or your interests change in the future?

[ollypenrice]

Hi,

I don't have a picture f the Mesu 200's feet but they are conventional threaded devices ending in rubber circular pads. They just stand on a concrete floor which is, on average, about 6 inches thick with a lot of hard core under it. Although I was going to bolt tie-downs to the floor to hold the beast in place the whole thing is so massive that we haven't yet got round to it. It really isn't about to go anywhere. Here's Yves next to the setup.

I do have my piers bolted down because they are lighter and have more human traffic around them. Here's the top of the pier for my Takahashi mount.

It has tripod-like supports coming out from below there because I cut it down from an earlier, very high, pier for a large visual refractor. The feet bolt to the ground using treaded bar epoxied into the concrete. It works fine.

There's nothing wrong with monster piers, I just don't believe, from considerable personal experience, that they are at all necessary. You see them on the net made from two 40 gallon drums welded together and filled with concrete before being buried. I think this is crazy unless you are putting a truly enormous scope on a truly enormous mount. People come here with free standing AP900s, plonk them on the ground and take great pictues with them at 2.5 metres of FL. I just think the whole thing has lost touch with reality in some cases.

Olly

[rfdesigner]

As I'm going down the build a pier route I've found a few things out.

Firstly this will be my second pier, my first one was all concrete and was low and solid.

However that was built 12 years ago and was destroyed when we moved out about 6 years ago. Since then I've been pierless.

Observations:

1st.. concrete piers should be poured in one. My one wasn't and I was VERY lucky, when I came to demolish it the pier just fell away from the foundations as they'd been poured seperately. Thankfully it didn't do that with the scope on it.

2nd Ideally you want a light pier. Think weight on a spring, heavy weights sway slowly, light weights sway quickly, and quick here means the scope is held firm at low frequencies (where you or your camera might notice movement) So if you go for a hollow metal pier, don't fill it up to the brim with concrete.

3rd Stiffness is your friend. Steel is about 3 orders of magnitude stiffer than concrete.

4th a thin hollow steel tube appears to be the best that can be done, on a bang/buck ratio.

5th Get it as high as you can, every meter helps not only the view but also reduces turbulence just a bit.. over half the seeing can be due to turbulence and thermals at ground level, and turbulence seems to get worse the nearer the ground you are.

see: astronomical seeing, part 1: the nature of turbulence

notice the plot about 1/3rd of the way down, note the bottom scale is logarithmic.

(looking at the list above it's all pointing at Ollys pier being bang on.. no suprises there then)

I'm looking at a 3m long 10.75" diameter thinnest standard gauge galvanised steel tube with a single end plate, no levelling required. My scope + mount + weights is about 10 stone. This will be set 1 meter down into a nice big concrete plug all poured in one. Probably over engineered but the scope is no lightweight.

Derek

[ollypenrice]

Good point about the ground seeing. In Namibia (Tivoli astronomy farm) they have giant tripods for the observatories to raise them above the low turbulence. I'm just not in that market even if I were inclined to have a sort of War of the Worlds theme park outside my house! The importance of this will be closely related to your focal length, the longer the FL the more you need good seeing, of course. I really have no idea what effect it would have to raise Yves' big scope a couple of metres but I can't see myself finding out either, really!

Olly

[samtheeagle]

Excellent thread! I'm hoping to be able to build myself an observatory this summer, finances permitting, and the first consideration is the pier. It's very interesting to hear how people with loads of experience suggest that gargantuan pier foundations aren't the be all and end all... I was quite worried about aspect, and it seems to have been over nothing Also very interesting to read about the levelling aspects with regard to polar alignment, which I was already aware of, but for whatever reason I hadn't joined the dots up to piers and the adjuster plates a lot of them have atop of them.

Lots of good things to ponder!

[laser_jock99]

Deep foundations and a tonne of concrete seem to come from observatory builds in the USA. The reasoning being they have much harsher winters over there where the ground is liable freeze and cause frost heave

Frost heaving - Wikipedia, the free encyclopedia

Frost heave could theoretically cause your pier to shift if the foundations were shallow. However- it's unlikey to occur in lowland UK sites. Maybe in the Scottland and upland areas constructors should take acount of this.

Frost heave occurs ocasionally near my location in upland Wales. The peat bog at Bugeilyn Lake freezes over and churns up neolithic flint tools from the depths of the bog if condtions are right. That said we have had two very cold winters in succesion where I recorded temperatures as low as -22 deg C up here.

So- if you built an observatory up here, in peat bog, you would need deep foundations!

I keep looking but have never found any flint arrowheads here- other people seem to pick them up though!

[samtheeagle]

"When I first came here, this was all swamp. Everyone said I was daft to build a castle on a swamp, but I built in all the same, just to show them. It sank into the swamp. So I built a second one. That sank into the swamp. So I built a third. That burned down, fell over, then sank into the swamp. But the fourth one stayed up."

[pete_l]

2nd Ideally you want a light pier. Think weight on a spring, heavy weights sway slowly, light weights sway quickly, and quick here means the scope is held firm at low frequencies (where you or your camera might notice movement) So if you go for a hollow metal pier, don't fill it up to the brim with concrete.

3rd Stiffness is your friend. Steel is about 3 orders of magnitude stiffer than concrete.

4th a thin hollow steel tube appears to be the best that can be done, on a bang/buck ratio.

5th Get it as high as you can, every meter helps not only the view but also reduces turbulence just a bit.. over half the seeing can be due to turbulence and thermals at ground level, and turbulence seems to get worse the nearer the ground you are.

Some interesting thoughts there, Derek. Though I do have some questions if you don't mind.

So far as a light pier goes, ISTM it's desirable to have the energy from any vibrations absorbed/dissipated as quickly as possible. Won't a light pier take longer to damp out those vibrations than a heavy one? For the ultimate in damping, maybe filling your hollow tube with a viscous liquid is the best solution? (I don't know the answer - I'm just asking).

You mention using a thin, hollow tube. I'm guessing that means thin walled?

Generally the stiffness of a tube increases (dramatically) with the tube's diameter. So a 20cm OD tube is nearly 4 times stiffer than a 15cm one. But increasing the thickness of the tube walls only increases the stiffness linearly, so a 4-times stiffer 15cm tube would need walls 4 times thincker (and therefore 4 times heavier).

What is the tradeoff between pier height, seeing and vibration? I hadn't thought of raising a pier to avoid ground-effect seeing, but I have been advised to keep piers low to reduce vibration, as the higher they are (with the weight at the top) the more prone they are to sway, or catch the wind. Now you've sewn the seeds of doubt .

[perfrej]

Level pier? Yes and no.

Olly, absolutely right that the universe doesn't care which way your pier is pointing, but there is one incentive for making the pier level. Many simple alignment techniques can be made very reliable if the mont is level. Take the NEQ6 and EQMOD. You center Polaris (yes, center, not in the little circle) and then move alt bolt only to bring her down to the six 'clock position in the large circle. That way you can easily align your polar alignment ring.

Another example of level pier requirements is my new 10Micron. After a three star align it can tell you how many turns to do on alt bolt and ra bolt respectively. Or, it can slew to a star and have you center it with the bolts. The first method defininitely doesn't work if the pier is off, while the second may work- have to experiment on that one

/per

[Spikey]

This is all valuable information ! I'm considering building a small observatory in the garden and will be wanting some form of pier.

[samtheeagle]

So are there any engineers in the house? From a structural point of view, is a cylinder better than box section for this application? The old man has managed to rustle me up a couple of potential bits of pier material, one is a 4" round tube, and the other is a 5" box section.

Given the choice, which would you choose?

Also, given the above discussion on the merits of having a perfectly level mount, do I aim to try and build it level in one lump, or add the adjustment plate on the top? But how much weaker does the adjustment plate make the whole thing? By quite a lot I would have thought, 4x bolts vs solid wall of steel...

[rfdesigner]

Some interesting thoughts there, Derek. Though I do have some questions if you don't mind.

So far as a light pier goes, ISTM it's desirable to have the energy from any vibrations absorbed/dissipated as quickly as possible. Won't a light pier take longer to damp out those vibrations than a heavy one? For the ultimate in damping, maybe filling your hollow tube with a viscous liquid is the best solution? (I don't know the answer - I'm just asking).

You mention using a thin, hollow tube. I'm guessing that means thin walled?

Generally the stiffness of a tube increases (dramatically) with the tube's diameter. So a 20cm OD tube is nearly 4 times stiffer than a 15cm one. But increasing the thickness of the tube walls only increases the stiffness linearly, so a 4-times stiffer 15cm tube would need walls 4 times thincker (and therefore 4 times heavier).

What is the tradeoff between pier height, seeing and vibration? I hadn't thought of raising a pier to avoid ground-effect seeing, but I have been advised to keep piers low to reduce vibration, as the higher they are (with the weight at the top) the more prone they are to sway, or catch the wind. Now you've sewn the seeds of doubt .

Vibration timing: it's springyness and weight. The heavier the weight the slower the ringing will be, and the longer it will take to die away. A light scope on a stiff pier might 'ring' at 10 or 20Hz, which means if it damps down over 10 rings means vibration has gone after 1 second. If you fill the pier with concrete you won't improve the springyness but you will increase the weight, so suddenly it's vibrating at 5 hz and 10 rings will take 2 seconds. (you will get the same loss per vibration)

Tube thickness: we're talking here about the best place to put the metal.

a 10 inch tube with 1 inch thick walls will be stiffer than a 10 inch tube with 0.5 inch walls. But a 20" inch diameter tube with 0.5 inch thick walls will be stiffer than a 10" with 1" walls.. and will consume the same amount of steel.

So given a fixed amount of steel, go for diamater at the expense of thickness.

On the issue of how high.. If you look at UK planning, you can get an obsy with a double pitch roof with a ridge at 4meters high and eves at 2.5meter, without needing planning permission (we'll ignor the domestic authorities at this point). So given this it's clear that a pier in the region of 2~3m tall is a viable option. Given a pier this high, how thick/strong does it need to be to be strong enough. The answer appears to be in the 10" to 12" or so diameter range. There are some standard size pipes at 12.75" and 10.75", using the thinnest wall pipe in these ranges gives a reasonable level of stiffness such that a 5lb force on the scope will only give a 0.7 ~ 1.4 arc second pier movement.. that ought to be enough for most people.

I found a good article here:Pier Design Fundamentals - How To

note however there is an error in the 12.75" pier numbers.. it actually bends about 0.7 arc seconds.

PS: I'm an enginner: electronics, but I have a good physics grounding and have to play mechanical games occasionally... just don't ask me to model the forth bridge ;-)

Derek

[rfdesigner]

So are there any engineers in the house? From a structural point of view, is a cylinder better than box section for this application? The old man has managed to rustle me up a couple of potential bits of pier material, one is a 4" round tube, and the other is a 5" box section.

Given the choice, which would you choose?

Also, given the above discussion on the merits of having a perfectly level mount, do I aim to try and build it level in one lump, or add the adjustment plate on the top? But how much weaker does the adjustment plate make the whole thing? By quite a lot I would have thought, 4x bolts vs solid wall of steel...

Reading around the subject it generally boils down to which is heavier.. a box section has some weaknesses and some strenghts. Ditto the tube. But a box section with 0.5" walls will outperform a tube with 0.2" walls and vica-versa

Given that you already have some metal, and assuming they are both mild steel, then go for the heaviest per unit length. My guess is you'll end up with box section as it's larger width than the pipe.

Adjuster plates.. not too much of an issue if you can keep the bolts thick and short and done up to the correct torque... the bolts should be at the perimeter to get maximum strength. I would probably be more worried about the plates themselves flexing if they are over large compared to the pier diameter.

Derek

[laser_jock99]

Also, given the above discussion on the merits of having a perfectly level mount, do I aim to try and build it level in one lump, or add the adjustment plate on the top? But how much weaker does the adjustment plate make the whole thing? By quite a lot I would have thought, 4x bolts vs solid wall of steel...

The other option is build the pier level in the first place and not worry about having to adjust it later! If you access to a spirit level this should be possible even if you a casting your steel pier directly into the concrete.

Another way is to have a bolt-on flange arrangement at the base of the pier and heavy duty threaded bars sunk into the concrete. That way there is a small amount of leveling adjustment on the studs.

[samtheeagle]

Thanks very much for the help guys I shall get my hands on them soon, so I'll size them up accordingly.