Pier: Concrete or Steel?

[spinnaker]

G'day all,

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.

[bojan]

Concrete is cheaper... and equally good.

[sheeny]

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:D. Whatever you prefer is the correct answer.

Al.

[allan gould]

Have done concrete before and am just about to do it again. I use PVC piping of 6 - 8" diameter with reo reinforcing. Works well.

[spinnaker]

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.

Cheers,
John

[Moon]

If you are going to use concrete, it's worth the time to read this (http://www.cloudynights.com/ubbarchive/showflat.php/Cat/0/Number/168463/page/0/view/collapsed/sb/7/o/all/fpart/1/vc/1)first

James

[bojan]

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).

[GrampianStars]

Come in Spinna
I went with 8" steel buried 4ft in 3mt of concrete:thumbsup:
with the original 8" LX5
now it rides a GPS12 with no problems

[spinnaker]

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 :eyepop:. But it did provide some insights into possible pitfalls of a concrete pier which I'll keep in mind as I develop my plans.

[spinnaker]

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.

[Bassnut]

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 ;-)

[Doomsayer]

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

[Jeffkop]

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

[Paul Haese]

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.

Steel is better in my opinion.

[Quark]

Hi John,

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.

Regards
Trevor

[Moon]

I disagree. It's more about the diameter. This is another good reference that discusses the topic in some detail.
Link (http://www.cloudynights.com/item.php?item_id=1275)
James

[Paul Haese]

James,

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.

[Moon]

I would rather have a 12" pier.

[Moon]

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.

[Paul Haese]

James,

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.

Anyway, make your own choices.

[rally]

I'll add - in fear that I have provided fuel to the fire !

Wall thickness does increase the strength of a pipe for a given diameter but it is the increase in outside diameter that provides the greatest increase in strength/rigidity (or rather lack of flexure).

Vibration at the frequencies we can hear is almost completely irrelevant.
Its only very, very low frequencies that might cause a problem.

Yes, I know, I have read all the various posts and threads on astro forums over the years by would be engineers who say it matters or are just repeating what they heard.
It does not matter if the pier is otherwise rigid.

The real issue is the purpose of a pier in astronomy ?
In partiocular astrophotography because for visual astronomy these things are not so important.

The piers purpose is to provide a rigid support base that is mostly free of LOW frequency vibration that has a high enough amplitude to affect the image on a CCD (these are not audible sounds in the 1-5Khz range, they are very low frequencies usually well below 50Hz.

But more specifically and most importantly it is designed to ensure that there is the least amount of deflection when lateral loads of the order of a few Kgs are applied to the top of the pier/mount etc - either as a result of wind on the OTA, imbalance of the Mount and OTA as it moves throughout its range and to resist the opposing forces when fast slewing.
Being rigid means it transfers these forces into the ground where they will do the least harm and allow the system to settle very quickly.

Since we are asking the pier to resist deflecting by arc seconds or less - the pier has to be substantially stronger and more rigid than ordinary engineering would dictate on the normal basis of material limits and stengths for what are in effect quite trivial loads.

We want the pier to deflect less than the image scale of the system or maximum resolving capability of the system - preferably half or better.

Filling your pier with sand may make you feel happy that you have done something about it and it will change and subdue the resonant pitch of your pier at frequencies that have nothing to do with a pier deflecting arc seconds under a lateral mechanical load.

If you fill a hollow steel pier with concrete you will increase its strength (improve the modulus of elasticity) and that will help, but the preference would be going an extra couple of inches larger in diameter.
Loose sand doesnt add any strength !

I would start at 10.75" and look to 12" with 1/2" wall thickness or greater, but if you dont have a suitable mount (without backlash) and very solidly built and an optical system that is already rigid and without flexure all the way through - eg threaded adapters, solid focussers and rotators etc, solid attachment to mount etc then it may be a moot point what your pier construction is.

Calculating pier deflection (cylindrical beam) is a trivial engineering exercise for those who have studied it or wish to repeat it - the relevant function for calculating the moment of inertia OD (Outside diameter) in inches and ID in inches - pi = 3.142

i = pi * (OD^4-ID^4) / 64
deflection angle in radians is
(W * L^2) / (2 * E * I)

Where
L is the length of the pier in inches
W is the lateral load in lbs
i is the value from the first equation
E is the modulus of elasticity of steel = 3.00E+7 (Concrete without reo is approx 3.60E+4)

As you can see the function shows that it is proportional to the diameter to the fourth power ie (OD * OD * OD * OD) that means that a small increase in OD yields a large improvement.

Hope that helps

[mill]

Ok what are we doing here i will ask.
Do we want a pier for professional astronomy with a 100" scope or are we just amateur astronomers making pictures for ourselfs?
The bottom line is that when we make a pier strong enough to hold a car, what more do we want?
And who is stupid enough to hit a pier with a hammer when imaging?
Whether we make pictures at a star party on a say EQ6 with the tripod or at home with a pier made out of steel or concrete, there is no difference.
And i am personally sick and tired of people saying a steel pier is better than a concrete pier and vice versa, they are both good.
PS: i wouldnt image in 20Km/H winds even when i would have a pier that can withstand winds of 100Km/H.
Just my two cents worth :shrug:

[h0ughy]

OK i will step in here - at starparties when you are imaging and you get people walking around it does cause vibration into your mount, and sometimes it can be horrendous. The pier at home should be very sturdy and stable as possible - and separate as much as possible from walking traffic - even if it is you walking around - and i am no tooth fairy with wings so i need all the help possible. that's why i went for a very large mass pour of concrete - but instead of a steel pipe 8-12" diam i have a square section pier. it does vibrate ( especially when extended) but this is shielded very much by the pier base i have. It really becomes an individual thing - and very much dictated by the terrain. We all have different ideas - some work some don't. for the record i did one night as an experiment to see what moved the image - a light tap, rap and bump and to see how long it took to settle ;)

[allan gould]

I must admit I don't get a lot of these arguements. In the field I've used an HEQ5 and an EQ6 for imaging. As long as you don't bump the tripod then they are adequate for imaging. Now setting up an 8" PVC pipe with reo And concrete has to be better than the Chinese tripods and after making several of them I've found them to be excellent with no observable flexture or resonance. Why? Probably because I don't run into it or hit it with a hammer while imaging. All your arguements have some reason and rationale to them but practically ..........?
Just my 2 cents worth

[bojan]

Practically, in those arguments I do not see the concrete numbers for specific case, only formulas - which are of course just indication of what is better and what is worse..

So, bigger diameter is better than smaller one (that is not a surprise for me)..

But what would be good to see are some absolute numbers, not comparisons: what kind of flexure (in arcsec) we have at a specific case, for example: 16cm dia concrete pier, 1 meter high, with 10N force applied to the side.
Otherwise some people will always think that their pier is not *quite* rigid enough.. and most likely it is already 10x overkill.

I will try to talk to a friend who is civil engineer, and I will give you guys some specific numbers when I get them.

[troypiggo]

I'm a structural engineer. I wasn't going to say anything, but I feel there's some facts, figures, and equations getting thrown around here that are all getting quoted out of context. I'm not saying they're entirely incorrect, just that you need to look at the big picture rather than get caught up in detailed formulae[1].

Yes, Young's Modulus (or the Modulus of Elasticity "E") of steel is higher than concrete. More like 5 times higher, not 10, but that's neither here nor there. However it's almost irrelevant unless you're comparing a steel hollow tube with a concrete hollow tube, or a concrete solid column with a solid steel column, and both are the same size and height. Reason is that shape, size, and height also affects stiffness, and to a much higher degree than Young's Modulus alone.

Yes, wall thickness is a factor too. A thicker walled tube will be stiffer than one of the same outside diameter, same shape, same material, same height, but thinner walls. But change any one of those other properties and things change and you're comparing apples and oranges all of a sudden.

Concrete versus steel? I reckon it's more about what you have readily available or have more experience with rather than it's structural performance. Each will work adequately.

Other factors to consider would be what are your ground conditions - sand, clay, gravel? If loose sand or gravel, you'd be best going with a pad footing. If any clay and the sides will stand up, I'd go with a piled footing.

If it was me building a permanent pier, I think I'd go with casting hold-down bolts into a piled footing, then bolt down a steel pier. Reason is you can adjust any out of vertical in the pier, and remove the pier if you ever move so you can just lay a new footing at the next house. But then, I have access to local steel fabricators and would know exactly what I want and how to specify it. If it's exposed to the weather, make sure bolts and pier is galvanised or you use a very, very good paint.

You might prefer to just buy some PVC tube as formwork for the pier, some stock lengths of reinforcement, and pour concrete yourself.

What sized pier would I use? Depends on what you're going to throw on it. Now and in the future. It's too hard to calculate exactly what sized pier to use. Too many variables for precise equations to be worth it. I'd just pick a size by feel, and then maybe double it. Sorry if it sounds vague.

[1] Take care if you decide to go against my advice and try to use that formula rally posted above. The formula itself is correct, although what it calculates is the deflection of a cantilever under a point load - like how much a diving board would deflect down from horizontal if you were standing on the end of it. Not sure it's relevant for what you're trying to achieve. I suspect rally was quoting it to point out the effects of size and shape rather than use it to help you size your pier. I also suspect there's something off in the numbers he quoted for E of conc and steel - looks like one is metric and one is empirical and you shouldn't try to mix and match those.

[Moon]

Bojan

Have a look in the second link I posted. Here it is again:
http://www.cloudynights.com/item.php?item_id=1275


Extract:
72 Inch Height, 8.6" Diameter, Deflection 3.4 arc sec
72 Inch Height, 12.75" Diameter, Deflection 0.1 arc sec


James

[troypiggo]

From an engineering point of view, that article looks pretty good. Shame he skims over some parts, but all in all it's pretty good article.

I'll have to check some of the figures he's quoted. As per rally's post above, some of the Young's Mod values don't look right, but they're empirical not metric. I live in Australia in the year 2010, I'm all metric, baby! Haven't got a feel for empirical values off the top of my head.

[spinnaker]

OK, interesting stuff being posted here! Can I pose another questions? Assuming I were to build a steel pier (with diameter of say 200-250mm), is there any advanatge or disadvantage or using a circular steel pipe v a square-section pipe?

I suspect the answer may lie somewhere in the formulae quoted above, but I'm no engineer ;-)

[bojan]

Thanks mate, for pointing this out.
However the problem is specifying the lateral push, I am assuming it will come solely from imbalance (centre of gravity of the mount with telescope not aligned with vertical axis of the pier).
Also, the pier itself does not have to have constant diameter - if it is larger at the base and slimmer at the top (conical) , it will have almost the same deflection for the same load but it will be much lighter.

[troypiggo]

Assuming same wall thickness, same material, and the sides of the square section are the same as the diameter of the circular hollow section, the square is stiffer by some 75-80%. It's a more structurally efficient shape in bending.

But it's also slightly heavier by some 25-30%, therefore uses a bit more steel, therefore is probably slightly more expensive. Steel is usually priced kg/m give or take.

[bojan]

I think it it will be better (in terms of deflection due to a variable load), especially if the diagonal of the cross section is aligned with S-N line.
And if the diagonal of the square pipe is bigger than diameter of the round pipe.

[hatman]

I simply can't believe the rubbish I am reading.

I have seen several piers made of 4-6mm wall thickness and there is no scientific evidence of lateral deflection at all.

You really have to ask the question: You have a 900 mm high (average) pier that is anchored by 4 anchor bolts that are each designed to hold several tons of weight/force.

How much force is it going to take to laterally move it at all? Lets get in touch with reality.

In all cases I have seen deflection in steel piers it has been because of the foundation moving or even more common, the "stretching" of the threaded anchoring bolts!! Yes threaded rod does stretch and will do so long before a metre long tube welded to 12 - 16 mm plate ever will.

I don't beleive that this has been looked at at all in the research and thus creates flawed conclusions.

Lets deal with another fact. we all spend lots of time with lots of astrophotographers who produce images of an incredible optical quality (many which are published)and they all seem to use steel piers, usually filled with sand and usually around 700 - 1000 mm high and generally have wall thicknesses on the tubes of 4 - 6 mm.

There is no evidence at all in any of their images to date to support any argument for lateral deflection! If this was the case then why are we not seeing that movement come out it in these images.

Think about for a second.

As a person that works with working and manipulating steel daily, I can say with great confidence that it is absolute overkill to construct a steel pier under a metre long of any steel thicker than 6mm wall thickness unless you are constructing taller pier supports.

To the contrary, to go for a thicker wall thickness in an attempt to try and make the shorter pier more rigid can actually introduce extra vibration problems - Physics 101

In other words, you have to be careful of being too unforgiving.

High rise buildings in eathquake zones are not built with rigid foundations but rather with dampeners at their base. Why? Scale it down to a pier and you get the picture.

Piers need to have a minute bit of give to absorb any external vibrations, This minute deflection usually occurs with the fittings "absorbing" the deflection, not the pier itself. If the pier moves, it is because the vibration is being transmitted through the anchors or not sufficiently dissipated through the anchors to the ground!

The question poses itself, If the theory of thicker and more rigid is better was true, then why is it that we place dampening pads under field tripods to reduce vibration? Bigger is not always the best solution.

Any person with basic engineering knowledge would also know that concrete fractures under the smallest lateral force compared to steel. More importantly, concrete and is a better conductor of vibration than steel.

If you really want the low down on piers then check out Brendon Downs' latest pier with a Paramount ME mount and Meade 14" on it.

Better still, go and have a look first hand google at the Pegasus piers in at Sirius Optics. These ones have 4 gussets top and bottom fully welded on 12 mm plate. I think Brendans is 16 mm thick plate from the same maker.

I would love to see what size truck could cause those bad boys to deflect. :D

[rally]

Hatman,

What deflection calculations are you using to justify all of this ?

With only 2.5kgs of lateral load applied - this will cause a 1m high 5" dia, 5mm wall thickness steel pier to deflect approx 2.3 arc seconds - that is a function of the laws of physics.

Now for someone with an image scale of say 0.4 arc secs per pixel - eg Your example - Meade 14" f8 scope with a KAF8300 chip
Thats 6 pixels in just one direction.

So if we have some quite light breeze gusting on a 14" scope and the pier and mount etc that is going to cause it move in both directions maybe yielding up to twice that movement.

So your small stars just got huge.
You just lost ALL the detail of the pillar nebulosity in the Rosette nebula.

But wind is not the only thing that provides a small lateral load to the top of a pier - cable drag at one end, load imbalance, just the mount with OTA.
Ever undone an EQ5 on a pier and watched it flop to one side - they are naturally unbalanced.

So its a question of what sort of imaging you are doing and what sort of results you are striving to get with what sort of gear.
The one thing you don't want is the pier (a very cheap item in the whole scheme of things) to be the thing that lets you down. Just overengineer it and know that it will not be the factor causing you grief.

This may not matter to you, but it does to some.

Astrophotography pier design calculation is not an engineering exercise in strength - piers could easily support a small building - its an issue of deflection and that requires by all accounts quite massive construction.

Very small lateral loads do occur and thus deflections do occur and they will shift your image.
If it reduces your FWHM by 2-3 arc seconds or more then you may as well pack up and go home.


In answer to all your various points in order.

- You shouldnt discount all the rubbish you read without at least verifiying some of it and ensuring that you havent in fact contributed to the rubbish yourself.

- The scientific evidence was the formula used by every engineer and engineering graduate for the last 100 years listed earlier in the thread.

- The force required to move such a pier enough to seriously degrade an image is actually trivial as calculated herein.

- Maybe the bolts will stretch ! but not as much as the pier will bend. The bolts are already under considerable tension.

- Most piers are held down by numerous bolts all appropriately tightened - so it has been considered otherwise we would be using contact adhesive or staples.
2-10kg lateral load at the OTA will not cause multiple bolts each having tonnes of clamping force to measurely deform enough to affect imagin.

- Astrophotographers tend publish their good results not their bad ones as a rule. In any case its difficult for anyone to be able to accurately identify (with all the parameters involved) exactly which problem caused them grief - they will simply take another sub and reject the bad ones.
If you look at what most of the Pros" and commercial institutions have installed for a pier its usually of a substantial nature and often that is inside a perfect environment like a large dome.
Its a question of improving your odds - your choice by how much.

- It is clear you state this with great confidence - that does not mean you are correct - you are applying incorrect logic to the problem at the very least.

- Since your additional vibration problems statement is so vaugue and generic - its impossible to offer a comment on that. The idea is to make a pier substantial enough that all vibrations are carried to ground and are hence dampened as quickly as possible.
The vibrations you can hear that are dampened by sand have nothing at all to do with pier deflection and the problems that causes. Such frequencies have such small amplitudes that they do not (themselves) affect our imaging.

- Earthquake proof construction is another science altogether (and I dont have any expertise in that field) - we are talking about building a pier for astroimaging in this thread.
We dont want that (pier) to be able to move on a flexible interface and . . if an earthquake occurs then I will reject that particular frame as part of my normal post processing but since it is only likely to last a second or two in a 1-10 minute exposure - chances are it may not even matter.
I don't get that picture at all - I don't want a wobbly pier on a non rigid foundation !

A pier is not designed to absorb vibration at all - it is designed to transmit all it can to the ground.
Its principal design function is to defeat deflection.


A field tripod is not a rigidly designed contruction that is anchored to the ground, it has a different purpose - that being lightweight and portable - these are not the ideal platform choice for astro imaging, but if the conditions are ideal they can still perform, but never as well as a pier under less than perfect conditions and I recall reading a good essay about the comparisons made between a pier and a tripod - but I cant prove that !

Any person that knows concrete knows that a 2.5kg lateral load on reinforced concete is not going to fracture it at all - it will just deflect a little bit, and that little bit is more than a steel pier of equivalent diameter with a suitable wall thickness. But it is cheap - works very well, easily accessible by most amaterurs and is usually solid verses a steel pier
Is concrete really a better conductor of vibation than steel ? - But that matters not - it is not the vibration that is the main issue - if the pier is so flexible that it cant support a a small lateral load without bending, then I guess it will vibrate a lot - but thats because its simply inadequate for the purpose.

Please provide engineering and structural details of this pier and the results of its performance or how it its better or worse than alternatives discussed here - that would be constructively useful to members.

I looked at Pegasus Piers in at Sirius Optics - I didnt see any technical specifications on pier deflection listed
But I can say that 1000mm high 165mm OD 6.5mm wall thickness pier (without gussets) will have a deflection of
just under 1 arc sec with only a 2.5kg lateral load !
Flexure is not eliminated as they say - that is impossible.
The sand will not change the flexure unless its is mixed with cement and water - then it will add a small improvement.

I would estimate that a standard sized Tonka Toy truck used by most 4 year olds without a load of sand on board will in fact deflect a pier by some amount if it was hung off the end of the average OTA assembly causing an imbalance.

Hope that helps address some of the issues you raised.

[PeterM]

This is a very interesting thread. Thanks to all of you for the informative posts.
In my case I am doing multiple short movement, scripted slews to many galaxies for 25 second exposures with an alt az mounted 12inch LX200, I allow an 8 second damping time once I arrive at the object. I was using a 850mm high homemade pier of 165mm od with no gussets and dampening seemed quite ok. I have recently bought a Pegasus Pier as replacement (basically bought on looks, construction quality AND that it has the gussets). I can't give a report yet as only went in on the weekend and skies are crap, but will do as a comparison to what I had, don't know what value that will be given my needs over say those doing longer exposures.
After reading carefully the above I am not sure if we are comparing apples with apples here when it comes to steel piers, that is a section of pipe with plates welded top and bottom versus one with gussets (as I originally had).
Rally notes that the deflection on a pier example of 1000mm high /165 od /6.5mm wall as about say 1 arc sec (2.5kg lateral load) and notes this is without gussets (again, pretty much what I had).
As an example all the Pegasus piers I have seen (and most others I have seen) are gusseted (4 top and bottom, extending to about 50mm across the bottom 12mm plate and about 140mm high and about 6mm thick) with substantial welding, so in order to compare apples with apples does anyone know what affect does this have on lessening deflection as compared to a pier with no gussets? ie does it effectively act as an increase in the tube diameter (footprint at ground where the major issue is) and how would this be taken into account in measuring deflection?

PS where's Mark Bolton when you need him, he teaches this stuff.

Thanks

PeterM.

[rally]

Peter,

Yes you are right - the original comparisons were between ungussetted mild steel piers of a cylindrical shape.
Using higher tensile steel - eg Gas pipeline improves the bottom line.

I am not sure what effect these gussets will have, that is why I calculated it without and stated accordingly.
A structural engineer would need to model that one.

But if we assumed they were perfectly rigid (which they are not) and completely shortened the pier by their approx length becasue of this perfect rigidty - I still calculate a deflection of 0.51 arc sec, so I am guessing its somewhere between the two measurements for 2.5kg lateral load

Remember this is with only a 2.5kg load applied, the reality is that if the load is applied at the OTA (which is most likely) the increased leverage will increase the amount of deflection.
It is extremely difficult if not impossible in practical reality to properly balance eccentric loads on larger systems - which are a normal occurrence.
The imbalance is usually at the very ends of the OTA - eg a STL11000m with 8 filter wheel and cables is quite eccentric and it hangs out nearly a metre rearward from the centre of Dec rotation on a large refractor.
Focuser motors, Rotation motors and other items such as guide scopes and guide cameras hang off the sides or tops of the system in most cases.
The entire camera system is rotating about the end of the telescope too - so the eccentricity is a dynamic problem.
Thus as the telescope swings through the full movement of travel it goes in and out of ideal balance in many different directions - although this is happening slowly and guiding can take care of that, it does cause problems with Polar Alignment and pointing accuracy and its variable.

People can go to a lot of trouble with getting their Polar Alignment below 1 arc minute in RA and Alt in a pemanent observatory situation and then create a many point star model to get the pointing and tracking accurate and apply PEC correction.
A pier that deflects through its travel simply adds to your woes - I would argue unnecessarily.
Spend $100+ extra on extra steel or concrete on a system worth potentially $10,000's and this problem is for the most part nearly eliminated.

Rally

PS
If you want to get fancy - as is done in many military pier designs they use a cone shaped pier and I am aware of at least one amateur astronomer who has applied this engineering design to his pier.
It means it takes up a larger footprint but the increase in rigidity is enormous and the wall thickness can then be reduced down from 10-15mm.

[bojan]

Guys,
This lateral load you are talking about is more or less constant.
So what if there is a constant deflection present ? Most of us are not in the positional astrometry business.
The only things that really matters are fast variable loads (cable pulls, wind, walking around the pier etc).
I have a feeling we are grossly oversizing things here..
Not that this is bad per se, but it all adds the cost and time to the whole operation.

What about people that use tripods in the field?
Surely any tripod is inferior compared to 180mm dia concrete pier.

[rally]

Bojan,

I am only offering the calculations for those that are interested, nobody is wrong if they dont use them !
Rigid piers are definitely not needed by everyone.

But the problem is the loads are changing and this means the flexure is different at different camera rotation angles and at different positions in the sky

Wind tends to gust - even a light breeze will push a large OTA around quite a bit.

One set of deflection affects Pointing and Polar Alignment, the other set affects the position of stars in the image frame during the image sequence

When exposure times can be very long this can be an issue.

I think the people to whom this matters will know who they are, for the rest its not as relevant.
Planetary imaging is unaffected by the pier design for the most part

At the end of the day it boils down to degrees - to what degree do you want to reduce bloating your stars.
For many, gaining an extra 0.5FWHM does matter if it can be achieved relatively easily and cost effectively.
For them that may represent an increase of 25% in image quality from a tripod setup (I dont have calculations to show that).

Big scopes with long focal lengths are obvously more affected than widefield - but the final measure is image scale - I guess if you are at 3.5arc secs per pixel or greater you can afford to relax a little, if you are at 0.5 arc secs per pixel or less you need all you can get !

I agree a 180mm concrete pier is superior to a tripod in the field. But that doesnt mean that on occasion you couldnt get a good image from the tripod - under the right circumstances you can.

I just say that you should get the largest diameter heaviest chunk of pipe you can manage (physically an within your own budget) at the local scrap yard and use that !

Most people end up with a relatively longer pier than 800-1000mm though - because they need to allow for the extra height that their Obs is above ground level.
2700, 2800, 1900 and 1600mm are some that I have checked.

I think I have said more than enough toward the OP's original thread so I'll sign off unless someone wants to start a new thread about pier deflection and how it affects your FWHM and image quality.
It would be nice to have two comparable systems setup side by side - one on a solid pier and one on a less rigid support - eg tripod or small pier and measure the results across a range of tests.
At least there would some tangible results to argue over !

Rally

[troypiggo]

As a structural engineer, it's very difficult to model and analyse these piers with any high degree of precision. I have all the relevant software right here on my computer, but would need to know the ground conditions, site location, size and weight of scopes, height of pier, preferred construction materials and methods, what is your acceptable deflection, is the pier in an observatory or out in the open, and more. There's just too many variables to make it worthwhile. What you'd do is make assumptions here and there, and in doing so all precision is lost, but you end up with a slightly conservative solution in the right ballpark. That's what I was trying to point out in my posts above.

Peter - not sure if this answers your question or not. It's not strictly correct to say that adding gussets to a pier is effectively increasing the diameter of the pier. Yes, it increases the stiffness because the gussets increase its Moment of Inertia, which is directly proportional to stiffness.

If you've got any specific questions, happy to answer them - maybe PM so this thread doesn't get too off-topic. I do have a little bit of structural engineering experience after 18 years of working as one, and now a partner and director of a structural engineering consultancy. :)

[koputai]

I'm not an engineer, but I did listen at school when spelling and grammar were being taught!

Dampened = wet
Damped = reduced

Dampening = the process of wetting
Damping = the process of reducing

Just taking my pet ant for a walk.

Cheers,
Jason.

[bojan]

OK, lets make the following assumptions:

1) The telescope with mount and counterweights has 60kg. The whole thing is balanced, which means there will be no change of the centre of gravity during the observation/imaging.

2) The equivalent surface of mount (from the wind's point of view) is 0.3 m2, Also, lets assume the mount is perfectly rigid, so it does not contribute to the flexure (and FOV shift)

3) the speed of wind gushes is 25km/h (is that severe enough to pack up and go home)?

4) There is no earthquake, walking around pier and/or pulling cables etc.

I think the question may be something like this:
What is the minimum diameter for concrete or steel pipe pier 750 mm high, that will suffer flexure less than 0.3arcsec under above conditions?

[Bolts_Tweed]

Marples - what do you mean you wish Bolton was here - first time I've ever heard anything like that out of your mouth (usually its - damn Bolton is here).

This turned into a tome do dont bore yourself reading from here on if you arent interested - musing of an old Engineer on piers.

I do note that you have an accomplished Structural Engineer in the discussion (Gday Mr Piggo) and probably others. I agree with Troy that the variables involved make it a very complicated calcultation. Troy and I (and probably others - sry I only know Troy) have finite element packages on our computers in front of us that could model it quite well IF we had time. And as I have said before out at the farm it is no good modelling structiral elements unless I know the loads I going to put on it. What scope - how big - weight - GEM or fork (sry if you've discussed this but im at wrok and dont have time to read all the posts - I have to pretend im intersted in undergrads). i'll read them all tonight.

I agree with Bojan that diffeential flexure is the problem - If I have a scope on one side of the mount on say a GEM at a max distance from the mount (ie max moment at the top of the piers) and im taking a 2 hour image the distance the load is from the centre of the pier is not going to change much in 2 hrs - ok there will be slight difference in distance (a bit like the PDelta effect on columns in reverse Troy I suppose) at least based on my empirical experience. If it is a fork mount there is going to be next to no change in moment. In fact thinking about a correctly balanced frok mount should have bugger all moment at the base or top of a pier.

Anyway when I get time and if you give me the dimensions of the piers you want to compare (leave out gussets Pete - I think I know the answer already and I suspect you will be surprised how low the defelctions are). I will do a simple static analysis like - the pier is sticking out horizontal from the wall with a moment at the end and you can compare deflection of conc and steel like the thread says (and this cantiliver model will be far worse than a real world model) - I do think however at the loads I am thinking about - say a 12 inch Meade on GEM (forget about forks it wont happen) the differential deflection and thus differential flexure is going to be anything at all.

I have a 4 inch water pipe with top and btm plates bolted to the floor of my observatory with no conc pier or anything under it - I have a Astro Physics 5 inch, A Tekevue 4 inch and an Orioin ED80 with 4 large skywacther counterweights, motorised filter wheels, CCD cameras etc etc hanging off it and I get no (ie zero) differential flexure over periods of up to 4 hours of imaging the same object. I do realise that others have far more load however but how much are wetalking about -as much as you weigh maybe sry Pete - no joke here.

Imagine your pier solidly bolted to a wall - now stand on the end of it and measure the height from the floor - note that the moment you induce in the pier from this will be far more than it will ever get from a scope (unless you have a 36 inch RC or something on a GEM) - now decrease the moment by stepping say 300 mm closer to the wall and measure again - the difference in these 2 final measurements is what is important- it wil be extremely low and tranforming this change in deflection to arc seconds in a 'normal' focal length telescope (say 2.5 to 3 m focal length) is what we are talking about. Remember too the pixel scale of your equipment (2.5 to 3 metre f/l) when comparing this - is your set up capable of imaging at 1 or 1.5 arc second resolution? maybe but well thats another variable.

I think we as astonomers get a little carried away in design - is bigger better - yep definately but I have never seen pier flexure causing problems in a normal amateur observatory - I dont doubt it has occurred - just i've never seen in it 30 years. What I see more often is flexure induced by overloading (or incorrect side by side loading ) of GEMs.

My 2c anyway. Im sure some will disagree but I can only comment on what i've seen. If i'm off topic - well im old and I spend my days rambling incoherently.

M

[AlexN]

Good points there by Mark... Im a fan of steel (being an industrial metal worker and all)

Only thing I will point out in Marks' post is that even if you have a 36" RC on a GEM, you will still not get as much flex in a pier as if a person stood on the end of the pier when its bolted to the wall.. This is why we use counterweights (along with the fact that we dont want to burn out our mounts motors).

Lets say, you have a mount on a tripod with a 20kg scope, the mount and tripod weigh 40kgs, and when the mounts RA axis is horizontal, the scope's load center distance is say, 80cm off to one side... Can anyone tell me what happens to the tripod if you remove the counterweights at this point? The tripod falls over, or becomes very unstable.

The same goes for a pier.. More often than not there would be minimal lateral force on it, because as 20kgs of telescope starts to hang off to the western side of the pier, 20kgs of counterweight is going out to the eastern side.. Provided you balance your setup correctly, your pier is not under a great deal of strain...

What am I using? I'm using a steel H beam girder for a pier with top and bottom plates welded onto it, and the bottom plate is bolted to a 2'*2'*3' its solid as a rock, and the section of girder I'm using weighs more than the mount, the scope/camera setup, myself and my laptop combined. Better than a round pier? At this level, who really cares? Do I ever notice problems in my images on account of the pier being inadequate? Thats a no.

[Bassnut]

BTW, if you want to get really fussy, do a Tpoint model. Pier flex and sag is automatically measured and modeled. There are terms both for german EQ and fork mounts.

[troypiggo]

If the scopes, mounts, and counterweights are all balanced as they should be, ie within normal working range of the mount, the lateral loads on these piers is nominal. Even wind loads will be nominal.

Using Bojan's figures above, 25km/h wind is about 7m/s, and using assumed shape factor of 1.2 for scope/mount etc and applied over 0.3sq.m area produces a force of about 0.01kN. A 100kg person weighs about 1kN, so we're talking 1/100 of the weight of a person. You can see how nominal these lateral loads you're talking about are. And as Bojan intimates, would you be imaging in those sort of winds?

That CN site is recommending about a 219x6.4 CHS section (I adopted sections available here). Putting 0.01kN lateral load on a 750 high pier of that size yields a deflection of about 0.12 arc seconds based on my very rough analysis.

Seriously guys, don't get caught up in the numbers. Use that link above from Cloudy Nights, and just adopt the pier sizes recommended there. They'll be well and truly sufficient and then some, and that's about as much time as you need to spend on it.

[AlexN]

You gotta love software compensation! :) Tpoint is the goods.. Perhaps a discussion for another thread, but it will compensate for almost anything.. even SCT mirror flop

[Bassnut]

For what its worth, Here (http://fredsastro.googlepages.com/observatory) is my concrete, slightly coneular pier. No reo, no steel, no welding, cheap, and a fairly weighty PME/12" SCT sits on it with no bother I can detect.

[PeterM]

The thread may have drifted slightly from concrete versus steel piers but I think the overall information presented here from all the combined knowledge is very valuable indeed. I have learnt a lot. Again thank you all.

PeterM.

[spinnaker]

Although my original question was concrete v stell, the resulting discussion has been extremely interesting, so no complaints from me about thread-drift.

However, I'm still uncertain whether I'll build a concrete pier, or a steel one ;-) (But pretty certain I'm stick with my original plan of a steel pier, built as solidly as I can make it!)

[PeterM]

As promised I present my initial results on my new Pegasus Pier (I had best declare here and now that a friend of mine also an amateur astronomer manufactures these piers). I have now ran the scope (12inch LX200R on alt/az) on the Pegasus Pier through a hefty multiple galaxy script (105 galaxies in 2 hours) and all seems very well indeed. No evidence of any deflections in the pier causing any issues whatsoever. The night was very steady with the odd 1-2km breeze that would have caught the flexible dew shield extending above the observatory walls, nothing was noted in the images, but they are only 25 second images. My pier is 900mm x 12mm plates top and bottom with 4 gussets top and bottom. The pier has been filled with dry white fine sand. The pier is solidly dynabolted to the concrete floor with 4 x 12mm x 60mm dyna bollts. There are no engineering specs provide as such re the deflection but I think from all figures presented here and my own run last night it would be below 0.5 arc seconds (realising there are other factors to be taken into account as Bojan has correctly noted above). The pier looks superb, it's professional look and sturdiness compliment the telescope. A good investment I am trusting to support my excellent optics. Spinnaker I don't think you will have any issues with a steel pier.

PeterM.

[rally]

Troy,

You might like to add extra in for load imbalances and cable drag.

As the mount swings through meridian the cables and any other eccentric load (side mounted) guide cam, finder scope, etc) can change by double.
You cant balance these out completely except with a very complicated set of eccentric counterweights - which are not practically used.

When the camera rotates 180° after a meridian flip a similar thing can happen.
If you are using a large camera, say with 8 filter wheel and depending on your camera angle (the eccentric load may be pointing out or in toward the pier and its cables protruding yet further out or in) this eccentric imbalance can end up being quite a lot more than your wind induced loads - I would estimate by a factor of maybe 100 or more based on my own experiences.

Depending on where the scope is pointing that imbalance differential can be a little (say at zenith) or a lot (say lower on the horizon) - so it is very dynamic.

I am not sure where you calculated the position of the wind load, but depending on the OTA, it could be well in excess of 1m above the top of the pier and I suspect that will increase the deflection in your calcs.
But I agree 25kmh winds mean the end of any imaging session !

750mm height for a pier may be OK for some piers with small scopes where the floor level is also the pier footing level, and low walls, but for most amateur observatories constructed of wood above the ground, and for larger scopes the pier needs to be taller by maybe 300mm to 1000mm to get above the obs floor and maybe another 500mm to provide clearance for the scope and wall clearance to get a reasonable view (say 30-35°) above the horizon.
(much depends on the particular Obs design)

Since the deflection is proportional to the square of the pier length (height) this will affect things considerably, Amateurs need to know that they cant just scale it linearly.
Double the length and the deflection increases 400%, triple it and its up by 900%.

So I would argue that the 8" Nominal Bore pipe (8.625") you used 219mm OD with 6.35 wall thickness may not be sufficient for many situations.

I am truly not trying to be a trouble maker !

But the issue is not always so simple and I am not only trying to talk theoretically here - these are all problems I have had to deal with either for myself or calculations of real life piers that have been constructed at lengths greater than 1.75m - even as much as 2.7m.

Yes Alex - TPoint is wonderful - but it doesnt understand camera rotation yet ! (aka eccentric load rotation). Only rigidty can help that one - in the OTA, adapter Mount and pier !!!

Cheers

Rally

[troypiggo]

Absolutely. I've been saying all along that there are many variables that affect the outcome of this, and it's just not worth the effort of going into detailed calcs. What I was doing above was directly responding to Bojan's model for the sake of it. It was a quick and simple model to post.



I'm not denying that any of those actions occur. However, this thread is about the pier design. My thoughts on this are:

Wouldn't your mount normally be pretty well balanced about both axes when in a static position? If not, you'd be stressing your mount quite a bit.

If your balance is out by enough to affect the pier, I'd seriously be concerned about your mount. In your mount is one of those massive ones like a Paramount ME with huge capacity, we wouldn't be talking about such small piers and again the pier wouldn't be an issue. It's all relative.

I think the actions you are talking about are perhaps momentum of the moving mount? If all of your gear was reasonably close to perfect balance, there should be negligible lateral loads on the pier. The reaction at the connection of the mount/pier should be practically nothing in shear. There may be some torsion, but that is not a lateral load.

I'd expect there'd be far more movement/slop in the mount than there would be in the pier. By the time the mount settles down, the pier would well and truly be static.

And if you're talking about side by side setups versus piggy-back setups, you need to consider deflection in the plates and all connections too. It's all additive.

And you wouldn't be imaging in those milliseconds/seconds after a bit slew anyway, would you?



As I said above, I was simply addressing Bojan's numbers directly. If the centroid of whatever is catching wind is higher, the deflections will indeed increase by the difference of the square of the heights.



If you look at that CN link we keep referring back to, he correctly provides different pipe dimensions for different heights, for the reasons you mention here.

The 219 diameter pipe I mentioned was quoted specifically for the example used for Bojan's model. I think I've said this many times in this thread before. Change the height, or change the wind speed, or change the area, or change any of the other factors used in the calcs, and the pier size should be adjusted accordingly.

But again, it appears to me that the CN article adequately addresses most of those issues for amateur imagers.



No worries. As others are saying, it's an interesting discussion. I'm learning too.

[hatman]

One thing that seems to have been forgotten in this whole discussion is that the so called figures being used are based on piers without gussets and more importantly, there is no mention of the hundred variables that could affect the test results.

Variable such as scope tube movement, a mount that is slightly loose or of inferior quality and so on.

The reality is that the figures are usleless if the tests were not done on piers independently in a test machine and free from any mount and, attachment to the ground or any other outside influence.

If the results have been gathered from piers attached to the ground and with scopes already on then there are so many variables that could have influenced and skewed the resultts it is not funny.

Surely no one can disagree with sound scientific analysis which it seems the figures some are using as gospel, were not derived from.

Since this thread has started I have looked through many scopes on different piers and noted no movement or deflection at all attributed to the pier on those scopes mounted on gusseted piers.

In fact even on the scopes that I viewed that were on un-gussetted piers the only movement came from inferior mounts and in most cases, scope vibration from the tube which died settled within a second of testing.

It is obvious that the figures eveyone is basing there assumptions on are not based on proper testing or on gussted piers either.

Too much emaphasis is being placed on pier peformance when it is clear for those who look closely, there are clearly other factors which could have skewed these results.

It is OK to question and I would think it is bad practice placing too much importance on partial reasearch and then trying to apply that as a standard. :shrug:

[rally]

Hatman,

You say you have looked through many scopes and verified this to all be nonsense.

The whole point of this pier execise is about astroimaging not visual observing !
You have completely missed the point.

Can you tell me what the seeing was in arc seconds on those occasions and how did that affect a long exposure ?
What is the limit and range of your observational skills in determining the FWHM of the stars you see through an eyepiece to validate you claims ?
How much did the FWHM differ according to the diffreent scopes and piers you used ?

For visual observing you can use any old tripod and pretty much any old mount or not even use one at all, the eyes cant detect the small displacements we are discussing here which are in the orders of 1- 6 arc seconds and even if they could, the eyes and brain will compensate for any system movement and even substantial target movement.

The camera will not compensate - it will faithfully record every microscopic movement at whatever your image scale allows and every error - it will bloat or smear your stars and destroy the fine detail in nebulosity by adding noise.

I recall a Seminar where the speaker showed the difference between stars and galaxies that were taken in different seing conditions ranging from about 2-6 arc secs - 2 being good and 6 being about the end of the road for good astroimaging - Most imaging systems (all other things being equal which they usually aren't) are capable of resolving between say 0.4 and 4.0 arc seconds.

He then showed a picture of a human eye that was treated to a blurring filter that effectively added the noise representing the same amount that a perfect star could be blurred to represent the same range of arc second seeing.
The human eye at 2 arc secs was sharp and clear, at 6 arc secs no longer had any iris detail and the eyelashes were no longer recogniseable.
This corresponded nicely with the loss of detail in his astro images and makes for a good analogy.

The astro imagers job is to try and get their system down to a point where the atmospheric seeing is the limiting factor, not the scope, not the focus, not the guiding, not the camera read noise, not the mounts tracking (PE and pointing), not pier induced noise etc etc

Astroimaging requires these levels of stability otherwise we end up with noise in our images.
Any additional noise from mounts that are loose or of inferior quality (as you say), OTA flex and OTA movement due to poor attachment etc etc etc simply add quadratically to all the other noise.
They are all separate problems that each need to be resolved separately and independently.
One does not simply mask the other it adds to all the others.

What we have been referring to is trying to solve the noise in our images and pointing inaccuracies etc that are caused by lack of pier rigidity in observing/imaging conditions and environments that are less than perfect.

The astroimagers biggest job is to eliminate all the noise making problems they possibly can within their own budget and the actions taken in order to get the best image they are comfortable with.
The Pier and its footing is just one of many, but it is the first of many sources starting from the ground up !

It is also the easiest and cheapest of all of them to eliminate.

Only today I was asked about the pipe dimensions necessary for a 48" pier for a requested deflection that was 1/3 to 1/4 of the Mount's approximated flexure under a defined load the person concerned believed the lateral loads we used for his system was probably conservative !
This criteria (given his optical and imaging system) meant that his system would be "mount limited" and that is what he needed to do make the system, such that the weakest link was now his expensive mount because to go the next level would have cost a fortune.

The steel pier dimensions required given its height was 14" Nominal Bore with 15mm wall thickness - based on off the shelf pipe.
He had already done his own calcs and was aware that the pier required was necessarily large.

You seem to have a great understanding of these things - maybe you can provide details of the test machine you refer to and the constraints we need to work to - eg what is minimum movement of the telescope in arc seconds to achieve the resolving accuracy you need for your particular optical system based on your preferred sampling rate and the image scale and the types of targets in mind together with the affects of imaging system rotation and their unresolvable OTA imbalances impacted by pointing in different areas across the sky from the initial "balance".

When it gets down to the numbers we can then talk in "scientific" terms and provide some answers based on historically proven engineering principles rather than rely on hearsay and fundamental misunderstandings of what the issues are.

My only argument throughout all of this is this :
- The pier is an integral part of the imaging platform and it is a ridiculously cheap item to make rigid. In fact apart from maybe some of the cables - it is probably the cheapest major component of your system.
- The Imaging system (Scope, Mount, Camera and accessories) cannot always stay in balance (no matter what you might think) if rotation is involved or pointing across meridian is involved - except in a relatively few special cases and as such needs to be supported by a pier that will not flex beyond a certain limit.
- Other factors affect the stability of the imaging system - wind on the OTA and the pier needs to transfer these to the ground and resist flexure.

1. You can choose the limits of flex and the ultimate image quality you want your system to resolve
2. You can choose the amount you want to pay to achieve this
3. You can choose the amount of effort you want to expend
4. You can choose to believe the engineering maths or not.

1,2,3 are very optional and everyone's situation is very, very different - their system, their budget, their needs, their desired image quality, their observatory, their viewing conditions, their pier height etc . . .
1,2,3 ultimately define how the pier will perform according to your choices and the maths.
4 is more akin to believing the earth is flat or not, you don't have to believe it, but your pier and therefore your optical system will deflect and react and give your images extra noise just the same whether you do or dont believe it !

Its just a question of how much and how much you are prepared to live with - that is all.
Obviously we need a pier (or tripod) so we are really arguing about how much pier we need !!!
It just happens to be a bit more in diameter and or thickness than we think. ;)

Cheers

[troypiggo]

I'd just like to make one last post here about these piers. Been having some PM discussions with another member here about the sizing of these things, and more about the variety of different gear going on them. I feel that I need to qualify some more of the assumptions behind my comments just so we're all on the same page.

If you've read this far in the thread, you can tell there are many different factors to consider when designing these piers. If you've read my posts in this thread, you should understand that I've been making certain assumptions on the gear being analysed. I usually try to state what all of the assumptions are, but sometimes they get missed.

So what I've been assuming is that the mount and scopes being supported by the pier are completely balanced about all axes. Think I did mention that a few times, but it's important to reiterate. I'm talking about mounts like EQ6 type scale or smaller. Ones that won't take huge eccentric loads, and won't support bigger and heavier scopes like, I dunno, 12" or bigger?

It has been pointed out to me that some of the larger arrangements may have big eccentric loads - whether it's due to the mount, scope, or cameras supported. These will induce bending moments in the piers as the mount/scope moves, and the pier size will need to be bigger to limit that.

Bottom line... it is very, very, very difficult to accurately analyse all different load cases and combinations on these there. You only need to forget one little thing, or make one assumption, and the detailed analysis is useless. It's not worth skimping on the pier size trying to get the absolute minimum size you think you need.

[hatman]

rally,

Whilst i would respect your opinions greatly you really need to re-read the comments I have made again, slowly.

I am in fact referring to imaging as well as visual. It is YOU that has missed the point most respectfully.

Your own comments have more than clearly demonstrated my points and given weight to my concerns.

There are far too many variables that not have been addressed or mentioned.

I have raised several questions which still go unanswered by yourself and others and which play such an important part in how the "test results" were arrived at.

I am happy to back any printed test results as long as they are scientifically validated and all I am asking for is this before making decisions.

I am sure others share the same view.

The request for answers would be considered more than reasonable before investing money on someones unsubstantiated hearsay.

For some reason the simple questions I have asked are worrying some people and being ignored or criticised which really surprises me being that we are all astronomers and should not automatically accept claims without scrutiny.:question:

Rather then being on the defensive, just address the questions raised.

I am sure many people would agree that the answers to these unbiased and neutrel questions have a bearing on how much importance is placed on these so called results.

I again ask the following;

1. Have the figures been obtained from gussetted or ungussetted piers?

2. Were the test results based on a pier attached to the ground and with a mount and scope on? or

3. Were the results based on a pier that was free of outside influence; (ie was it tested independently in a machine or device that could scientifically measure any defelecture or was it the backyard method?)

4. How were the measurements taken and determined? By eye, a computer, camera or stanley tape measure or some other method???

5. rally brings up a very good point about seeing. What were the seeing conditions when these tests were done?

Rally himself indicates that this could make a difference.


This whole thread is based on some figures that were published without scientific backing and without adddressing the above very important facts that without any question DO INFLUENCE THE FIGURES.

Until someone can answer the above questions I think it is silly to place too much importance on figures that have no scientific validation.

In my own practical experience (yes I do have some like other astronomers) :) I have seen no deflecture in any piers under 1 metre high. Incidently these same piers have gussets and generally have a wall thickness of around 6 mm. I have also seen first hand, multi hour images on the same like piers of many hours duration and there is simply no perceivable defect in the images attributed to any pier deflection.

From my observation the wall thickness means stuff all on piers under a metre high, contrary to unproven claims.

The proof is in the pudding as one would say and I have seen no evidence of any pier deflection in any multi hour images that I have seen. Perhaps someone could provide an image that purports to be pier deflection and with it, details of the mount, scope and pier as well as other details that could play a part in the image making.

Seeing that pier deflection is unproven and so far undetectable in multi hour images, just how far do you want to go and how much do you want to spend to get the NASA standard.

This is just a whole lot of rubbish. Before accepting unvalidated figures quoted as gospel, one really should ask the questions and check the facts before being critical of others. When the simple answers to the questions I have asked have been answered, then we can all have a deep and meanigful discussion:D

[coldspace]

I also think that if you look at many,many award winning images posted around the place alot of these were done using tripods as well in the field. So I think that a well made, gusseted steel pier fixed properly to a solid foundation would have far less deflection than some of these tripods been used.

I will have alot more chance of getting a great image with my gear using one of those well made Pegasus piers that I have installed in my observatory.

My mount, alignment, focus, and I will say focus again, balance, and I will say balance on all axis, seeing and local winds, and user experience are going to let my imaging fail well before my solid professionaly built Pegasus pier will.

99.99% of imagers that use steel piers including some of the worlds best, including quite a few guys I know that get images published are using well made steel piers some home made and some professionally built like the Pegasus pier and they will all tell you that most image flaws are due to seeing, focus and I will say focus again, user error more so than microscopic pier movement.
I have never been told that "last night was crap my images failed because I had 3 arc seconds of pier deflection on every image I obtained yet my mount, focus, balance,seeing,local winds were all perfect and mister worlds best imaging guru was at his best last night".

I have always been told that " My images failed last night because focus, weather, seeing ,mount,balance were having issues,or mister average stuffed up".:lol:

Unless we can setup on top of Mt Mauna Kea or the moon for that matter then a well made gusseted steel or concrete pier is going to be more than enough for 99.99% of people.

I am more than happy with my Pegasus pier which is way better than my Giant field tripod that I was using.

Just my 2 cents worth.

Matt.

[hatman]

Well said Matt. :thumbsup:

I still haven't had the questions answered though from others and with all the hullabaloo it is sad that when challenged with direct questions, asking for scientific validation there appear to be no answers forthcoming on the subject.

Where are those images that show pier deflection or movement?? Heeeellooo..................?

Silence is a loud answer! :zzz2:

The point is well and truly proven that 5-6 mm tube has no issues at all, contary to ill informed belief/opinion.

I am curious though, I keep hearing about 12 mm wall thickness steel tube so perhaps someone can please explain where you can get some of this magical 12 mm tube. Please don't say the scrapyard or Council depot because the chinesse are buying every bit of scrap steel around and forget the Council, you have to go through so much paperwork it is not funny.

There seems to be no tube of any sort anywhere unless it is 4". The commercial major suppliers of steel only sell tube up to 6 mm wall thickness which, you guessed it, comes from China! (cheapest cost tube) You can get the aussie steel but be ready for the 30% increased bill at the end.

You get what you pay for though and that is what people need to remember. The commercial pier and mount makers build to what they think the market can afford and tolerate. If you want a NASA quality pier that is gold plated with anodised fittings and all the gadgets, then you will certainly pay a lot more. The sad thing that has crept into our society is that people don't value quality anymore and all they want to do is pay the cheapest price and cut corners and then whinge when it goes wrong or something doesn't fit.:screwy:

For what it is worth the the local group pier is good but then you have the Pegasus pier which is obviously far higher quality. I sure if you pay enough dough you can get a better pier again that has the stubby holder, electronic anti earthquake control device as well as the 4" plasma screen and DVD player thrown in but you will pay for it.

I am not sure if it is Pegasus or the other maker but I hear on the grapevine from a reliable source that there is apparently some testing going on by one of the makers to provide some genuine accurate figures on tube flexure/plate flexure (if any) for that pier and once completed, apparently will be published for that pier so I await with great interest on the results, not that it makes any difference.:)

[troypiggo]

I don't know about the other guys, but I didn't take too well to your 2 comments about what rubbish you were reading. Perhaps the tone of your posts has yielded the replies they deserve.

[Paul Haese]

I got my pier made up with that wall thickness. In fact I think it might be 13mm thick. You can get engineering firms to get hold of that material. I paid nearly a grand for my pier. I don't know where you got this information from but clearly this is incorrect. I also know someone else who has a 2.1m length of this for his pier. I hope you are not suggesting we are not telling the truth???

I do agree though that most errors come from everything else other than my pier. I think that I would need a considerable amount of equipment to cause any deflection that would affect my images. Just my opinion of course and anyone is free to disagree. Images tend to be the proof.

Carry on chaps.

[Waxing_Gibbous]

Pleas don't think I'm being fatuous. I'm not.
The best pillar combo I can think of is an old railway rail, or small 'I' beam, pounded into the ground with a fence post pounder, surrounded by formatube and filled with concrete.
Typically the rail should be driven into the ground 60/40 (60% below ground).
I rather discovered this by accident while trying to remove one.
It required a 30 tonne excavator and 400 grammes of TNT.

The advantage to this set-up is that rails / beams are practically free, the concrete required is minimal, and the end-result is almost immune from vibration short of an earthquake.
The disadvantage being one needs a fencing contractor with a tractor and post-pounder (about $80 p/h) and a bit of space for him to move around in. The bloke who removed it remarked that it would probably only cost $100 - $150 to put in but it was costing about $400 to get out.
Were the pillar not in a completely veiw-less position, I would have used it in a minute.

[spinnaker]

Agree totally!

[hatman]

"Ditto" Troypiggo.

The rubbish being referred to was with regard to the blind faith some continue to place in unproven pier deflection figures. Listen mate all I want and a lot of others is for the answers to really simple questions that is all. Don't go and get all precious on me now. :)

There have been simple questions asked that still go unanswered, and whenever they are asked, attract criticism and insult. Why? Thats unfair and not in the spirit of this forum.

After such a lenghty discussion and so much reliance on unsupported figures of so called pier flexure, one has a right to ask questions that reveal the full facts and should not be riduculed.

When these findings/figures are placed under simple scrutiny there is a reluctance to substantiate the figures? Why?

Feels like we are in a UN climate change meeting and Penny Wong is running it.

All I have asked is simple questions in a non offensive way and I would ask respectfully for someone to provide some answers.

If no one wants to answer them thats OK but the reluctance to address simple queries on test results only raises more scepticism on the figures quoted.

I am sure I am not the only one who challenges them.

Instead of firing bullets lets respect each others opinion, get into the nitty gritty and tease out the questions, get answers and come to a well informed outcome. How about it:question:

[bojan]

Well, hatman, this is because no-one knows for sure, or so it looks like.
Of course, one single rule applies here and that is: bigger the better, you can not miss that way.
But, unlike some others on this forum, I can not afford to spend un-necessarily huge amounts of money and effort for something that is not really needed for what i want to do as amateur astronomer... I want to spend my available resources rationally, I want the biggest possible bang for a buck and obviously I am not the only one who thinks in those terms.
After all this discussion, and based on my so far experience with piers I have build in the past, my conclusion is that *anything* fixed is better than removable tripod.
Why?
1) Because it is fixed. No need to align every time the telescope is used.
2) Also, because the mount and telescope are balanced, so there are no changing lateral loads, even during very long imaging sessions.
3) Then because we are not in positional astrometry business, where the stability of mount over plolonged time periods is paramount (and then, who would use EQ6 or even much better for this job anyway???
4) And when it is windy, it is not good to do anything anyway. No mount (except the most massive ones) could cope with this - pier will be the tiniest problem in those conditions.
5) when there are people walking around, it is show time and not imaging or measuring session.

Everything above this is just a gross overkill, un-necessary spent money and/or effort.
Well, I guess this is not the problem for people who could afford it.
But the importance of correct information is essential, still.
Because from A to B you can get in Ferrari or in Datsun 180B, in the same time, with the same effort. I would always choose Datsun 180B, and the rest I would spend on better things (even beer in good company is way better)

[troypiggo]

It's not that nobody knows. It can be calculated providing every single dimension, weight, size, eccentricity, wind load, slew speed, founding conditons, acceptable deflection limits etc are all quoted. It can all be modelled and analysed using very simple priciples and software. The problem is noone ever provides all the information, and all of the different combinations are almost infinitely variable and consequently so are the results.

[troypiggo]

Putting a smiley face after saying I'm precious is not really helping your cause in getting questions answered. By me, anyway.



I'd like you to cite where I criticised you and insulted you. If you're referring to someone else, was that before or after you said "what rubbish" you were reading? Do you think perhaps your tone and choice of words may have been what attracted it rather than the questions themselves?



All figures I quoted are fully supported by engineering principles/calculations/analysis. I said what I based those on in my posts. I have not ridiculed you.



You are probably mistaking an inability to substantiate the figures with the motivation/incentive to do so.



I find this a bizarre statement on many levels.



Sorry mate. They are not simple questions to answer because, as I've said repeatedly in nearly every post I've made in this thread, the variables contributing to pier deflection are vast and wide, and not one person, including and especially you, have provided enough to carry out a detailed analysis to be accurate to the sort of levels required for the magnitude of deflection limits being set. It would be utterly impractical to do so. You'd be better off being conservative and oversize things to take care of the things you haven't or can't allow for.

I would also dispute that you have asked the questions in a non-offensive and respectful manner.



Again - you are probably mistaking an inability to substantiate the figures with the motivation/incentive to do so.



I won't be firing any more bullets in your direction. By bullets, I assume you mean responses?

[hatman]

Thanks guys for taking the time to respond and I appreciate genuinely your comments. A I have learned a lot from all the discussion and and hopefully have been able to provide something meaningful. Do you think the idea of a stubby holder on a pier might take off?

As a matter of interest, Is there anywhere on the IIS site where one can view images of members observatories or setups specifically or is it randomly spread throughout the different forums/site? It would be inspiring to see what others are doing with their setups, perhaps under a specific page dedicated to images of that?

[bojan]

I do not think so..
The problem is, people do not know really what is important so they want to calculate in everything that comes to their mind, which is of course waste of time and effort.
Engineering and design in general are all about making a right compromises, to satisfy some minimal requirements for the task.
My concrete pier (16cm in diameter, concrete filled PVC tube and reinforced with 3 1 metre threaded rods) was quite adequate for my 10" reflector. True, I did not use it for imaging then, but all problems I had then were due to everything else but pier (mainly it was the way I mounted the Newtonian tube on the DEC flange, I had a weak point right there.
But for visual, it was perfect.

I plan to do it again soon in my backyard, and will certainly post my assessment of how it behaves... it will be biased report of course ;)

[troypiggo]

My point throughout this whole thread has been that it's not worth designing the absolute smallest, most economical sized pier. It can be done, but it's not worth it. I wouldn't do it if it were my pier. And I'm a structural engineer.

I totally agree with you about eng'g and design being about making the right compromises. That's what I've been trying to say, maybe using different words, but the meaning/result is the same.

[Paul Haese]

Troy, I was just wondering if you calculate my pier defection if I supplied the information? I will understand if you don't want to, but I thought it might be good to work the numbers with an actual pier in place and working.

height: 1300mm
diameter: 200mm ID
Wall thickness: 13mm
Base plate: 12mm at 450mm diameter and flange welded
Gussets: 4x 150 base tapered to 20mm at distance of 450mm from base and welded both sides of gussets.
Top Plate: 12mm thick and welded on top of pier.
Pier support: 8m x 1m x 1.1m reinforced concrete and support bolts caged to a depth of 600mm below concrete level.

If you need any more info please let me know. I think that is everything I specified at the engineering firm.

This should be interesting to see what comes of it.:)

[Moon]

Paul,
The CN link will give you a rough idea. In your case the closest match on the table looks like 48 inch high, 8.4 inch ID results in a 1.5 arc sec deflection if you push on it with a 5 pound force.
Bump it up to 10 inches = 0.6 arc sec
Bump it up to 12 inches = 0.0 arc sec.

Of course this is very rough, but it gives you the order of magnitude which is all you need to know when deciding how big the make the monster.

James

[troypiggo]

G'day Paul. I've PM'd you.