Pier Design

[avandonk]

Here are a couple of vids with some insight.


http://www.youtube.com/watch?v=AOeLnecOMeg&list=PLDB5EF0B84CBB0438

http://www.youtube.com/watch?v=LI3PkC0aT2E


I can tell you what he is keeping back.

This vid helps

http://www.youtube.com/watch?v=ahm_WMW1xqo

Bert

[Steffen]

Thanks, I enjoyed those!

Cheers
Steffen.

[avandonk]

The simplest way to think of a pier and its load of mount and optics is a vertical spring with a weight at the top. This system will have a resonant frequency that can be energised by any energy source such as a motor.

To minimise resonance of a simple cantilever parallel beam, the tapered ribs will make the system have resonances that are multi modal. That is it has many more possible resonant frequencies so making the amplitudes far smaller and damp far more quickly.

The dull thud says it all!

Below is an image of my pier.

Bert

[wasyoungonce]

Hi Bert....these are exactly the videos I referenced my pier design to.

I even emailed them on costs...no reply. My ribs went a little too high but I can fix that but the principle of not requiring the adjustable "rats cage" is appropiate and adding longitudinal ribs lower flexure by holding in tension and also alter resonate frequencies.

I went thru the pier calculators on CN but could find no reference to the effect of adding ribs (past my engineering skills...although something to do with the 2nd order/derivative of momentum)...although knowing they would improve the performance as mentioned. I like your work...noting the placement of the pier footing bolt holes...near the ribs! Well done a lot of thought into that I can see.

I too recommend these videos for people but the difficult thing is finding appropriate pier pipe...now that that's the key!

[bojan]

Yes, he is spot on on almost everything (except the part where he doesn't care about DIY.. and I think he is mystifying his work a bit too much, which is consistent with his business model)
However, sometimes you have to compromise in designs.. I know I did. The pipe is definitely the main issue.
And bigger and fatter, the better.
http://www.iceinspace.com.au/forum/attachment_browse.php?a=130093
http://www.iceinspace.com.au/forum/attachment_browse.php?a=105341

[PeterHA]

Whilst the piers with ribs are one good approach, the explanations and expressions used in the videos are to some extend just wrong and do not reflect the fundamentals of mechanical engineering principles.
Some is there to mystify...

The analogy of a vertical spring (pier) with a weight on top and induced vibration is spot on.
Some ways to tackle this are:
- dampening the vibration with suitable design, a path often used in the automotive industry as it saves weight.
- modifying the design to shift the resonance frequency

I also like bigger is better, often true in respect to shaft sizes and piers.

[avandonk]

When I said I can tell you what he is holding back I did mean his reluctance to give any real info and the mystification of 'energising' the pier.

Any good design takes into account a balance to match all components. Two extremes are a flimsy pier and large mount and optics or a very solid pier with a very light mount and optics.

In my case I wanted to maximise the travel past the meridian to not have to do a meridian flip. My western sky has far more light pollution than my eastern sky. So the size of the pier near the mount should not be too large. I can get more than two hours past the meridian without the mount or optics hitting the pier.

It is really all about minimising resonance. Putting sand or any other material even lead shot into the pier only adds to the 'weight' on the 'spring' and does more harm than good.

The simplest way to minimise resonance of any pier is tapered ribs that are continuously welded to the pier tube. The bolts holding the pier to the concrete base should be very close to the ribs and as far from the pier tube as practical.

Bigger is NOT better! Some of the worlds loudest bells weigh many tons!

I designed my pier about five years ago for the EQ6. I was just surprised to find a video that endorsed some of my ideas.

There is no 'correct' answer. It all depends on the configuration of your setup.

Bert

[wasyoungonce]

Well I had my pier ribs caterpillar welded along their length.

I had this done using the theory this would create node points along the pier tube...... altering natural resonance to shorter wavelengths.

Well that was the plan.

[avandonk]

If you take this approach to its limit then a continuous weld will produce an infinite number of nodes!

There would most probably be not much practical difference.

Bert

[Logieberra]

When I built my pier - I received the same advice from a classically trained engineer: 'the terminology that this UK guy uses in this video are off'. I went with my engineers specs instead, used large SHS and have never looked back.

[wasyoungonce]

Thanks for the info Bert...didn't really think of this..no great loss on my pier but I suspect you are right.

[mill]

Hmm the spanner in between the pier and the floor is to get rid of foreign resonances :P
I have my own pier made by using a normal pier (steel) and putting a 12" dob part around it and then filled the gap with concrete.
No resonances when i kick it, just a thud, i have to try it when making a long exposure and see if it resonates.

[Steffen]

I suppose piers can be too massive, too. If a pier is much heavier than the load it carries then it will efficiently shield the latter from ground vibrations, but any vibrations generated in the mount (or wind grabbing hold of the scope) will reflect right back. It will then be entirely up to the damping capabilities of the mount-scope combo to kill them. I don't think mounts with scopes and counter-weights attached are very good at that.

I reckon the best pier would be:


statically rigid, i.e. not flex with shifting static loads
matched to the load in mass, so it can eagerly take up vibrations produced by the load, instead of reflecting them
turn the received vibrations into heat as quickly as possible, transferring as little as possible back to the load
isolate the load from ground vibrations


Number 1 seems to be what most people go for. Here bigger (heavier) is better.

Number 4 can be achieved with large mass and/or soft, absorbent suspension, which conflicts with 2 and 1, respectively. Therefore I think 4 is best kept off the pier and left to a very massive base.

Number 2 is probably the least intuitive to grasp. Matching a pier to the load is not easily done without experimentation. A super-massive pier will reflect vibrations coming from the top just as badly as a light-weight insert ("rat cage").

Number 3 is hard, vibration absorption is not easily achieved without violating number 1, rigidity. Spoiling the modes of the pier by welding on ribs etc is a bit of a cheat, but effective if it makes the pier oscillate at a variety of frequencies each at a lower amplitude. It also seems obvious that the pier should be dead at the resonant frequencies of the mount/scope/counter-weights contraption. How does one determine those? The often cited lead shot fill sounds like a good broadband dampener, and the large mass still wouldn't lead to reflections at the pier head since the lead shot is only loosely coupled to the pier. All energy not dissipated here will go right back into the load, the super-massive base will mostly reflect it.

I would conclude that it is impossible to design and build a good universal pier. Too much depends on the intended load and its dynamic properties. Some things should be avoided for sure, though, such as guitar-string piers (unspoilt metal tubes), super-massive piers and rat cages.

Just my 2 cents…

Cheers
Steffen.

[bert]

Another factor in designing a pier:

If you use a lot of mass to control damping, you will get a lot of thermal inertia, and that means local seeing effects as the pier stores heat and releases said heat through the night.

This pretty much precludes using concrete for a pier.

Like logie, I am a big fan of square section hollow piers, they are just easier to work with.

[Logieberra]

Biggie loves SHS :)

[kinetic]

My 2c,

I've done exposures like this at high FL with a 12" OTA sitting on
a piece of 2" waterpipe for a tripod pier.

Yes 2" waterpipe........

It has traditional outrigger legs, gusseted lower down, see the screengrab
from dome-cam.
I think some pier designs are a bit overkill.
Sure, if you are going to need pinpoint , repeatable positioning
at high FL, night after night, maybe even via remote, you will need
a pier that minimises all of the factors, like pier flexure, vibration
dampening, expansion etc.
If your exposures are 1 hour long, sure flexure will come into it.
But how much would seeing and tracking/guiding/PEC have had an effect by then also?

The seeing and focus were the factors that determined how sharp this image was.
The pier flexure didn't come into it
The expansion didn't come into it.
The vibration didn't come into it.

thoughts?

Steve

[Kunama]

I was very impressed with this pier Logan. I doubt that you will ever need an upgrade..

With all this talk of resonances etc, I wonder how many of the imagers actually tap, kick, rap etc their piers during imaging ....................

[Steffen]

Probably very few of them ;) The vibrations to dampen would be mostly those produced by stepper motors and drive mechanics, and the ones travelling through the ground.

Cheers
Steffen.

[bojan]

.. Not a single one. Not me.

[RobF]

I think Steve has said what I was thinking. Many of us with EQ6s sitting on a pier probably have a hell of a lot more trouble with backlash and PE than movement from the pier. I suspect even a stable tripod has minimal influence on the final result when you still have mount issues to guide out.

For those with permanent observatories and PME/AP mounts, MIGHT be worth considering, but really, if you rpier is isolated from floor vibrations do you really think vibration from the mount motors is going to resonate that badly? :question:

[wasyoungonce]

Of course not...but basic physics and engineering dictates we can minimise influences cheaply and easily during design that all combat successful imaging. Influences that are harder to fix once built.

Worst case...a pier that has a natural frequency harmonic in sync with applied average wind force...in other words this causes it to hum or resonate...which in turn adds amplitude to the natural freq....just sayin!

Motor resonances...no these are not an issue at play here.

[el_draco]

The thermal mass of a pier is a significant issue. Concrete always bothers me and I was recently advised to fill my new pier with sand. Soooo much extra mass!

The system I am currently assembling has a 33cm diameter steel pier and to dampen this down I'll be using expansion foam... Next to nix mass/thermal retention, but past experience has shown it to be excellent in dampening vibration.

Cow of a problem all round really. Hot concrete slabs, massive equipment and then we wrap it all up in a metal dome/oven? :screwy:



I stubbed my toe on mine ...... does that count? :rofl:

[avandonk]

My reason for posting these vids was to show how many fallacies become gospel in the amateur astro community. These vids do go some of the way to dispel the myths about pier design.

My background is in Applied Physics and we prefer to analyse any system from first principles with a minimum of assumptions. We then take the simple step of measuring this putative system to see if our ideas are valid.

I only wanted people to think about what was happening with their system.

Is your continuous poor astro photographic 'seeing' due to pier and/or mount oscillations?

It was never about denigrating any system.

I tightened the bolts holding my pier to the concrete block far higher than I did initially. The rms errors in guiding have nearly halved to better than 0.5" ! It could be that I have suddenly been blessed with better seeing. My exposure for guiding was two seconds so this should eliminate seeing. Only time will tell.

Bert

[avandonk]

If I touch any part of my optic train even very gently while watching the guiding in MaximDL the corrections become larger and oscillate. Wind has the same effect. It is only about a few seconds of arc.

Drive motors will start bad oscillations when there is any tiny resonance in the system between the motor and system.

Remember any unwanted vibrations/oscillations ADD to the seeing and inherent mount drive variation.

The worst case of wobbles I had one night was when a possum decided to jump onto the telescope from the observatory wall. It was a small ringtail possum. I had to go out and coax it to get back in its tree. When the RoR roof is open possums use the exposed tops of the observatory walls as highways.

Bert

[RobF]

Thanks Bert. Interesting to hear how top end gear behaves at the extremes of performance. And mental note to self for future too - steer clear of possums :lol: