Pier Rat Cages

[Poita]

I've started this thread up so as not to hijaak the other thread on pier design.

I don't believe in putting an adjustable plate/cage on top of a pier, purely because I see it as unnecessary as you should get the pier levelled and aligned during the construction phase, and the mount-head should have enough adjustment within it to pick up from there.
I think adding extra materials, complexity and $$$ rarely makes sense if there is no real payoff, or can be avoided.

I am interested from a purely knowledge gaining point of view though whether they weaken the design in enough of a way to affect imaging, or or example in lengthening the damping down time if the mount it slewed quickly (like when traking satellites) or focusing etc. (Just like I was interested in actual Losmandy vs Vixen bar compares)

Or are piers just generally over-engineered, and way exceed the specs required?

Has anyone got any empirical evidence of a cage on top of a pier causing problems with imaging?

[Poita]

Oh, and Brendan, I've taken it outside before they throw us out :P

http://www.iceinspace.com.au/forum/showthread.php?p=855252#post855252

[bmitchell82]

And just to add to this thread. If you have read something somewhere that somebody had read something of something else that their fathers brothers sisters twice removed grandmas nieces best friends dad said. Please keep it to yourself as it means squat.

Keep it to empirical evidence. Your own actual results and a method that is scientific and can be peer reviewed. If you have actual formal training and experience say so. if you have no experience in the area be prepared to be challenged!

That aside :) I look forward to some cold hard steeely evidence!

P.S. The youtube vid that relates to "the rat cage" the guy is a fabricator not an engineer, there is a vast difference and you can tell by the language he uses as engineers find it really hard not to use engineering expressions and explanations. He is hyping up his product, does it work yes how add more steel! I could go into an analysis why some of the things he does are right and most of them are just hype but :) not today!

[alistairsam]

It did sound like a lot of hype.

[Zaps]

These rat cage things are totally unnecessary for EQ mount users, so why use them? Especially since they weaken the entire footer-to-optics system. Why willingly introduce that weak link? Because others misguidedly do so? I hear stung egos at work.

[koputai]

No, I'm not a mechanical engineer, but as has been said, EQ mounts don't require it, so why do it? Disregarding whether it's as stiff or not, it adds more work, more cost, and more fiddlyness to the whole thing.

Sure, if you have an alt-az mount on a wedge then you might need it, but for an EQ, you don't.

Think of a pier with a top that's not perfectly level. Now transpose this plate to somewhere else on Earth not too far away, and it will be perfectly level. All the adjustment to get perfect polar alignment is there to use, everything below the polar axis is irrelevant.

Cheers,
Jason.

[Poita]

I'd like to focus on do they compromise the mount rather than are they required.
I don't see the need for one, but maybe some mounts have connections that require a raised plate, or you are a wedge user or already have a pier with a top plate.

Question is, does it cause a measurable/visible detriment to imaging, and why?

[bmitchell82]

Zaps you are making this very easy for me to prove that you do not know what your talking about and furthermore to the situation are inadept at comprehension and reading. For your sake please go back to my posts and read what i said without letting your brain cloud what it is that you think you are reading.

You hear stung egos at work. No. I have mearly proven to you in a mathamatical form (yes nearly all science is mathmatically based) that the flexure and "Vibration" that you are supposedly quoting as gospil does not exist. This you cannot accept and persue to see if im actually telling you truth.

You are just quoting from what you think is truth, be it read on a forum somewhere, or a magazine. I have known my university lectuers to get it very very wrong!

Basics 101 of research do not pin all your tails on the same donkey! Its bound to buck you off at the most in oppertune moment.

We are not saying that the top adjustment plate does not increase the complexity of the mounting system because it does. What Peter and I are asking is does it actually make a difference to the end result. To which my reply is if done correctly NO IT DOES NOT, I stand by this 100% untill somebody can show me other wise. If you put a few piss ant bolts there yeah it will matter. If you don't know what your doing, yes it will matter.

So please Zaps before you post again stop breath think and formulate a logical response with some proof, theory or something other than i told you so.

This thread is about proofs and peer review to get the truth not just another arm chair expert/engineer i think this is what it means, if you want one of them go elsewhere!

B. Mitchell

[Poita]

I think Zaps is right in that most people put one in there because they see them on other commercial designs, and they would be embarrassed to pony up and say "I put it on there because I didn't know any better".

So far everyone seems to be in agreement that it would be silly to add one to a pier unless there was no other option.

However, many piers have them, so are they compromising the mount?

Now I hear about people 'ring testing' the mount by hitting it on the side with a hammer.
Brendan points out that we rarely do this while imaging, but that doesn't mean it is a totally pointless exercise (though it may be). We often test things in extreme ways to find out their potential strengths and weaknesses.

So what does the hammer test tell us?


I know if I walk near my EQ6 on tripod that I get vibrations that affect the image, I can quantify that quite easily.

Has anyone tried imaging with and without a top-plate and gotten a different result? Or has anyone managed to induce vibration in one with a top-plate that has been fixed by removing it?
Or has someone got the math that would show some potential problem with the top-plate.

Again, I think they are a stupid idea (though I could be wrong), but I'd like to know how much they affect things, and under what circumstances?
(I had similar questions long ago about vixen vs losmandy bars, everyone told me losmandy was better and stronger, many people spent good money upgrading, but amazingly I couldn't find a single piece of empirical evidence of its superiority in use).

So do the rat-cages screw things up or not? If you have a 3m long scope and windy conditions, could it make a difference?

Brendan, you have said that they don't if you know what you are doing, can you elaborate on what knowing what you're doing entails from a design standpoint?

[Zaps]

I repeat the only logical question worth asking:

[rally]

Brendan,

Vibration in mounts and their optical systems are quite a real phenomena.

I was exposed to a significant vibration problem with a large telescope mounted on a P-ME.
In order to counterbalance the scope it was considered easier to make one large counterweight and add that to the end of the counterweight bar rather than many extra weights all the way along the bar where they served less effectively - which would have meant extra mass was needed and that was the purpose of the exercise - to keep the total mass contained.
The single large weight introduced a significant vibration (probably a resonance) into the system that was unable to be controlled even by AO.

The mounts motors are effectively being pulsed or frequency controlled and other components such as AO and fans introduce vibration into the system, if a component or arrangement of components end up in resonance then its very difficult to dampen the vibration in some cases - noticeably that one.

Wind gusting and Wind induced vortex shedding vibrations are also a cause of vibration in telescope systems.

Rally

[Poita]

I think we have already answered that, (i.e. don't use one if you don't need one) however it isn't the only question worth asking.

For instance if you already have a cage on your pier, it is worth knowing whether any advantage exists by removing it. So knowing if an adjustable plate actually can cause problems makes it a question worth asking.

Plus I'd just like to know.

[GrampianStars]

Answer = Absolutely "NO"
As there is "NO" measurable adaptor plate movement there can be "NO" visible or measurable detriment to imaging :thumbsup:

p.s. I had to add the adaptor plate as the Pier top buckled when welding it on, therefore there was "NO" flat surface for mounting a scope..... :rolleyes:

[bmitchell82]

Like i have said a few times now.

Using M16 bolts and holes that are 18mm is pointless, there is ... to much air

Using M20-24 bolts that have minimal clearance and not setting the top plate too far away (like i do see on on some piers). My opinion is i wouldn't go further than 100mm difference between the plates.

if you spread the bolts sufficiently eg 300mm+ and use a decent plate 12mm or so the top plates are sufficient enough to withstand any forces applied to them and transfer these forces out to the bolts.

Its the same premise why truss tube telescopes work so well. Intentional design putting elements into their strong points eg. Tension and compression. Geometry plays a key point here in putting mass where it is required and taking mass out where it is not.

Like i have said before a pier I designed, fabricated and installed clears the ground by 150mm or so... there are no issues running at 2000mm FL.

On your point of testing your rig to extremes. If you really wanted to do a test that actually has merit not just oh i hit it with a hammer which will never ever ever occure (your test has to be meaningful).

Get a spring weight and hook it onto the top of your pier. Maximdl reports in seconds of arc so you can grab the pier and quickly put 50kgs worth of force at the top of the pier.

Take a reading of the arc second error the guider throws up. upon release measure it again. do that 30 times to gain a good statistical average. That is an extreme case and is measuring your static deflection which is the maximum the pier can move at a said force. This is how I theoretically determine my "vibration" in my normal day to day work by looking at what force produces what deflection.

As for dynamic vibration thats a whole new ball park. It takes into account Natural frequency of the structure, Frequency of the dynamic force, Dampening and a whole host of other things.

Think of a combustion engine sitting on a flat floor with an out of balance crank shaft or even your car the vibration you feel is dynamic it changes with respect to time.

So thats my view point on the dynamics of it and a design that will work.

B. Mitchell

[Poita]

How do you know there is no measurable adaptor plate movement?

[avandonk]

For very large piers leveling adjustment at the top can be useful. It does need to be very robust to not introduce a source of flexure under varying loads. This unfortunately can limit the amount of travel past the meridian before a flip is needed.

There is no correct answer.

This is my solution and it works for me. I never need to do a meridional flip as I can go more than two hours past the meridian. My western sky is more light polluted than my eastern sky. I can adjust the level by the eight bolts at the bottom of the pier.

Resonance is the major problem if you are not careful. The tapered gussets on my pier give the pier a low 'Q' for resonance.

Bert

[Poita]

This is the part that interests me the most, I trialled a very large mount with a lovely smooth PE, but it had a vibration in the images that wasn't there with my EQ6, and I didn't have it long enough to track it down.

I think mounts and piers generally are way, way stiff enough to handle issues of flex. But with autoguiding and PE correction etc. I wonder if there can be issue with resonance (not sure if I am using the right term) in the pier, and if a top plate can make that worse, and if it can, what design considerations would you do to combat it?
I know people use tapered gussests to minimise it on the pier itself, but what about on the plate?

[bmitchell82]

Thankyou for your reply Rally,

So your issue was high frequency vibrations forming a dynamic resonance. Is this a issue of the pier or is this a issue of the mount?

To me it would be more a case of in the pier, with adjustments and tolorances. Possibly due to the telescope being sufficiently big enough coming upto the PME's rated capacity and hence any dynamic vibrations start to affect.

If this where a real case and it happend all the time wouldn't mount design take this phenomena into account?

I also think that placing the CW further out would make the issue worse not better. Yes the leaver arm is bigger hence less weight but the ability for a weight further out where your natrual frequency decreases and hence has more chance of coming into resonance.

It is a tricky one high frequencies because they are hard to measure unless you have some really expensive tools.

[GrampianStars]

The new peir under construction for another scope is reinforced 500mm round concrete with 4x22mm SS rod embedded 3ft. and a plate on top.It will be super! :thumbsup:

[koputai]

What has wetting your pier got to do with it?

Are you sure you're an engineer Brendan? I know lots of them, and none would EVER confuse 'Damping' with 'Dampening'. :)

Cheers,
Jason.

[bmitchell82]

Tapered gussets do three things.

Geometrically changing your section
Increase the stiffness of your base plate
Changes inherit natural frequency

1. Geometrically changing your section.

I'll quote square sections as i know the formulas off the top of my head

Second moment of inertia of a Square section BD^2/12*(A*h^2)
B= Bredth
D= Depth
A = Area
h = Hight from the neutral axis

For SHS sections the second term is negated as it is doubly symetrical.

by adding gussets you are in term adding depth in turn that relates to a higher Ixx value which is the measure of Stiffness.

2. The gusset goes into tension and compression hence stopping the base plate from bending (often used in structural steel connections to increase the capacity of the connection).

3. Is a byproduct of the previous two points. the stiffer the section the higher the natrual frequency. it can be simplified and calculated by the use of matrix manipulation in the form of eigen values. This can shoot you in the foot as if your natural frequency matches the induced frequency (motor mounts fans etc etc) the sinusoidal waves are coincidental and hence additive (much how noise cancelling works it throws a inverse wave to make the -ive appear as the + appears 1-1=0. 1+1=2. thats what the whole meaning of resonance. the only way you can kill resonance is dampening. You see the little rubber doova on a tennis racket..... :) Dampening of the strings.

[bmitchell82]

I never said i was good at spelling :D numbers are my game!

[Poita]

Most browsers can have an in-line spell-check now, I must turn mine back on so I look less like a doofus!

[Poita]

Brendan, can you also combat resonance by making the pier less symmetrical?

[koputai]

So it's full of water?

[bmitchell82]

Jason its a method of damping :) have the pier completely submerged in viscous fluid :D

Your a funny bugger ;)

[bmitchell82]

Answer to your question Peter, It would possibly help in the axis with less stiffness, but would still be present in the stiff axis..

[Poita]

Speaking of viscous fluids, what about the practice of filling mounts with sand or oil or other 'non rigid' materials in an attempt at damping?

[allan gould]

I find this thread slightly be-musing. When I put my pier in for the observatory I realised that since it was located on ash residue from mining and clay it was unlikely to remain level either after it set or with the passing of time. So I buit it as shown in the thread
http://www.iceinspace.com.au/forum/attachment_browse.php?a=72246
It has proven its worth over time for ease of adjustment in its initial setup and subsequently as it appears the whole shebang is slowly sliding down the hill due to the soil constitution.
I dont experience any resonance with my G11 or heavy scopes imaging at 2500mm for 15-30 min.
The maths may be there to say it should flex, resonate or whatever but my experience says this is not the case.
Its a bit like bumble bees mathematically proven not to be able to fly.
Just my 2c from experience.

[bmitchell82]

Oil or viscous fluid would help with high frequency vibrations and as such could combat the high frequency experienced in Rally's situation.

[Poita]

This was actually why I started the thread, my own experience is similar to yours. I had the same thing when using Vixen rails as well, everyone told me they were spaghetti and I should have losmandy ones, but I never had an issue, and suspected the vixen rails were more than stiff enough for the loads experienced with most amatuer gear.

My gut feel is that when it comes to piers, rails etc. that they tend to be capable of well in excess of the strength required, but there is so much opinion around about these topics and people spend big money sometimes on them, that I thought I'd like to know some actual factuals.

[bmitchell82]

I think the dyanmic resonance we are talking about are in the catagory of 40-50kg telescopes + all their equipment.

[jenchris]

I use a plastic/concrete/steel combination pier with 2x 15mm steel plates forming a ratcage on top for my alt/az on a wedge.
The only vibration I get is across the forks on the top - nothing is apparent in live view x10 on my laptop.
Maybe the thee different materials of the pier damp the effect of each other in the presence of a vibration. I've not noticed resonance or any resonant frequency of the pier at all - it should be very low and very quickly dispersed by the mass of the differing materials, but I have only empirical evidence of this and no way of measuring.

[Poita]

Anecdotally I find the wooden tripods recover more quickly from a bump than the metal ones do.

[rally]

Brendan,

It was a real case ! and I think that it does happen with other large OTA's

The P-ME was "built", rather than really engineered and designed at the sort of levels mooted here !

We can only assume it was a resonance, not necessarily high frequency, but that depends on what you mean by high.

Whilst it could be the pier or the pier as part of that particular system, I believe this is unlikely, it occurred in different situations and appeared to be unaffected by any dampening applied to the pier or the piers ground connection.

It seemed to be an OTA/Mount system related problem, that was able to be remedied by changing the counterweight mass around.
That was a solution but not necessarily a fix of the source.
It wasn't specifically an overloading mass related problem because the total mass was ultimately increased to solve the problem !

I didn't solve it, just supplied some of the pieces.

In any event the purpose of this example was to illustrate that vibration in telescope systems does exist and does cause real problems in all sorts of unexpected and complex ways.

Rally

[Nico13]

Not directly related to vibration but what if you lovely new peer has been installed with an East/West cant??
Surely that can't be adjusted out without the adjustable top plate although ok if you can shim the bottom, just another area for flex?

[koputai]

That's the whole point, yes it can. The base of EQ mounts can be adjusted in altitude and azimuth.

So, imagine you have set up your mount perfectly and have your polar axis absolutely perpendicular to the axis of the Earth.

Now, keeping the axis parallel to that of the Earth, you can undo the alt and az locks and whiz that mount 100 miles away, and the base will adjust itself to the different plane of the Earth, but your polar axis is still parallel to that of the Earth, and you still have perfect polar alignment. Do up the locks and you're done.

That's the thing. The mount can be used anywhere on Earth, so of course the base does not have to be flat, you could have it at 30 degrees tilt if you wanted to, because that is exactly what someone 30 degrees away from you does.

Cheers,
Jason.

[Screwdriverone]

Another thought provoking and interesting thread Peter (Brendan & others....)

I've often thought myself, that it was a little strange there is SOOOO much concrete and stiffening in the pier with Xmm thickness steel and gussets etc etc and then there are 4 piddly bolts holding up a steel plate with 20-40Kgs of spread load (especially at Zenith when say a newt reflector is horizontal and the counterweights and tube are at opposite ends of what essentially looks like a see saw at rest)?

It seemed to defeat the purpose of all that mass and rigidity to then introduce a point of (my initial thoughts) weakness simply for what I thought was ease of mounting and perhaps clearance for the base of the EQ mount.....

I know it is more complicated than this, but as a few have said and Jason has mentioned recently, as the EQ base is inherently adjustable, I didnt see the point of having so much air gap and "unsupported" load simply resting on the compression properties and rigidity of some bolts sticking out of some concrete or steel?

I suppose its all relative, based on how accurate and tolerant the system is to vibrations induced by walking, motors, resonant frequencies, torque introduced by the optics and mount etc etc. so I suspect for most amateurs who simply want the convenience of a mount that can be left set up and is more stable than a tripod (ME) and dont need the accuracy to image above 1000-1500mm FL, a simple pier without an adjustable top plate/rat cage, would probably do?

Based on my limited Engineering Science knowledge from High School, I would assume that the vertical bolts that hold up the pier plate, would benefit from some sort of structural truss design that "tied" them together, a bit like a bridge truss where the triangle is stronger than the individual girders and can handle more compression and bending stresses induced on them?

I wonder if this would produce any more rigidity in the system than simply four bolts pointing vertical? At least the plate could still be there for adjustment (or whatever its needed for) and the assumed weak points could be reinforced?

Or have I got this all wrong? Is the system as it stands with the EQ mount on top all centred and balanced about the vertical axis of the pier and therefore, any bending moment or stress induced into the base would be evenly distributed down into the structure and not transverse to the mount and pier?

I think I confused myself more....oh well, it may be a while before I get around to a pier, so I shall read on with interest in the meantime....

Cheers

Chris

[bmitchell82]

Rally, im with you 100% and the first thing i said it would be a scope/rig that was on the borderline of the PME's capabilities... ;) My point more assists to the Pier and Vibrations induced and that a simple static deflection will assist you in finding the maximum a pier can move due to a force. Harmonics or resonance applies a force generally according to the right hand rule or a reaction in the opposite direction of the rotation of the harmonics which the mount/piers ability to resist is affected by the youngs modulus of the material, geometry of the section and the length.

Higher Youngs modulus = higher natural frequency
Higher stiffness (second moment of inertia) = higher natural frequency
Longer length of pier = lower natural frequency

By high frequency im talking in the 30+ Hz probably more. at the end of the day what you where doing by moving the weight away was changing the length and hence the harmonic properties of the CW bar.

The stiffer the components the more chance you will induce vibration into a system as it can develop easier into a harmonic. Unlike G11's, EQ6s and the like that have to have back lash for gear meshing high frequency vibrations cannot set them self up in to the natural harmonics.

For this reason is why my EQ6 doesn't use a CW extension bar. I bunch the weights up as close to the mount as possible. Short CW bar = extremely high frequencies and hence the system cannot react fast enough and so the vibration doesn't pose enough movement to see or quantify either that or they cannot get to the natural harmonic.

for instance. get a slinky pull it out a bit. and then start oscillating one end up and down. slowly go faster and faster you will get to the point of the first harmonic, that is it looks like a pure sine wave. get faster and itll go nuts for a bit and then itll get into the second harmonic you will see two sine waves. on and on it will go. What i am trying to demonstrate is the system needs to hit one of those harmonics for you to see anything the higher the order of the harmonic the less amplitude it has aka the peaks are smaller. It makes it easier to track down your issue! :)

Possibly and this is more an idea than too much science behind it. to install between either the pier base or the adaptor (see logiberras pier that is one of my designs although Logan unscaled due to section availability and cost) high density industrial rubber only a few mm thick this may take a lot of the vibrations out. Once again this is more ideas to design than pure mathematics.

BM.

[bcoote]

Clearly the larger the diameter of the pier, the greater the stiffness and lower the vibration potential. For me however a thicker pier really gets in the way for imaging. I prefer to image all the way past meridian until the camera hits the pier if the target is far enough south so as usual you have two conflicting requirements. My first "decent" pier was an RTA traffic light pole which I mounted on four x 25mm threaded rods set into concrete floor slab. Vibration was never a problem I was aware of but the slab shifting with water content variations in the clay underneath remains a real problem especially with polar alignment.
Brian
www.bcoote-astro.com (http://www.bcoote-astro.com)

[Logieberra]

Here's a few pics of the pier that Brendan mentioned. I went as big as I could afford which has worked out for the best. I was planning on an AP Mach1 / 900GTO but ended up with a MX. Either way, you can't go wrong with a pier that exceeds your current requirements... it will serve you into the future.

http://db.tt/N1aq7N5l

What you can't see from the pics is the floating top plate. This top plate is connected to the pier top plate by 4 x M16 bolts. I am currently getting the floating top plate tapped to accept the MX base plate knobs (4 x 3/8" @ 24TPI). Again, as Brendan mentioned, with a change of mount I can always get a new top plate cut and tapped.

[bcoote]

Looks completely solid. What I don't understand is why you need a floating top plate added to this? What is the benefit?
Brian

[Logieberra]

When I say floating, I don't mean rat cage with room / air between the two plates. They are sandwiched together, 340x340x20mm plates on top of eachother, secured by M16 x 4 on the corners. As I said, I can butcher the top plate to suit my current mount, drill, cut, paint, tap - whatever. With a new mount down the track I only need to replace that upper plate. The pier itself remains perfectly blemish free. Suppose my local machinest stuffs up the tapping of the MX holes, the only damage is that single top plate, not the overall pier. Makes sense to me... Long term pier, short term top plate.

[Logieberra]

Sorry for spelling, hard to type on Android phones...

[bcoote]

I'm sorry I was thinking you were going for a plate standing on four 16mm threaded pillars.
I did basically the same as you intend with the top plate for my AP1200 GTO mount. The mount was 2nd. hand and I got a machined top plate with it from the vendor. Apart from anything else this gives a dead flat machined plate for the mount which you cant be sure of with a welded pale on the column.
I did stand my old traffic light mount on 4 x 1 inch shafts which I used to adjust to try and keep the column vertical. This was only to try and minimize Polar alignment drifting without any great success. This mount is only use for short FL stuff mainly with FSQ or even camera lenses. With short FL imaging Polar alignment is hyper critical so that for me anyway even with perfect guiding stars in the corners of a big chip show definite rotation on anything longer than 5 mins whereas 30min integrations on 2.8 meter FL is no problem.
Can't see you having any worries with that mount and column
Brian