I have a ringing !!!

[beamish]

I've just completed the job of adapting my pier that had the HEQ5 on it to the EQ6, but I seem to have inherited a ringing problem:)

The pier BTW is 6" dia and concrete filled and didn't give me a problem before unless I gave it a kick but, and Im guessing here, that because I've added the extra height I've induced the ringing which seems to take an age to damp out.

My Q is, would I be correct in thinking the damping would improve if I relinquish my desire for extra height and cut the M16 rod down. The gap between the two plates is currently 200mm and so there's plenty of room for manoeuvre since the retaining bolt for the head doesn't take up much room beneath the upper plate.

Thought I'd take some advice prior to getting the hacksaw out again

[barkis]

Ringing might seem to indicate vibration, but the Forth Bridge rings too .

How does a star look in a high power eyepiece after a kick, have you tried it Karlo?. I had an A Frame welded up years ago, it supported a 2" diameter Polar shaft, located in self aligning bearings.

It rang too, and it caused the scope to resonate at the same frequency as the Synchronous motor driving it in Right ascension.

A light touch with my fingers on the focuser, was sufficient to stop it, and observing Jupiter for example, resulted in a much improved image once the blurring had stopped.

I changed to a stepper motor drive, and it was cured.

As you know, most mounts can be subject to vibration. The key is how long it takes to die down once you have give it a kick .

My current setup is very similar to yours, except the diameter of my Pier, is 31 centimeteres, and 26mm thick wall and filled to the top with concrete. The cables coming out are USB's from the laptop.

Ron.

[gary1968]

Karlo,

You have PM mate......... all you need is a couple of bits of some nice hard wood!!

We'll let the others try to figure out what to do with the dods of wood.....

Gaz

[beamish]

Thanks guys, Gary I've replied

Ron, my problem appears that when the mount is touched the vibes take a while to damp out which used to happen very quickly in the past but then the mount etc were nearly a foot lower down. I want to try and avoid a new pier for many reasons Gary's idea seems to have done the trick with Craig's mount and pier so I'll try that first, I've got a fair bit of 100yr old 3" x 10" joist still hanging about in the house

I suppose failing that I'll chop down the rods and invest in an ext pier to go on top of the head.

[beamish]

Just had an idea , I've got some seasoned 6" dia "telegraph pole outside I think I might just bring that into play as the grain will be running the right way.

What dya think?

[beamish]

Thanks for the pm's and suggestions guys- would this work as an idea ?? if it were to be repeated for the bottom plate too ?

[barkis]

You'll get no argument from me on the benefits of using wood.

It's a great medium for many constructions, and fine for mount uses too.

If you have enough length of that telegraph Pole, why not cut it to length, ensuring parallel faces, then rebate the base to accomodate those four nuts at the base, then

tighten the top plate hard down on to the top of the wood.

Levelling might require shimming to accurately level the top.

Not sure I would be happy with a rubber damper sandwich mate, although it might work.

Ron.

[Psychobilly]

I'm wondering if it's the lengths of the threaded "rod" thats are ringing... rather than the plates on the end...

[beamish]

Appreciate all suggestions, but it would seem that the wood idea isn't going to be an easy fix due to the need for access to the retaining nut and the blasted nuts on the top of the base plate:mad:

How about sealing the baseplate and then add a collar ( of which I have) and filling with sand/cement to about 2" from the top of the upright threaded rod ???

Also -is it worth adding rubber "O" rings btween the washers and the plates?

after this it looks like I'll have to get the hacksaw out !

[barkis]

Out of interest Karlo, how is the base of the pier held. Is it embedded in a concrete base, or attached to a concrete pad using captured threaded bolts. I'm thinking possibly extending the pier fro t'other end, so the top could be reduced in length.

Ron.

[Psychobilly]

Also -is it worth adding rubber "O" rings btween the washers and the plates?

after this it looks like I'll have to get the hacksaw out !

If you did you would want to make sure that theres clearance between the bolt and the metal plate...

[Gina]

Those long threaded rods will move sideways with springiness, the plates and mount provide the mass and hence a recipe for ringing. The threaded rods don't provide much horizontal rigidity, that's the problem, I think.

[Bizibilder]

The problem is most likely resonance if the damping takes an age to die down. Are the lengths of the new bolts an exact multiple of the pier height? if so you have introduced a harmonic and the whole thing will resonate beautifully!! The cure is to shorten the studs just enough so as to kill the harmonic and not introduce another one!!

A second option is to make a third disc with four holes and bolt it securely somewhere between the top and bottom plates but not exactly in the middle (or 1/4 or 1/3 of the way - see harmonics above!!). You may even find that just putting a few extra nuts on one or more of the studs will cure things - do them up against each other and it will change the stiffness of the studding thus changing the natural resonant frequency. Hope this helps.

[beamish]

A) the pier is buried into the ground into a "well" of concrete, then the pier is filled with more.

I think Gina/Peter are pretty much on the money, well you all are really it's a result of the longer studs.

C) the "harmonics" is a good idea and probably comes into play too.

I noticed earlier on that if the rods were braced to each other the ringing/quivering/vibrations from a touch damped out almost immediately. So before I do any cutting (which I want to avoid cos I want the extra height), I thought of adding two pairs of opposing wooden braces, perpendicular to each other or perhaps diagonally ?

It looked so nice too

[barkis]

Why not intoduce a couple of 18mm thick plywood discs equispaced along the long threaded rods. 12 extra nuts and 12 large plain washers would hold them in place. You could make their diameters a bit greater to allow the drillings for the rods to have enough material for them. That should firm things up a bit.

Ron.

[beamish]

Thanks Ron that's another good idea=braces the rods and can break up any "harmonic ":icon_salut:

[Gina]

Wood acts as a damper too.

[nightfisher]

the wooden brace i.e a plywood disk at a certain point on the studs should do the job well, good luck karlo

[RogerTheDodger]

The wooden disks should sort it, but don't make them equally spaced, put them at an offset or you could be introducing higher resonances.

[beamish]

Thanks for all the input guys, out of interest, how long would you expect is a reasonably acceptable damp time to be- perhaps I'm expecting too much? I'm currently getting about 2.5secs with the addition of changes this am.

[beamish]

Latest dev.

better but not quite as stiff and damped as I'd have liked. I'll have to see what it's like with the scope on otherwise it'll be plan B and a reduction in height.

The blue stuff between the ply boards are yoga blocks (sure the missus wont mis 'em )

I've also dropped the bottom plate right down onto the pier.

Again, I tried a ratchet strap around the rods and tightened it up and that seemed to improve things- so if I could figure out a way of doing that perhaps in a more elegant way I'll try it, but I'll see how this gets on first.

Don't get me wrong it's not wobbling all over and I don't intend banging the pier or scope but just a bit concerned over the damping time

[barkis]

Two and a half seconds is no time at all Karlo, and you probably have reduced that even further with mod.

I'm sure the mount on the original Tripod would oscillate for longer.

It looks like the setup is not in an Obs, so your biggest enenmy is going to be wind, and you would not Image in any appreciable wind anyway I'm sure. Is your pier a metal one, or plastic pipe by the way ?

Ron.

[Coco]

Slip another tube over the lot and pore in some concrete...

My 6' fence posts vibrate when I tap them, the short ones dont..

Use a hammer drill as a vibrator to get the concrete free of air pockets..

[Merlin66]

A couple of turnbuckles from B&Q diagonally from the top plate to the bottom will significantly change the tuning!

[malc-c]

Why not shorten the rods to just a couple of inches and then use a extension tube as shown in Barkis's post ?