mount vibrations

[Eric Watkins]

Evening,

I have just test driven a new set up, an original ME mount which supports a 16″ Newtonian reflector.  This system is mounted on top of a 1m tall  fined steel pier which is in turn bolted to a concrete block 1m and 0.75m square  below the ground surface.  I find that there is some induced road traffic vibration from a road some 35 m.   I also have a 14″ sct on an ME II mounted on a similar sized concrete block, but with a cast concrete pier and situated 40m away from the same road which  I have used for many years with no such problem.   Both ‘scopes are used for CCD imaging and photometry.

I  plan to remove the steel pier and replace it with a 1m tall concrete pier cast onto the existing concrete block.  Further  I will

place the telescope mount plate onto 4 vibration pads made of rubber or sorbothane similar to examples attached . 

I would be interested to know if others have  had a similar problem, tried similar solutions and how they got on

 Best wishes,

Eric

Attachments:

1. 

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[Dr Paul Leyland]

Something which worked well for us back in the day was to get a 10 foot long cast iron pipe, complete with drilled flanges at each end, from a scrap yard. It was painted inside and out with bituminous paint. A hole of suitable dimensions was dug so that a plug of concrete 3 feet thick and three in diameter could hold the bottom of the pipe, which was buried to slightly more than half its length. The inside of the pipe was then filled with sand. The top flange was ideal for attaching an equatorial head.

The fundamental vibration mode and small harmonics were heavily damped by the concrete and through being clamped with back-filled sand. The internal sand quenched high frequency vibrations very effectively.

It was possible to have one person placing fingertips very gently on one side of the pipe while another hit it as hard as possible with a length of 2×3 on the opposite side and still feel no vibrations.

[Eric Watkins]

Thanks for the info Paul.  The ror observatory is built and the pier positions already set.  It would mean a total re-build and positioning.  I have another smaller set up in the same observatory and could try filling that pier,  which has an open top, with sand  an see how that performs.

Eric

[Martin Lewis]

Sorbothane is good stuff in the right application but here won’t it make the mount very sloppy? 

There is a great application guide for Sorbothane here: https://www.sorbothane.com/Data/Sites/31/pdfs/product-guides/Sorbothane-EDG.pdf and an even more useful design calculator here: https://www.sorbothane.com/Data/Sites/31/edg/vibration-calculator.html  Generally what is important is to compress the Sorbothane by 15-25% for best performance – so you pick your area to give you that compression for your given load.

Martin

[Eric Watkins]

Martin,

thanks for your comments and link to the calculators I shall have a play with these.  I have read that road traffic vibration frequencies are  about 7 Hz.  I have seen some discs that have threaded bolts isolated from each other within the discs so that  I may be able to bolt down onto the pier and also bolt down the mount adapter plate so that the system is isolated with the mount resting on 4 such pads.  There are rubber discs of various sizes, the Sorbothane of this type are smaller.

Eric

[Colin Hall]

The steel pillar with a mass on top will have a fundamental frequency at which it will vibrate, along with a set of harmonics. If you strike the top of the pillar with a hammer it will ring like a bell. The note you hear is it’s fundamental frequency. It may be that the road vibration is exciting the steel tube. You can do one or all of three things: 1)  change the mass of the system by say filling the pillar with dry sand. 2) change the stiffness of the pillar (add stiffening webs etc.). 3) increase the damping (strap/bond  hard rubber to the pillar). Lookup ‘Mass Spring Damper Systems’ on the internet for a full explanation. Option 1) also increases the damping. Your other mount will have a different fundamental frequency so, you were just unlucky.

When I was still at school many years ago one would annoy the teacher by pinging a wooden ruler on the edge of your desk –  we were doing an applied mechanics experiment Sir!

[Eric Watkins]

Colin, thanks for taking the time to reply.

I tried some more imaging sessions last night and had no obvious vibration effects on images.  I not sure if there was less traffic passing – I had also tightened up the RA and Dec axis slackness a little, there has to be a little on the ME otherwise it jams when slewing.  I’ve read a lot about adding sand, but no reports actual improvement  of before and after sand.  Have you actually tried this method.  The pier does not sound a  ringing tone  when struck .  I shall try out your suggestions.

Eric

[Colin Hall]

Eric

A heavy mass (telescope) at the end of a cantilevered metal beam will be very prone to low frequency vibration. Your concrete pier will have greater mass and stiffness so it will have a higher fundamental frequency – which is why it does not ‘see’ the road noise. Sand filling will move that frequency higher but you may need a more drastic shift.

Best of luck.

Colin    

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