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Observatory base / elbow pier foundation advice


tooth_dr

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I have some direct experience in this matter.
My 10' dome is raised on a timber building to second storey height.
The massive, pyramidal timber pier is almost immune to vibration.
I even had my computer desk directly mounted on it for a while.

I have just removed my 7" f/12 refractor from the massive GEM mounting.
This was because it was catching the wind through the observation slit in winter.
Not even a dewshield to act as a spinnaker and the cell was a couple of feet inside the dome surface.
Just the bare objective cell on a long, smooth, 8" Ø metal tube.

Early morning solar imaging in the three warmer months avoids the prevailing south westerlies.
As the day progressed the slit and telescopes move steadily westwards.
Beyond lunchtime was usually a problem despite the meridian flip.
[When the telescopes were now pointing into the wind instead of sideways on.]

The solar seeing conditions in windy weather are rarely much good anyway.
But the general lack of sunshine means I like to try whenever it is shining.

A bent pier is just one-legged tuning fork.
A perpendicular dropped from the centre of mass must pass down through the pier into the ground.
With a GEM that is the crossing point of both axes.

Even an offset mass, like a typical fork mounting and hefty OTA [SCT?] will place serious asymmetric loads on a vertical pier.
Asymmetry means easily induced vibration to every touch, footfall or breeze.

P1360328 rsz 600 upright bright.JPG

 

P1360558 rsz 600.JPG

Edited by Rusted
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What Peter and Gina said.

Choose a deep, rectangular cross section for a bent pier.
NOT an I-beam or U-beam which both twist far too easily unless physically restrained.

A deep box helps by increasing the moment of the beam perpendicular to the applied loads.
Stiffness increases as the square of the depth of the beam or hollow rectangular tube.
Which is why floor and roof joists are always set on edge. Not laid flat. Nor made square. Which is wasteful.

Round tubes and poles are far more "springy" because they have so little resistance to bending.

Moreover, the "leverage" of any offset telescope mass is always trying to pull the pier [or tripod] over at ground level.

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Hello Adam

Going back to the concrete question - I noticed this morning that just up the road from us a field has been dug out for foundations for a house - this site is on a severe slope and is at the edge of a moss - parked up was a car and men in hi-vis jackets labelled up 'Concrete Masters' - so I went and a had a blether with them and told them about your situation - their unanimous suggestion was straight poured concrete - no fibres or reinforcements - pick a week when no frost is forecast and cover it with  tarp and keep it wet for a week. 

Also a bit of personal experience - my pier is not isolated from the floor - it was all done in one pour - I am never in the obsy when its in use but as a test I jumped up and down right next to the scope whilst imaging and the PHD2 trace did not record any anomoly. The pier is filled with kiddies play pit sand.

Some photos here that may or may not be useful - https://www.davidbanksastro.com/micro-observatory

Good luck!

 

 

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