Marchal,

2 cents worth:

I would strongly recommend that you avoid the knife edge concept for 
the boom pivot. The ground motions and/or the required stability for
a useful long period require controlling the fixed end of the boom
in the sub-micron range, which is significantly less than the roughness 
of the surfaces of the knife-edge idea. Seismometers have always 
used a flat flexure(s) or a taught wire for pivot points: nothing moves
with respect to anything to make micro-positioning noise.

For the boom, I would suggest aluminum channel material, like a U-shape
0.5" high by 0.75" wide (USA dimensions) with a wall thickness of at
least 3/16". Aluminum stock is widely available at most hardware stores.
Even handier would be threaded rod at least 1/2" inch diameter; you can 
find brass, aluminum, or stainless steel and matching nuts (at a Tru-Value
store): common steel rod is magnetic and will not work. In fact, the 
only place for magnetic material in a seismometer is in the magnet 
itself. The large-diameter (I would use 1 1/2" material or larger)
soldered copper tubing frame sounds like a good idea, although again
I would prefer to use large aluminum channel or angle (2" x2" x 1/8" wall)
that are bolted and epoxied at the joints. I would keep in mind that
I want something sturdy enough to stand on without damage, as this
would probably provide long term stability and support for a horizontal
seismometer. The big LP seismometers use aluminum castings 0.5" and up 
in thickness, and triangular supports for the boom mast.

Regards,
Sean-Thomas

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