archive: Re: SETI Yet another hare-brained scheme for building an antenna

Re: SETI Yet another hare-brained scheme for building an antenna

Corey Ashford ( )
Mon, 31 Aug 1998 17:43:26 -0700 (PDT)

Hmm.. not exactly a vortex because there's no
net vertical motion of water.

This technique for making paraboloids has been
exploited for making very accurate optical mirror
surfaces [I recall reading an article in Scientific
American a few years back about a mirror made from
a spinning vat of mercury.
The rotation rate has to be controlled very
accurately for it to work for optical surfaces and
vibration has to be minimized,
but is otherwise a workable technique]

Here are some other problems that occurred to me
today has my brain was both driving and thinking nnas:

1) Evaporation of water causes change in focus because
of two effects - water dropping lower focus point.
Having less water will probably mean that the rotation
rate will increase further shortening the focal length.
So some mechanism would have to be installed to
maintain a constant water level.

2) debris such as leaves would tend to mess up the
surface... disconnecting the balls from each other.

3) The walls and bottom of the pool need to be as
smooth as possible to minimize turbulance and
friction. Ideally,
the whole pool, including the walls, ought to rotate
on a huge lazy suzan, totally eliminating turbulance.
However, this is impractical.

4) Some care in the placement of the pump would be
needed to ensure smooth rotation of the water.

This last point may be a real stickler

I just now tried spinning a large pan of water on
a lazy suzan, and you do indeed get a very nice looking
dish shape in the middle.

I tried stirring the water to see if I could get the
effect, and it doesn't look as good. I think the
friction with the side of walls and the bottom screws
up the angular velocity - i.e. the center ends up
spinning slightly faster than the edges.

So maybe... if one were to use a pool that's much
larger the reflecting surface (e.g. confine the
balls to the center 2/3 of the pool), the distortion
at the edges may not be too bad.

Ok.. it's getting wackier by the minute... perhaps
this is a dead end.

---"Dr. H. Paul Shuch" <> wrote:
> At 02:17 PM 8/31/98 -0700, you wrote:
> >
> >
> >Picture a round, above-ground swimming pool about
> >14 feet in diameter, filled about half way with
> >and floating on top of that a coating of aluminized
> >ping-pong balls.
> Clever concept, Corey. Now please remember that
the vortex has to make the
> water surface *parabolic* (I'm not sure what conic
section would normally
> be formed by a vortex), and it must hold accuracy
to within 0.1
> wavelengths. At the 21 dm hydrogen line, that's
2.1 cm or 0.82 inches. If
> you can keep things accurate to that tolerance, you
have a winner!
> H. Paul Shuch, Ph.D. Executive Director, The
SETI League, Inc.
> 433 Liberty Street, PO Box 555, Little Ferry NJ
07643 USA
> voice (201) 641-1770; fax (201) 641-1771; URL
> email work:; home:
> "We Know We're Not Alone!"

Get your free address at