[Laser] Simple optical beacon experiment in San Diego

[J. Forster]

TWOSIG at aol.com wrote:

> Kerry

[snip]

> What are the light sources in the area?  Sodium vapor, Mercury vapor,
> incandescent, neon?  It may be easier to find a filter for the gas  discharge types
> than for incandescent sources.

Most places have a mix of light types. I'd get a narrow BP filter matched to the source.

> There may also be a  component in those noise
> lights that relates to three phase power.  I doubt  that any single light will
> have a 180 or 360 component, but there may be lights  that group into three
> sets of phase relationship.

I used a PMT and telescope to look at the night sky some years ago. There was plenty of 60
Hz and quite a few harmonics (up to several hundred Hz) Discharge lamps turn on and off
every cycle.

As to three phase, I think you will find all 3 phases in close proximity. It's not like the
east side of town will be on phase A, the south side on phase B, etc. You might find a
whole block on a single phase, but not large areas.

BTW, It's fairly easy to measure the phase content of the stray light. Just hook up a
medium fast detector to a lock-in amplifier with the line as a reference, then tune the
phase from 0 to 360 degrees. Lock-Ins, like Ithaco 391s are pretty cheap these days.

I'd use a carrier frequency above a few hundred Hz and NOT related to 60 Hz by any integer
or even ratio of small whole numbers and a BP filter for the mod frequency.

-John