[Laser] Re: Lunar Eclipse

[Chuck Hast]

On 9/12/07, James Whitfield <n5gui at cox.net> wrote:
> It has been interesting trying to follow the discussion.  There is a lot of
> thought going into the synchronicity of the the power grid and delay time of
> light from the limb of the Earth ( as seen from the Moon ) versus the point
> on the Earth's surface along the line from the centers of the Earth and
> Moon.
>
> It seems to me that no one has considered the "wave shape" of the light
> source.  There is talk of the 60 cycle power grid and three phases, which
> results in a 360 Hertz pulse rate of the light.  Assume for the moment that
> we only had to deal with Sodium Vapor lamps and only one time frame, that is
> all of the lamps are equal distant from the Moon, which acts like a mirror
> that returns all of the light pulses to the test sensor at the same time.
>
> The Sodium Vapor Lamp, being a gas discharge type, will fire when the line
> voltage reaches the trigger point ( Does anyone know what "angle" that is
> from zero crossing of the power line? ) and will have its current limited by
> a ballast of some sort.  Is the light output more or less constant until the
> line voltage drops to the point that the gas discharge is quenched?  What is
> the "angle of the quench?
>
> I know that the assumption of constant light output for the duty cycle is
> shakey at best, but if it were true and if we can assume the start angle is
> about 75 degrees and further if the quench angle is about 60 degrees, then
> what does that leave for the composite waveform of the light emitted from a
> bank of lamps balanced on each of the three phases?    If I got my figures
> right, the pulse starts at 75 and goes to 120, then the next starts at 135.
> I think that gives 45 to 15 duty cycle or 75 percent on, 25 percent off.
>
> I am not suggesting that this guess is valid.  But if it is "in the ball
> park" ( to use the American idom from baseball ) it may be the effects are
> as important as synchronicity and travel times from curved sources and
> curved reflectors.  If you overlap the effects, it may be more difficult
> than was prevously assumed.
>
One thing to take into account, many HID lamps (Sodium, Mercury and
metal halide) now use electronic ballast, these devices are switching power
supplies and are designed to switch based on the best operating frequency.

I have worked with both HID sodium and metal halide, both of them used
a 105hz switching frequency. Mains power is run through a full wave rectifier
and then a switcher, many of them do NOT have filter caps so the 100 or
120Hz base switching frequency is still there. Also others switch at about
20-30 KHz, so as you can see you have a lot of frequency spectrum to play
with. The old magnetic ballast are fast disappearing, the cost of copper and
the weight of the iron core is pushing them out of the market place.

The Chinese make them very cheaply, so you will find that most of these
devices are now made in that country. Oh yes, the non-filtered ones have
subcarriers every 100 or 120 hz above and below the switching freq on the
high freq switchers. Usually there is a strong 3rd harmonic that falls in the
40 -70 KHz range. Look at one on a spectrum analyzer, it is usually a
scary view. The quick fix is to run them on DC power (since they rectify
the ac anyhow) if it is nice clean power you will see only the spectrum of
the switcher...


-- 
Chuck Hast  -- KP4DJT --
To paraphrase my flight instructor;
"the only dumb question is the one you DID NOT ask resulting in my going
out and having to identify your bits and pieces in the midst of torn
and twisted metal."