[Laser] How much power? Ideed!

[James Whitfield]

----- Original Message -----
From: "Art" <KY1K at verizon.net>

> You wouldn't see a laser based on the Moon. Although the laser is
> collimated, the few (at best) photons that arrived on Earth would be
> masked by the Moon's reflectivity of the sun (light that bounces off
> the Moon and makes it's way to Earth).

I did not suggest that you could see a 5 mW laser shining from the Moon with
the naked eye.  However, IMAX is fond of advertising that the projection
lamp they use could be seen from the surface of the Moon.  I can only assume
that something equally bright on the Moon could be seen on the Earth.  And I
assume that both the observer and the lamp in respective "night" conditions.
Being able to "see" a laser, or any other light, on the Moon from the
surface of the Earth is only a matter of how much light is generated, how
much of it you can gather, and how sensitive is your sensor.  If you have a
bright enough source and enough aperture ( to borrow an amateur astronomy
term ) you can detect it with your eyes or with instruments.  And with
enough brightness and aperture you can even see it in broad daylight.

Is that 5mW, I don't know, but somebody should be able to calculate it.  The
current lunar ranging experiment uses a 2.3 watt laser that splashes a spot
on the Moon about 7 Km in diameter, is reflected by mirrors less than a
square meter.  That power ratio is more than 3.848e7.  A 5 mW laser ( a
different kind of laser and not pulsed the same way ) on the Moon would be
perhaps 84,000 times a bright as the beam reflected from the mirror.  It
would also be spread wider, being a 360 mile spot instead of 20 Km.  Thats
about 830 times the area.  Roughly 100 times as bright.  That is five
stellar magnitudes, which is enough to bring an object from "not visible
under the best of conditions" to "visible even in a suburban area".  But not
enough to make "detectable in a 0.7 meter telescope" into seen with the
naked eye.

> The only hope of seeing a laser on the Moon (from the Earth) would
> happen only if the laser was located on the dark side of the
> terminator. In such a case, the field of view would have to be very
> narrow, about the same as the divergence of the laser transmitter, or
> around 2 to 5 milliradians. It would be just as difficult to aim
> receiver as it would be to aim the laser. When you consider that both
> the Moon and the Earth move relative to each other, both ends would
> need to be actively tracking each other.

Considering the large number of quite successful astrophotographers there
are, this problem would seem to have been solved.  "Amateurs" have been able
to take pictures, first with long exposures on film and more recently
sequenced digital, of such objects as the polar caps of Mars,  eclipses of
various moons of Jupiter, the Cassini division in the rings of Saturn, lots
of comets, star clusters, even a few asteroids.  Some even take pictures of
the Moon, which is about 8.33 milliradians in diameter.  If the beacon was
located on the central peak in the crater Copernicus, it would be on a
landmark that is easily seen with 7X35 binoculars.  Copernicus is less than
a quarter of a milliradian in diameter.  Not only can it be seen, but much
of its inner detail can be imaged with a very modest telescope.

The tracking system needed on the lunar side would be quite different from
that needed on Earth.  From the perspective of the Earth, the Moon moves
about 372 degrees in 24 hours.  If I did the arithmetic right, that is about
75 microradians per second.  From the perspective of the Moon, the Earth
moves 18 degrees east and west in 29.5 days, about 10 degrees north and
south per year, and yet another cycle about every 16 years.  The motions are
sort of sinusoidal, fast in the middle, slowing to a stop at the ends, then
reversing.  I think it peaks at less than one degree per day, roughly 400
times slower than the Moon relative to the Earth.  One could move the beam
about once a day to a spot on the Earth, preferably in nighttime, and let
the Earth rotate under.  In the example I used of a 360 mile diameter spot,
Equitorial cities would have a view as short as 20 minutes before the Earth
moves them out of view.  If the spot was closer to one of the poles, cities
might have a longer viewing time.

As an alternative, the spot could be centered on a specific Earth location
from Moonrise to Sunrise, or Sunset to Moonset depending on which part of
the month.  Or you could program a complex pattern to allocate view time by
population density or even the amount of cloud cover.

> I think receiving a laser based on the Moon would be difficult and a
> 2 way QSO between the Moon and the Earth would ne next to impossible
> for amateurs.
>
> Receiving an Earth based laser from the Moon would be even worse, the
> Earth is a much better reflector than the Moon is, so the background
> ambient light level would be much worse.
>

I did not suggest, nor do I advocate any amateur optical uplink.  Without
trying to explain why I oppose Earth to Space amateur optical
communications, I want to say that I am in favor of satellite and lunar
optical communication down to the Earth as a way to spark the interest of
people, perhaps a billion people, to look up and marvel at space and our
wonderful world.  I believe that there will be, and perhaps soon, commercial
and governmental use of optical systems.  Those will be tight beam, secure,
high speed systems.  They may not be Earth based.  I am sure that weather
and other factors will make optical systems unsuitable for mass
communication.  That leaves the door open for the experimenter and the
educator to be able to use optical systems to motivate and inspire.  I am
sure that it will happen, I just hope that it will be more than a "gee we
did it this once, now we can forget it ever happened."



That has been a lot of time on my soap box.  I hope that it has some ideas
worth thinking about.


James
 n5gui