[Laser] Re: photon flux for magnitude 6 stars

[Tim Toast]

thanks Yves,
It looks like those figures are in the ballpark. I was
using the solar constant for the flux above the
atmosphere (1370 W/m^2) so my figures were for no
atmospheric absorption. 
Using a 70% insolation, I used 959 W/m^2 at earth's
surface to come up with 7.96 x 10E-11 W/m^2 for a mag
+6 star. Which is about 5600 photons/sec into the 6mm
dia eye. Still, many more photons than I thought a mag
+6 star should be, but there are other things going on
here i guess - like background sky noise maybe?? 
I think i read somewhere in all that, the sky
background can be in the range of 10E-11 W/m^2 under
'good' conditions - almost equal to the apparent
brightness of the mag 6 star. So given that, i can
understand better how it could still be difficult to
see even with thousands of photons.

Yves F1AVYopto at aol.com wrote:
>I found with my calculations something like 9.10E-11
>W/m² at magnitude 6. (Very near your 1.1 x 10E-10
>watts/m2 = 11 x 10E-11 watts/m2)
>


James, 
it looks like about 70% of the total energy incident
at the top of the atmosphere from the sun (or any
star) makes it to the earth's surface. And of that,
about half is in the visible spectrum inside the
broader optical window. I don't believe this figure is
corrected to that imaginary monochromatic 500nm
wavelength deal. So we're left with the total
radiation in the optical window that can make it to
the ground. 
I got several figures while looking for the info, and
these ranged from as little as 25% up to 72%. So using
the higher numbers, 1000 watts total radiation per
square meter on a clear day sounds about right for the
sun - 500 watts of that in the visible spectrum.

I was thinking if each of the 50,000 photons could
make a loud click in your headphones, then it has the
potential for a good high-fi signal :)

James TWOSIG at aol.com wrote:
>So my question is:  What is the fraction of a star's
>total energy output that is visible?  If the number
>is at least 10 percent, then the numbers in the
>original post should be in the right order of
>magnitude.
>This topic got me to thinking about "counting
>photons".  If I want to receive voice quality
>signals, can I expect to be able to do that if my
>system receives 50,000 photons per second?  
>








Tim Toast
http://www.aladal.net/toast/exp.html

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