[Laser] Re: street light EME

[Art]

Thanks Yves.

The photomultiplier is probably overkill and 
probably will get flashed unless the viewing time 
is limited to the few moments when there is 
totality in the eclipse. But, the extreme 
sensitivity of the photomultiplier probably 
enables VERY fast detection of the moons light 
output so short FFT's would be possible. If a 
shutter prevents the PMT from seeing light until 
the moment of totality, it might be possible to 
let the balloon payload spin with the elevation 
fixed and just look for fingerprint frequencies 
as the field of view passes (sweeps past) the moon.

I don't think any optics at all would be needed 
with a PMT-just some properly aligned holes or a 
bore sight with the inside of the bore set to equal the active area of the PMT.

So, during a period of total eclipse, we need no 
stabalization of alt/az and no optics (or VERY 
MINIMAL optics) to improve the light gathering ability provided a PMT is used.

So, I'm thinking a photmultiplier is best if the 
flight coincides with a total eclipse.

During non eclipse periods, a photodiode and some 
means of stabilizing alt/az is needed. I did 
consult a few in the ballooning community and 
their response was very interesting.

First of all, stabilization of the alt/az to 1/2 
degree or better is much more difficult than I 
thought. So, keeping the receiver pointed at the 
moon is not as trivial. One of the group 
developed a dynamic alt/az mount for the purposes 
of helping get good video at high altitudes, but 
it just barely works...not nearly good enough for our purposes.

The group pointed out that gyros are probably 
necessary to achieve the stability we need. Gyros 
are enormous power hogs and POWER is the enemy 
for high altitude balloons. Batteries are not 
light and they are not inexpensive and gyro's 
ain't exactly cheap either::> So, a photodiode 
based receiver with stabilization will probably 
require an FAA waiver to fly since it will weigh 
more than 6 pounds and due to the cost, it must 
be recovered-not trivial, especially for a long 
duration mission (3 or 4 hours flight time is necessary).

Further, one of the group mentioned some NASA 
balloon launches that happen regularly from 
Texas. These are in the 100's of pounds of lift 
category and do have lots of power and extremely 
good gyro stabilization (sometimes their missions 
put telescopes into the sky so they have to be 
stable).  It was mentioned that a group might be 
able to hitch a ride along with them on one of 
their launches. I found the idea very interesting.

If we are looking for street lights, I think a 
yellow optical passband filter is probably 
needed, regardless of the flight parameters or 
whether a PMT or photodiode is used.

I do have information on the upcoming eclipses if 
anyone is interested in pursuing this.

Regards,

Art


> >street light EME sounds like an ideal application
> >for an  amateur based high altitude balloon group.
>




>
>Art,
>It is a  fantastic idea
>Because the  very cold temperature I think the photomultiplier is not
>mandatory.
>At -60° all  the photodiodes have very low noise.
>A low  noise large area "blue enhanced" PIN photodiode will be very good for
>this  experiment because with this kind of diode the response curve is
>improved also  near the yellow.
>A rather low  cost OSD50-5T could be a good choice.
>The magnet is  a good idea and with a very short focal wide angle objective
>this aiming system  could be accurate enough.
>A simple UHF  FM radio link could be sufficient to forward the optical audio
>for a batch  FFTDSP treatment at ground.
>In other  words the cost could be rather low for a radio club group.
>It is a good  project base.
>73 Yves  F1AVY
>