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This project
was very unique and I believe quite significant in that has been rarely
done.
In a nutshell, the aim was to take a photograph in wavelengths of lights
different to that of what the human eye can see. The easiest way to do
this was to use a satellite dish which dictated we take the picture
using radio waves. This was the result, with the 11GHz image at the top
and the visible light image at the bottom for comparison:
Certain objects are visible - trees, path, wall but most notably the
geostationary satellites that give us satellite TV and their reflections
on the wet decking (bottom left).
For the details on how we achieved this, read on.
The following was written by Dad for the 'Starship Asterisk' forum, hence
the references to me and the better understanding than my own:
It was taken on 10/11 February 2010 between 20:30 and 02:30 GMT in
Scotland.
The picture is in two parts. At the bottom is the view across our back
garden a frosty morning. At the top is the same view taken at night
using 11GHz 'light' and it shows the arc of the geostationary satellites
across our sky. Here's the story of the image:
Origin
It came about from conversation with my son, Kevin, where we wondered
what the world would look like in frequencies of electromagnetic
radiation beyond visible light. IR and UV pictures were easy to find on
the net but we wondered about the TV transmitter 20 miles away. It's
spraying 400-800MHz across the landscape and we wondered whether, if you
had a very directional antenna, you could produce an image that would
show even the nearby hills being illuminated by this 'light'.
Discovering techniques, building our 'camera'
Then I realised I had a very directional antenna - a satellite TV dish.
It's a little camping satellite TV kit with a 40-cm dish and it includes
a LNB (low noise block - at the dish's focus) and a receiver. We bought
a simple signal finder meter and wafted the dish around the garden.
Though it has a conventional meter display, it does not directly measure
the signal level. Rather, it is a bistable device which emits a tone on
one side of a threshold level and sends the needle to the opposite end
of the scale, However, the level of that threshold can be set by a knob
and I realised this could be a way to gain signal level measurements for
low cost. Our garden trials were showing profound differences between,
say the lawn, and the path running across it. Since the TV sats transmit
around 11GHz, we would get a 'photograph' in that 'light'.
My son's a drummer and he used some of his hardware to fashion a rig
with which he could pan and tilt the dish. We fashioned protractors from
CDs with labels stuck on them sporting scales made on a drawing package
on the computer. That gave us azimuth and altitude angles. We made
another small protractor for the signal meter, bored a tiny hole through
its knob and passed a sewing needle through to give degrees of rotation.
Our plan then was to point the dish in a known direction, adjust the
meter to read '5'; the middle of its scale (and therefore in the middle
of its threshold) then take the angle of the knob's adjustment as a
measure of the signal level. The meter's frequency range is 930 MHz to
2.4 GHz. The LNB is downconverting the 11GHz into a frequency in that
range. I don't know how that translates into the bandwidth of the system
as a whole.
Collecting Data
We set up and started at 20:30 one evening to take measurements for a 66
x 30 pixel image - 1,980 samples in all! It took 6 hours (!) of Kevin
out in freezing temperatures pointing the dish, and me in the relative
warmth adjusting the meter and entering angles into a spreadsheet. We
were shattered by the end; Kevin using his stamina as a hillwalker to
stay with it despite the cold. As we worked, I could perceive variations
in the levels that looked interesting.
Interpreting Data
The next day, we used Paint in Windows to directly transfer the angle
measurements into grey values in a 66 x 30 pixel image. Another 2 hours!
Once finished, the levels were inverted and stretched in Photoshop and
the image enlarged to make the final 'photograph'. I was delighted to
see the arc of the geostationary satellites. The bright splodges are us
looking right down the boresight of the TV sats that serve this area.
I'm guessing the dimmer splodges are leakage glow from other TV sats or
other downlinks at a similar frequency but aimed at other parts of the
world. I was really pleased to see the outline of the trees and a hint
of the path across the lawn. Also, there is an obvious reflection from
the powerful sats off our wet wooden deck.
Now if someone could provide a computer-controlled pan/tilt platform,
wideband signal measuring gear that will directly attach to a computer
and software to directly translate level measurements to pixel values,
we'd be laughing! I'm told that the Cassini probe was made to do a very
similar thing by tediously rocking the probe backwards and forwards to
use its large antenna to image in the Saturnian system in 13GHz.
Thanks for reading. |