Washington DC Trip
2009
External links:
- A novice-frendly introduction to radio telescopes
- Green Bank on NRAO web site
- Green Bank as on ESO "80 Telescopes" in 2009
- - - (try to sleep through the 2 minute lead in music) - Active Surface Architectures of Large Radio Telescopes Parker and Payne
- Holographic Measurement and Improvement of the Green Bank Telescope Surface Jul 2011, T. Hunter, et al
Green Bank Radio Telescope
1 Day Tour, September 24, 2009Entrance W - 79.8162008, N 38.430809
I *really* wanted to see the largest fully steerable single aperture radio telescope in the world.
National Radio Astronomy Observatory
which is in a 13,000 square mile "radio quiet" zone at Green Bank, WV.
(I'm a bit of a radio telescope junkie
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A goal was to also take the High Tech tour, offered the 4th Thursdays of the month.
We landed at IAD (Washington-Dulles International Airport) Wednesday afternoon, rented an SUV,
and headed for a motel in Harrisonburg, Virginia. The next morning we took the route that
both Mapquest and Google recommended (via Highway 250) and tackled the winding mountain roads :-((
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As we approached the observatory, we noticed the fewer radio stations were getting very weak.
(We were unaware of the quiet zone and thought it was only the mountains shielding us.)
And our cell phones could not find base stations.
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| A telephoto from the visitor's center :-))) |
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We were very early for the 3:30 PM High Tech tour, so had plenty of time to:
My e-mail contact, Cara Rose, was at the front desk. We had e-mailed each other so much it was like a high school reunion ;-))
There was intended to be a laser range finding system to help do a closed loop control
of the large reflecting surface. The low-tech guide says this system is now stalled for
undisclosed reasons. The surface is now under open loop control, errors are hard and slow to detect. Some of the early dreams for laser aided closed loop control are at:
Radio waves from external to our solar system were 1st detected, confirmed and published by Karl Guthe Jansky in 1933 while employed by
Bell Telephone Labs. The young radio engineer and amateur astronomer
Grote Reber got excited and
made a 9 meter parabolic antenna and eventually made the first radio map of the sky.
The "reconstructed" version is about 80 meters from the museum. The original components,
including the dish and elevation drive, are painted white, the rotational parts at the base, added later by NASA are painted red.
A major display in the museum
"In between Karl Jansky's discovery of cosmic radio waves and the end
of World War II, one man advanced the science of Radio Astronomy.
"That man was Grote Reber.
"Grote Reber was a young radio engineer and an accomplished Amateur Radio
Operator in Wheaton, Illinois when news of Jansky's discovery reached him. Reber
had built a short wave transmitter-receiver when he was fifteen and had "worked all
hobby. continents." By 1937, he felt that "there were no new worlds to conquer" in his hobby.
"In 1931 Karl Jansky discovered radio waves, or "cosmic static,"
coming from the Milky Way. Reber was enchanted by Jansky's discovery and foresaw the prospect
of putting his radio hobby to exciting new use.
"Reber contacted Jansky, offering to work for him, only to learn that Bell Labs had transferred
him to other projects.
Reber contacted leading astronomy observatories,
but could not interest them in radio astronomy , Like the "Little Red Hen," he thought to
himself: "Grote, old boy, if anyone is going to do it, it will have to be you."
"And so he did it. In his backyard, and at his own expense, he built the world's first
radio telescope. Its diameter, 31.4 feet, was dictated by the lumber available
at the local lumber store."
"Jansky had observed cosmic static of wavelength of 14 meters. Reber reasoned that the signal would be stronger of shorter wavelengths. He knew that hot objects
like stairs obeyed this rule.
"He built a receiver that operated of 9 centimeters wavelength
"He built a second receiver that operated of 33 centimeters
wavelength. Again the results were negative.
"Undaunted, he set about constructing a third receiver, this time for a
wavelength of 1.9 meters.
"By April of 1939, Reber was of at last successful in detecting radio waves from the Milky Way!
He made a complete survey 1943 of the sky, and produced the first maps of the radio sky in 1943.
Later, when Reber searched for the shorter wavelength of 62 centimeter,
his data showed weaker emissions. Clearly, he showed that radio objects
did not obey the laws of hot objects. Another process was at work.
"Grote Reber, lone pioneer of radio astronomy for over a decade continued
his practice of exploring unconventional ideas throughout his life. His
advice to young scientists everywhere and his personal philosophy:
""Don't accept current theories as absolute fact. If everyone else is looking
down, look up or in a different direction. You may be surprised at what you find."
"His accomplishments insured that Radio Astronomy became a motor field of research
following World War II. Research groups in many countries began building bigger and
better antennas and receivers to follow up on Reber's discoveries.
Across the museum isle, Reber's home made receivers.
We head back to Harrisonburg, VA, the way suggested by Cara - it is much better :-))
During the rest of the week, we toured
They have a fun museum and science display.
There are about 2,000 mechanical actuators to move corners of the "radio mirror" in and out
to help correct for the gravitational sag of the steel structure of the telescope as it tips
through various angles. This exhibit gives an idea of the operation of the system.
2 of me
1 of me
0 of me
and more recently
More on Grote Reber started Jan 2012
"Grote Reber - Grassroots Beginnings of a New Science
"...I was too young to know it wasn't possible."
"If at first you don't succeed, try, try, again.
"He detected nothing.
"Reber recorded his data by hand in the early days, often
staying up most of the night. When he finally succeeded in
detecting cosmic signals, he splurged on the chart recorder
pictured below. No longer did he have to sit up all night
recording data.
"Discoveries
From 1938 to 1943, Reber made his first surveys of radio waves from the sky. Reber
surveyed radio radiation and presented the data as contour maps. They show that the
brightest areas correspond to the Milky Way. The brightest part is toward the center of the
Milky Way galaxy in the south. Other bright radio sources, such as the ones in Cygnus
and Cassiopeia, were recognized for the first time.
As a point of reference,
at that time 20 meters (15 megahertz) was regarded as high frequencies by
most radio amateurs.
"First Successful Receiver, center frequency: 160 MHz, circa 1942" - about 1.9 meter wavelength.
This may be the 
tuned circuit
used to help generate signals to help develop the 160 MHz receiver above.
"Second Generation Receiver, center frequency 480 MHz, circa 1945" - about 0.62 meter wavelength.
- Wikipedia
- Reber researches in Tasmania
"Low Tech ;-))" Tour time
About 20 people got on this diesel bus to go to the main attraction :-))
We pass the R&D building. We will be in here for the High Tech tour later this afternoon.
This is the last stop before:
Oddly, this display is in the highly protected radio free zone. It is a real full scale model of
4 segments the mirror.
Another view. There are about 2,000 segments in the real mirror.
An actuator
It turns out this is a maintenance day - later some ?painters? started rappelling down
from the right side of the dish. The guide says that it takes 6 minutes for the massive
assembly to turn 1/2 circle. This
explains the benefit of the more expensive offset design. (Executive summary: reduced defraction
due to no metal supports in the RF path gives much lower system noise level.) We are standing at
W -79.838459, N 38.432032
This is a giant machine - some say the largest, heaviest movable land object.
"High Tech :-))" Tour time
We are met at the Visitor's Center by a Receiver Engineer - The big deal is to
make/obtain stable ultra low noise receivers. This guy must be in the heat of the battle.
This is our High Tech tour guide, taking us by someone else's lower frequency receiver.
Now we are in our tour guide's work/play area. I shouldn't be so light hearted -
low noise is never low enough - the customer always wants less noise. The best is
not good enough :-((
Note the circular tube (wave guide) at the top, and an adapter to two rectangular
wave guides coming out the bottom.
Here is the middle section, note the detail of the two rectangular wave guides -
one for each polarization of the incoming waves. Polarization readings are very important to
radio astronomy. It gives information about magnetic fields where the waves are made.
Yet further down - more paired rectangular wave guide. We forgot to ask what those
wires control - so I'll guess :-(( Noise reference, and gain measurements are a difficulty
in radio astronomy. A method of estimating is to periodically block the receiver from the sky.
I'm guessing those wires control hidden solenoids to block various parts of the wave guide.
And way down here, near the green circuit board, are the special low noise transistors,
at liquid helium temperature for lowest noise, emitting electric signals from the RF waves.
I presume into coax - somewhere on the other side. The electric signals are eventually mixed
with another frequency, and the result modulates a laser to send the signal to the signal
processing equipment in/near the control room a mile away.
Ed (left) is holding the little 20 GHz feed horn where the focused energy from that 100x100 meter
reflector is focused - how about that!! The tour guide is explaining that the circular grooves
cut into the inside cone are 1/4 wavelength, and prevent some sort of currents.
Ed is trying not to look snowed!!
We are in the control room - a double shielded copper cage to keep radio noise
from the electronics in the room. The door had springy copper strips to block RF.
This is a view of the double screened window.
And this is where the operator sits, controlling that million pounds of steel a mile away.
and probably won't leave home again for a year !! ;-))