Return to main page
Washington DC Trip
- 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)
Green Bank Radio Telescope1 Day Tour, September 24, 2009
Entrance 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 - Allen Telescope Array in California - Very Large Array in New Mexico - Goldstone in California
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 :-(( We returned to Harrisonburg "the better route" as recommended by the folks at NRO.
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. The place seemed well funded - I presume due to Senator Robert Byrd of West Virginia, who has been in the senate since I was a kid. (Technically, the radio telescope has his name on it, as do most of the bridges and civic building in West Virginia.) W - 79.8162008, N 38.430809
A telephoto from the visitor's center :-)))
We were very early for the 3:30 PM High Tech tour, so had plenty of time to:
- to enjoy the visitor's center with nice museum and movies - take the regular tour - then take the High Tech tour.
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 ;-))
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
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:
- The Green Bank Telescope - Controlling the Green Bank Telescope - The Green Bank Telescope Laser Metrology Computer Control System - Active Surface Architectures of Large Radio Telescopes
and more recently
- Major Improvements to the Green Bank Telescope Surface Accuracy from Conventional Holography ( 2010 )
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.
Reber's original telescope could only view up and down, depended on the rotating earth to view east and west. This saved cost. Many early radio telescopes used this method. - Big Ear Memorial Website
A major display in the museum
More on Grote Reber started Jan 2012
"Grote Reber - Grassroots Beginnings of a New Science
"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."
"...I was too young to know it wasn't possible."
"If at first you don't succeed, try, try, again.
"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 detected nothing.
"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."
"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.
"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.
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. There is no sign, but this could have been the receiver Grote used in making the first radio frequency sky survey. Those copper pipe things on the right top are (I presume) resonant structures to help tune the front end.
This may be the tuned circuit used to help generate signals to help develop the 160 MHz receiver above. I bet the tube designer never dreamed they would be pushed to this frequency !! Grote may have removed the tube bases to save an inch or two of inductance. Looks as though one of the ?plate circuit? copper rods is popped out of its holder?
"Second Generation Receiver, center frequency 480 MHz, circa 1945" - about 0.62 meter wavelength. Maybe then, he was able to get WWII acorn vacuum tubes, which can work well at this frequency.
- Reber researches in Tasmania
"Low Tech ;-))" Tour time
About 20 people got on this diesel bus to go to the main attraction :-)) The tour guide says that old diesel vehicles have fewer electronic goodies, and are less electrically noisy. I bought 2 non-electronic cameras from the gift shop.
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: - turning off all consumer electronics, including digital and many analog cameras. - we go into the restricted area - through a barrier gate.
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.
We head for the R&D building, we can use regular digital cameras :-))
This is our High Tech tour guide, taking us by someone else's lower frequency receiver. This place is techie heaven, just gotta be - hardware of "all" descriptions all over the place.
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 :-(( This is the top section of "his" 18-22 GHz receiver.
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. Not shown is a Big Red Button that was not discussed ;-))
We head back to Harrisonburg, VA, the way suggested by Cara - it is much better :-))
During the rest of the week, we toured
- NSA National Cryptologic Museum - National Electronics Museum ( 1745 West Nursery Road, Linthicum Hts, MD 21090-2906 ) - Amish country near Intercourse, PA - Kennedy Center in Washington, DC - a spy tour of Washington, DC - Mt. Vernon, George Washington's home - the Smithsonian Aircraft Museum, near Dulles-Washington Airport
and probably won't leave home again for a year !! ;-))