The Robert C. Byrd Green Bank Telescope is the most unique of the giant single-dish radio telescopes in the world. Just one look at it, and you can see that it's not shaped like a typical radio telescope. Its designers wanted the dish to be the most accurate in the world, and their approach to achieving that was manifold.
A typical radio telescope dish is shaped like a parabola, which is a perfect bowl whose sides bounce all radio waves up to the same point above it, called a focus. Precisely at this focus, and on top of a pyramid of sturdy poles, radio receivers swallow all of the waves heading upwards.
Sometimes the distance to the focus is way too high above a large dish to install receivers that can be swapped or maintained easily. Telescope designers can split the distance to the focus by setting up a secondary mirror in the waves’ path upward. The rising waves are stopped by this smaller mirror and sent straight back down to the center of the dish where receivers nestle in a cradle just below the surface.
In both of these designs, metal arms hold a large station above the dish, and those arms block radio waves and scatter others, reducing the amount of signal that the dish receives. Radio waves from space arrive on Earth with a billionth of a billionth the power of a cell phone signal, so we need all of the waves we can get!
Here’s where the GBT differs from these classic designs: The secondary of the GBT is not sitting at the top of a pyramid of poles casting shadows down onto the dish. Instead, the secondary of the GBT juts out of the top edge of the telescope, looking like the Arm & Hammer icon, so that it does not block the dish at all.
To use this “off-axis” arm, the GBT dish is shaped as if it were cut out of the bottom section of a larger parabola, instead of the middle of a parabola. With its shape cut out from the bottom of the parabola, the top edge of the GBT's dish is where the center of the big parabola would have been. The focus, therefore, is 60 feet above that top edge, and that's where the end of the feed arm is.
Turret and Prime Focus
The 200-foot feed of the GBT is an enormous arm built from two angled towers that meet in a peak at the prime focus above the dish. There, a boom arm can either swing a secondary mirror or a giant receiver into the focused radio waves coming up from the dish.
When the telescope is being used with its secondary mirror, scientists have access to a suite of eight different receivers. Ranging from 100 MHz to 100 GHz, these receivers hang in a rotating turret that forms the roof of a small receiver room suspended on the feed arm. Scientists can switch to a different frequency with a spin of the turret.
Being attached to the feed arm, the receiver room tilts as the telescope tilts, and its contents are carefully secured to keep the equipment safe -- even the garbage can has been hinged to the wall.
State-of-the-art receivers for the GBT range in frequency from 0.3-100 GHz. Several array receivers have been designed for the GBT, including the MUSTANG bolometer array, a focal plane array, and an experimental phased array feed that slides into the prime focus. To keep these receivers cold, 1,000 feet of helium supply lines slither in and around the cartridges in the receiver room to the helium tanks below.
The GBT became the world’s largest single dish millimeter-wave telescope in September of 2006 when MUSTANG (the Multiplexed SQUID TES Array at Ninety Gigahertz) was used for the first time.
MUSTANG is a receiver package containing 64 detectors. When placed on the GBT, it turns the great big dish into an array of 64 mm-wave antennas.
The receivers are bolometers, which means that instead of digitizing a signal of radio waves, the 64 receivers feel the 90GHz, millimeter-wave radiation that hits them. You are a bolometer: you can detect infrared radiation as heat you feel in your hand. If you’ve ever tried to detect a fever on someone’s forehead, you know that your hand needs to be a lower temperature, or you will not feel the heat.
To pick up the billionth of a billionth of a watt power from the sources it observes, MUSTANG is cooled to nearly absolute zero.
Both inside the receiver room and back at the GBT's control room nearly two miles away, scientists and engineers have invented innovative systems for processing the radio data. A Zpectrometer, built by the University of Maryland, hangs next to a double-feed receiver inside the GBT. The Zpectrometer uses the combined views of the two feeds to create wide field observations to hunt for the redshifts of distant galaxies.
An availability of processors for gaming computers enables the Green Bank Ultimate Pulsar Processing Instrument (GUPPI) to do high-precision analysis of pulsar data. We use GUPPI to spot minute changes in a pulsar's timing caused by gravitational and relativistic effects.
The Versatile GBT Astronomical Spectrometer (VEGAS) is similarly designed with these fast gaming chips to harness the power of the new K-band Focal Plane Array by providing digital spectroscopy on up to eight dual-polarization inputs, each with a total bandwidth of up to 1.25 GHz.
Most Accurate Surface
Underneath each of the corners of the telescope’s 2004 surface panels is a small robotic motor called an actuator. The GBT requires 2209 actuators to gently push and pull on the 2.3 acres of surface to maintain its perfect shape.
The engineers who designed the GBT know its every bolt and weld, enough to be able to model the changes in the surface for every move the telescope makes and every change to the surface caused by sunshine.
These complex algorithms were fed into the computers that run the robotic actuators, giving this system the autonomous ability to nudge and tug on the GBT’s surface even as the telescope is observing. A scaled-down system exists up on the telescope’s secondary mirror.
The actuators on the GBT keep its parabolic shape to within the thickness of about five human hairs, making it the most accurate telescope of its size in the world.