The Coolest Part of Astronomy

Long, long ago… or if you’re an astronomer, far, far away…

STARS EMIT LIGHT Stars, galaxies, even dust and complex sugar molecules give off signals (signals = light = radiation) that is of GREAT interest to Earth-bound (for now) astronomers and engineers. 

STARLIGHT GETS FAINTER Travelling across the universe takes its toll on the signals, just a bit (sarcasm), and by the time the faint light sprinkles down on our planet the signals are so meager it takes a gargantuan effort – a VERY LARGE TELESCOPE – and the best cryogenic receivers cooled to ~ -450 degrees F to accurately pick them out of the technological noise blaring from our planet. 

A LARGE ANTENNA or SET OF ANTENNAS COLLECT STARLIGHT The incredibly faint signals are collected with an array of giant radio antennas or sometimes one single, very-large dish. The collection area of these antennas can range from 10 to 100m in diameter. The larger the reflector ( a 100 m dish at 100 GHz can resolve a 6.2 arc-second feature. A 4cm object needs to be placed 1.333m = 4000 feet = 0.8 miles away to subtend this angle.) This is called the resolution of a telescope, or the ability to see small features, to resolve it from another object beside it. 

The resolution can be further improved by using interferometers; an array of multiple antennas separated by large distances, their signals are digitally combined to synthesize a telescope as large as the furthest distance between the antennas. For example, the Event Horizon Telescope linked eight radio observatories from across the globe —basically, synthesizing an Earth-sized telescope — to be able to see the small details of the black hole at the center of M87, a galaxy far beyond the Milky Way, past our neighbours, beyond the Local Group and in the Virgo Cluster.

Remember the golf ball a mile away? With an earth-size interferometer, observing at a wavelength of 1.3 mm, the golf ball would need to be placed on the Moon to simulate the smallest object this interferometer could see (an interferometer with telescopes separated by 12,000 km can resolve a 22 micro-arcsecond feature. So, a 4 cm object needs to be placed 400,000 million meters away). Hence, the resolution of an interferometer depends not on the diameter of each individual radio telescope, but on the maximum separation between them. To put it another way: the angle astronomers can see is the ratio between the observing wavelength and the distance between telescopes.

COOLING THE NOISE TEMPERATURE Not only is the telescope able to see small features, but it is also extremely sensitive to faint signals. This is accomplished by using cryogenic receivers on the larger dishes, making the faint objects discernable. With a 100m telescope and a cryogenic receiver, the sensitivity of the telescope increases and we can detect extremely faint objects from beyond our galaxy.

How does cooling a receiver to cryogenic* temperatures (16 K = -257 deg C = -431 deg F) just above absolute zero, increase the sensitivity (i.e. allow astronomers to observe fainter and fainter signals)? All materials, due to their temperature and internally generated noise, contribute extra activity over and above the faint signals we are trying desperately to detect. By reducing the temperature, these internally generated noise levels can be reduced drastically, allowing fainter and fainter signals to be detected.

The cryogenic receiver is usually composed of a feedhorn that collects the light and transforms it onto another component called a waveguide. The waveguide minimizes the loss of energy by restricting energy transmission in one direction. But then we split the signal in two! The signal is separated into vertical and horizontal polarization pathways through another component called an orthomode transducer. From there the signal goes through many stages of filtering and further amplification before being detected and put onto fiber optic cable. Usually placed in a central control room, the fiber cables from a single-dish telescope (or all telescopes in the array) are combined to form an image that the astronomer can study.

*The cryogenic temperature range has been defined as from −150 °C (−238 °F) to absolute zero (−273 °C or −460 °F), the temperature at which molecular motion comes as close as theoretically possible to ceasing completely.

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About the Author:
Dr. Lisa Shannon Locke
Dr. Lisa Shannon Locke was born in Hay River, Northwest Territories, Canada. She received her Bachelor's degree in electrical engineering from the University of Alberta in 1997, and a Masters in electrical engineering from the University of Cape Town in 2001. For the next four years, she was a receiver engineer with the Arecibo Observatory in Puerto Rico. In 2005 she joined the NRAO in Socorro, NM, working on cryogenic receivers for the EVLA upgrade. In 2010 she started her doctoral research in electrical engineering at the University of Victoria in Victoria, BC. Her dissertation was on the design and construction of a K-band coherent phased array feed for use on large single-dish radio telescopes. After receiving her PhD in 2014, Dr. Locke was a postdoctoral research engineer with the Millimetre Instrumentation Group at the NRC Herzberg Astronomy and Astrophysics Research Centre. She led a team that developed a cryogenically-cooled 140-element phased array feed receiver (CryoPAF4), and helped design a single-pixel Q-band cryogenic receiver with possible application to the ngVLA. In 2018, Dr. Locke received an NRAO Jansky Fellowship, the first-ever awarded to an engineer. For her two-year appointment, she is based at the Central Development Lab in Charlottesville, where she is engaged in independent research on the electromagnetic design of antennas, feeds and orthomode transducers for radio astronomy.
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