One of the more powerful techniques of radio astronomy is the use of interferometry to combine the signals of several radio antennas into a single virtual telescope. Through interferometry we can make radio images with resolutions greater than that of the Hubble telescope. But how does interferometry work?

At a basic level, interferometry is simply the combining of signals from two different sources. If the two signals are similar they will combine to make a stronger signal, and if they aren’t they will tend to cancel out. Where this becomes useful for astronomy is that if two signals are out of sync you can shift them (correlate them) so that they are in sync. When the signal is strongest you know they are lined up.

A schematic of an interferometer system based on Thompson et al.

When two radio antennas are aimed in the same direction, they receive the same basic signal, but the signals are out of sync because it takes a bit longer to reach one antenna than the other. That difference depends on the direction of the antennas and their spacing apart from each other. By correlating the two signals, you can determine location of the signal in the sky very precisely. It’s the precision that you need to create a high resolution image.

Two antennas only give you one point in the sky, but with dozens of antennas (such as the array at ALMA) you can get lots of points, one for each paring of antennas. But even that only gets you a discrete set of image points. If the Earth were fixed in relation to the sky, then our radio image would look like a pointillist painting.

But the Earth rotates with respect to the sky, so as time goes by the relative positions of the antennas shift with respect to an astronomical signal. As you make observations over time, the gaps between antennas are filled to create a more solid image. This isn’t easy to do. It takes lots of observations and lots of computing power to combine the images in the right way. At ALMA, for example, it takes a custom built supercomputer that spends all its time correlating signals.

But the results are nothing short of spectacular.

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Chajnantor means “place of departure,” or more poetically “place of ascension” in the Kunza language of the Atacama region. It is a plateau about 5000 meters (16,000 feet) above sea level. Its elevation and arid climate makes for extremely difficult working conditions, but it also makes it perfect for the Atacama Large Millimeter/submillimeter Array, or ALMA.

ALMA is one of the first truly international astronomical endeavors. Rather than being spearheaded by a single nation with others lending primarily financial support, ALMA is a collaboration between the United States (NRAO), Europe (ESO), East Asia (NAOJ) and the Republic of Chile. It’s coordination has been likened to the United Nations. Given ALMA’s 1.4 billion dollar price tag, international collaboration was the only way the project was feasible.

A 12-meter antenna. Yours truly for scale. Credit: Tim Spuck

ALMA consists of more than 60 12-meter antennas as well as 12 7-meter antennas. The 7-meter antennas are designed to be closely spaced, forming the Atacama Compact Array (ACA). Since the antennas use interferometry to create images of the sky, the ACA creates a wide sky view, while the larger array of 12-meter antennas allows us to focus in on particular objects. The antennas can be moved to different locations to allow for different scales and resolutions.

The engineering of ALMA is incredibly ambitious. In order to combine signals from the antennas, a supercomputing correlator had to be built on the plateau. It is the highest altitude supercomputer on the planet. The correlator not only has to account for the arrangement of the antennas, but also the orientation of the Earth relative to the target object. As the Earth rotates, the effective separation of the antennas change. While this gradual change is a computing challenge, it also allows us to create a more complete image of objects.

Because ALMA focuses on millimeter wavelengths, it is perfectly suited to image cold molecular clouds, both in interstellar regions and surrounding young stars. Since it can image these clouds with the resolution similar to that of the Hubble telescope, it’s able to provide an incredible view of things like planets forming around other stars.

ALMA has only begun what is intended to be a 30-year mission to study the universe. As the largest international astronomy collaboration, it is perhaps fitting that it resides at Chajnantor, as it will likely be a place of departure toward some incredible astronomical discoveries.

This post was made possible in part by the ACEAP project, funded by the National Science Foundation.

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One of the advantages of radio astronomy is that you can connect observations from radio telescopes thousands of miles apart.  Done in the right way, this creates a radio interferometer that effectively makes a virtual telescope as big as the separation (baseline) of the individual telescopes.  The bigger your telescope (or virtual telescope), the finer the detail of your image.  When we talk about the detail level of an astronomical image, we usually talk about the angle of separation between two distinctly resolvable points.  So a resolution of a tenth of a degree would mean you could resolve two points of light (such as stars) separated by at least that angle.  

In modern astronomy our resolution is much better than degrees, and we usually measure things in terms of arcseconds. An arcsecond is 1/3600 of a degree. It comes from dividing a degree into 60 minutes of arc, and each minute into 60 seconds of arc. Yes, the terms stem from their historical relation to time measurements.  Beyond arcseconds, we simply divide them into parts by base 10.  So a thousandth of an arcsecond is a milliarcsecond and so on.

Given the baseline of radio telescope interferometers, the upper resolution is typically on the order of milliarcseconds. But now a new paper in the Monthly Notices of the Royal Astronomical Society has resolved a radio pulsar to picoarcseconds. That is, a trillionth of an arcsecond, or about a separation of 5 kilometers 2000 light years away.

The method they used is quite clever. As the pulsar rotates, it radiates radio energy in beams from the magnetic poles.  These radio beams sweep through the surrounding interstellar media.  As the radio beams travel through the interstellar media in the general region of the pulsar, they are distorted.  By measuring the motion of the pulsar itself, the team was able to use the distorted radio signals from the radio beams to create an astronomical interferometer. But instead of having a baseline of a few thousand kilometers, the effective baseline is about 5 AU, or the distance from the Sun to Jupiter. With such a precise resolution the team found that the emissions of the pulsar’s radio waves are closer to the surface of the pulsar than we had thought.  This could help us understand how pulsars generate such strong radio signals.

This trick will only work with radio bright objects like pulsars, but it does demonstrate how clever observations can produce results that are far better than we ever thought possible.

Paper: Ue-Li Pen, et al. 50 picoarcsec astrometry of pulsar emission. MNRAS (May 01, 2014) 440 (1): L36-L40. doi: 10.1093/mnrasl/slu010

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