Interferometry

Why do we talk about ALMA as though it were a single telescope, when in fact it is made up of several antennas spread out across the Chajnantor Plateau?

These antennas make up the array mentioned in its name: Atacama Large Millimeter/submillimeter Array (ALMA). Its modern design enables the antennas to work together as if they were a single giant telescope, more powerful than any individual antenna that can be built. Without this particular technology, it would be impossible to achieve ALMA’s ambitious scientific goals. This is due to a fundamental limitation of any telescope made up of a single antenna or mirror: its low level of precision.

The resolution (or level of detail of the image) attained by a telescope made up of a single antenna depends on both the wavelength in which it operates and the diameter of its antenna or mirror. A longer wavelength results in a lower resolution and a greater diameter results in a better resolution. In consequence, a telescope that captures longer wavelengths obtains a lower image resolution than a telescope of the same size that operates on optic or infrared lengths.

The European Southern Observatory’s (ESO) Very Large Telescope (VLT) on Cerro Paranal to study optic and infrared waves has 4 individual telescopes with 8.2-meter diameter mirrors. At an approximate infrared wavelength of 2 micrometers, these attain -thanks to the adaptive optics- a maximum resolution of approximately 50 milliarcseconds (just over 10 millionths of a degree). The 12-meter diameter of the ALMA antennas is 50% greater than the diameter of the VLT mirrors. However, ALMA observes in wavelengths on a submillimetric range, in other words, with wavelengths up to one thousand times longer than those of infrared light. This far surpasses the slight advantage that these antennas have over the VLT mirrors in terms of size, where an ALMA antenna operating on millimetric wavelengths has a resolution of 20 arcseconds.

In fact, for a single ALMA antenna to reach a resolution comparable to the VLT resolution, it would need several kilometers of reflective surface, whose construction would undoubtedly be non-viable. This is why ALMA is comprised of an array of antennas spread out across a vast area, working together under a method known as interferometry.

The resolution of an interferometer does not depend on the diameter of the individual antennas, but rather the maximum distance between these (baselines), because the resolution increases as the distance between them increases. The antenna signals are combined and processed on a supercomputer -the ALMA Correlator- to simulate an individual radio telescope. In other words, an interferometer works like a radio telescope equivalent to the size of the complete array.

As the distance between the antennas increases, the resolution capacity of the interferometer increases, enabling it to capture more subtle details. The possibility of combining signals from antennas separated by several kilometers of baseline is crucial to obtaining an extremely fine resolution and very detailed images.

The ALMA main array has 50 12-meter diameter antennas arranged in specific layouts with distances from 150 meters up to 16 kilometers. This array simulates a giant telescope, much larger than any single-antenna telescope that can feasibly be constructed today. In fact, ALMA has a maximum resolution that is even higher than the Hubble space telescope’s resolution on visible wavelengths.

Four additional 12-meter diameter antennas and twelve 7-meter antennas form the Morita Array or the Atacama Compact Array (ACA). The 7-meter diameter antennas can concentrate on a smaller area without interfering with each other. Due to the way in which the interferometers work, this arrangement provides a more general image of the astronomical objects being observed. On the other hand, the four ACA 12-meter diameter antennas are used separately to measure the absolute glow of the objects observed, a level that cannot be measured with an interferometer.

The different telescope configurations enable astronomers to study both the general structure of an astronomic source and its most minuscule details. However, to move from a more compact configuration to a broader one, the antennas must be moved. This is done with customized transporter trucks that are capable of lifting the antennas (which weigh over 100 tons) and transporting them along several kilometers of desert and then placing them on concrete pads with millimetric precision.

With interferometry, the numerous ALMA antennas operate together as a single scientific device, enabling astronomers to make observations that would otherwise be impossible to attain with a single antenna. This is why we refer to ALMA as a revolutionary telescope instead of a group of antennas.

To learn more about how interferometry works at ALMA, see the article entitled How will ALMA make images?, published in ALMA Newsletter nº 5.