A Day at the Very Large Array

What has 28 dishes, changes size every four months, and surveys the sky day and night? The Very Large Array, of course! After the conclusion of the 242nd AAS meeting in Albuquerque, NM, AAS Media Fellow Ben Cassese and I joined members of the media for a tour of this exceptional facility.

A Telescopic Tour

The Very Large Array, or VLA, is a premier radio astronomy observatory located two hours south of Albuquerque in the Plains of San Agustin. The VLA’s storied history began in the 1960s, when astronomers began to push for an array of radio dishes to complement the science being done with single dishes like the ones at Green Bank and Arecibo. Science operations at the VLA began in 1976. The VLA’s dishes work together as an interferometer, in which signals from multiple telescopes are combined to give the sensitivity of a single dish with an area equal to the combined area of the dishes in the array and the resolution of a single dish as wide as the largest distance between dishes in the array.

The VLA is sited on a flat expanse of desert surrounded by mountains, and the dry climate, high altitude, and isolation from civilization make it an ideal location for a radio observatory. The isolation is necessary because of the dishes’ sensitivity, which makes them vulnerable to terrestrial radio interference — compare the strength of a typical 5-Watt cellphone signal from someone standing nearby to a 10^-23 W/m²/Hz signal from a distant galaxy. Luckily, having multiple dishes working together provides another defense: signals spotted by only some of the antennas or that arrive at some antennas before the rest are suppressed.

Another feature of the VLA is its maneuverability. Every four months, operators guide the VLA’s enormous dishes into a new configuration, cycling through four configurations every 16 months. Yes, that’s right — the 220-ton dishes move, taxiing to their new locations on the back of a transporter that glides along railroad tracks at 5 miles per hour. In its most compact form, the array’s antennas are snuggled together within a square mile. At its most extended, the dishes span 22 miles. Because of the need to cycle through the observing configurations, observations for a single project can take more than a year if the project requires several different observing configurations.

Astronomers who use the VLA for research will be familiar with the process of applying for time on the array and eagerly awaiting the data if time is awarded — but what goes on behind the scenes to support our science?

Getting a Bird’s-Eye View

As our tour of the observatory grounds began, Rob Selina and Bill Hojnowski explained what goes into maintaining and upgrading the VLA. In the antenna assembly barn, we were able to see one of the VLA’s 28 dishes undergoing maintenance. While the VLA contains 28 dishes, only 27 are active in the array at a time, with the final dish undergoing regular maintenance in the barn. An antenna typically spends about three years taking data in the array in between trips to the barn along the railroad tracks. The miles of railroad track along which the dishes travel are a considerable source of maintenance work, with about 2,000 railroad ties and 30,000 pounds of ballast needing to be replaced each year.

Next, we were treated to a rare opportunity: climbing into one of the dishes! After donning mandatory hard hats, our group scaled a narrow metal staircase that zigzagged up to the top of the support structure — not recommended if you have an intense fear of heights — and emerged through a trapdoor into the dish itself.

From that vantage point, we could see the dish’s eight radio receivers, each of which is optimized for a different frequency range. As Rick Perley (the VLA’s first postdoc, who has worked at the VLA since 1977) and Rob Long explained, having multiple receivers allows for observations at frequencies from 74 megahertz to 50 gigahertz, depending on what the science requires. Up in the dish, one thing was clear: things can get hot up here! Keeping things cool is a major challenge at the VLA; the New Mexico desert gets hot, and the sensitive instruments need to be cooled to perform their best. Most of the VLA’s $3 million annual electricity bill goes toward the compressors in the cooling system.

The cooling issue is just one of many engineering and maintenance challenges that have been surmounted by the VLA staff over the decades. They’ve handled everything from finding the best way to paint the dishes without affecting their performance, to figuring out how to connect wires from a rotating dish to a stationary platform, and even managing the birds that build their nests in the dish support structures — and as the VLA expands in the coming decade, new challenges are sure to arise.

What’s Next for the VLA?

The next phase for the VLA, known as the Next Generation VLA or ngVLA, will include 263 antennas spread across 500 miles. Because of the immense distances involved, the dishes can no longer travel back to the barn when maintenance is needed. Instead, technicians will need to meet the dishes where they are, as far afield as Hawaii, Puerto Rico, and New Hampshire.

Each dish will be roughly half the size of the current VLA dishes but more sensitive, yielding about the same observing power as the current dishes. The result will be an array with ten times the sensitivity of the VLA and up to one thousand times finer resolution. The new dishes passed preliminary design review in December 2022 and will be shipped to the site in early 2024. If things go smoothly, full science operations should be underway in 2035 — and I can’t wait to see the great research made possible by the ngVLA!

Visitation Information

If visiting the VLA sounds fun to you, you’re in luck — the VLA is open to the public 362 days a year with guided tours on the first and third Saturdays of each month. Unfortunately, the antenna climb isn’t part of the standard tour package! Visitation details can be found here.