APOD: The Very Large Array at Moonset (2020 Oct 09)

[APOD Robot]

The Very Large Array at Moonset

Explanation: An inspirational sight, these giant dish antennas of the Karl G. Jansky Very Large Array (VLA) rise above the New Mexico desert at moonset. Mounted on piers but transportable on railroad tracks to change the VLA’s configuration, its 27 operating antennas are each house-sized (25 meters across) and can be organized into an array spanning the size of a city (35 kilometers). A prolific radio astronomy workhorse, the VLA has been used to discover water on planet Mercury, radio-bright coronae around stars, micro-quasars in our Galaxy, gravitationally-induced Einstein rings around distant galaxies, and radio counterparts to cosmologically distant gamma-ray bursts. Its vast size has allowed astronomers to study the details of radio galaxies, super-fast cosmic jets, and map the center of our own Milky Way. Now 40 years since its dedication the VLA has been used in more than 14,000 observing projects and contributed to more than 500 Ph.D. dissertations. On October 10, the National Radio Astronomy Observatory will host a day-long online celebration of the VLA at 40 featuring virtual tours and presentations on the history, operations, science, and future of the Very Large Array.

<< Previous APOD

This Day in APOD

Next APOD >>

[MarkBour]

Wow, that moonset looks a lot like a sunset! Is there any way to tell? Zooming in does not reveal any lunar surface features that I can see, it's just washed out ... If it is indeed the Moon, it looks like a full Moon, for all I can tell. Doesn't the reflection off of the Moon have a different spectrum than direct sunlight? But then it is filtered through our atmosphere a lot, because it is setting.

[heehaw]

Wow! Someone's making soup in that closest big dish!

[JohnD]

Will someone please explain the need for the railway tracks?

I can see (just) that a wider spaced set of dishes will give better detection ability, like a larger lens on a camra, but if the same number is used as when closer together, the resolution or clarity of the image will suffer. So, the dishes are redistributed according to the research requirements?

John

[orin stepanek]

Wow! what a gorgeous view! That moonset showing off the
array & tracks is really nice!

[zeecatman]

This picture is stunning, as is the size of both the dishes and the array itself! Human ingenuity is truly impressive. That said, something about the fact that the official name for this structure is "Very Large Array" is really funny to me.

[Case]

Click to play embedded YouTube video.

The VLA (along with Arecibo) is also known to be featured somewhat prominently in the movie Contact. (The movie Contact itself is based on Carl Sagan’s novel of the same name.)

[TheZuke!]

The VLA (along with Arecibo) is also known to be featured somewhat prominently in the movie Contact. (The movie Contact itself is based on Carl Sagan’s novel of the same name.)

IIRC, The VLA is also shown in the beginning of the movie "2010"

[neufer]

Will someone please explain the need for the railway tracks?

I can see (just) that a wider spaced set of dishes will give better detection ability, like a larger lens on a camra, but if the same number is used as when closer together, the resolution or clarity of the image will suffer. So, the dishes are redistributed according to the research requirements?

Any fixed spacing set of dishes will produce an artificial interferometric pattern array in the output image.

This can only be properly treated (in the North/South direction) by adjusting the spacing physically.

6 artificial diffraction spikes from a wye optical configuration

---

<<The Karl G. Jansky Very Large Array (VLA) comprises 27 independent antennas, each of which has a dish diameter of 25 meters and weighs 209 metric tons. The antennas are distributed along the three arms of a track, shaped in a wye (or Y) -configuration, (each of which measures 21 km long). Each of the massive telescopes is mounted on double parallel railroad tracks, so the radius and density of the array can be transformed to adjust the balance between its angular resolution and its surface brightness sensitivity. Using the rail tracks that follow each of these arms—and that, at one point, intersect with U.S. Route 60 at a level crossing—and a specially designed lifting locomotive ("Hein's Trein"), the antennas can be physically relocated to a number of prepared positions, allowing aperture synthesis interferometry with up to 351 independent baselines: in essence, the array acts as a single antenna with a variable diameter. The angular resolution that can be reached is between 0.2 and 0.04 arcseconds.>>

[neufer]

Click to play embedded YouTube video.

Wow! Someone's making soup in that closest big dish!

[JohnD]

Thnak you neufer!
But I thought that star spikes with the result of diffraction, not interference, as light glanced down the sides of whatever was holding the mirror or objective lens in the aperture of the telescope (3 or 4 legs?). Or in the case of a camera, the slips of the iris, 6, as in your pic.

There is, I suppose a virtual objective in a scattered array - does it have as many legs as the unit design, or as many legs as there are units?

John

[neufer]

I thought that star spikes with the result of diffraction, not interference, as light glanced down the sides of whatever was holding the mirror or objective lens in the aperture of the telescope (3 or 4 legs?). Or in the case of a camera, the slips of the iris, 6, as in your pic.

There is, I suppose a virtual objective in a scattered array - does it have as many legs as the unit design, or as many legs as there are units?

Diffraction spikes are admittedly a poor analogy here.

There are 3 artificial lengths that produce 3 artificial distortions of the image:

• 1) the dish size
  2) the dish separation and
  3) the full dish extent.

Movement along the rail tracks permits a natural way to deal, at least, with #2) the dish separation.

[JohnD]

OK, Thanks neufer!