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Do Antennas in an Array Always Need to Point to the Same Spot?

-- | July 29, 2014

Question:  There is a very nice animated clip of the array set as the bing.com background today (7-11-14).  I noticed that a few antennas are not rotating in sync with the rest (example: 1,2,4 and 9).  If the antennas move together to represent a single radio how is the information processed correctly, even if a single antenna is out sequence?  My first theory is that the antennas don’t have to rotate at the exact same time, and the information is rendered by software to represent a single antenna.  However, I would imagine that by not having a perfectly synced array, there would be a slight delay while waiting for the software to process the out-of-sequence data.  Can you please explain how this data is processed if antenna anomalies exist and can this affect data/image output?  – Avinash Lampman, 9th grade COVA

Answer:  When the antennas in an interferometric array like the Atacama Large Millimeter Array (ALMA) or the Very Large Array (VLA) are making astronomical measurements of an object, they have to all be pointed at the object.  If the antennas are moving from one object to another, though, they don’t have to all exactly track the same position on the sky.  The animated clip you saw was probably filmed when the antennas were moving from being pointed toward one object to pointing at another object in another part of the sky.  Now, the way that we deal with anomalies, such as subtle differences between antennas or in the way that each antenna tracks a source is by making observations of bright, point-like objects that we call “calibrators”.  Most of our calibrators are in fact quasars whose position, and in many cases intensity, we know to a very high precision.  By making all of the antennas in an array measure one of these calibration sources we can correct for any differences between how the antennas measure sources.  These anomalies, by the way, generally degrade the quality of an image, making it “blurry”.

Jeff Mangum