Daily Image

30-07-2010
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Hans' Surprise

Submitter: Oleg Smirnov & Ger de Bruyn
Description: In a previous submission http://www.astron.nl/dailyimage/main.php?date=20100223), we showed how solving for "differential gains" in the direction of off-axis sources can be used to improve images by removing artifacts due to direction-dependent effects (DDEs). It remained to be seen whether these solutions could be reliably associated with an actual physical DDE.

To test this, we requested a WSRT 21cm observation of a field with several bright off-axis sources (exactly the sort of field radioastronomers usually like to avoid!), and asked Hans van Someren to deliberately mispoint some antennas by a small amount. Exactly which antennas were mispointed and by how much was known only to Hans; we wanted to see if we could deduce this from our differential gain solutions. Looking at the solutions, we quickly (and correctly) concluded that RT1, 4 and 7 were mispointed, but it also turned out that antenna RTC had a more interesting story to tell.

Hans had decided to leave us a little surprise, by making the (mis)pointing of RTC time-variable! The differential gain-amplitudes indicate this in an obvious way. The above animation shows the evolution in time of the amplitude solutions for antenna C towards a number of sources in the field. Each frame corresponds to one solution interval of 30 minutes. Red circles indicate differential amplitudes below unity, and blue ones above unity, while size corresponds to magnitude.

If the antenna is mispointed "towards" an off-center source, we would expect that source to have differential gain-amplitude >1 (blue), and <1 (red) when it is mispointed "away". The animation clearly shows the area of increased amplitudes swinging around from vaguely NW to vaguely NE. Hans later confirmed that this was indeed the way the mispointing had been set up.
Copyright: MeqTrees Foundation
 
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