Daily Image
14-07-2009
First Fringes for LOFAR!
| Submitter: | Michiel Brentjens |
| Description: | During the night of July 8/9, LOFAR experienced its "First Fringe" moment: the first detection of correlated signals between different LOFAR stations from the radio sky! We observed Cyg A for 10 hours with four stations: CS010 (old), and CS302, RS307, and RS503 (new). The observation started at 21:36 CEST, just before sunset. The image shows the XX and YY visibilities, averaged over a 195 kHz subband centred at 41.7 MHz. Every frame shows one minute worth of data in the complex plane. Every dot represents a single second in that minute. Thus, every frame shows the time evolution over a minute, while the movie shows the evolution from minute to minute. Blue dots are XX, and red dots are YY. The left panel shows the data for a short (4.8 km) baseline, the right panel for a long (19.5 km) baseline. Thirty minutes to two hours into the observation, the ionosphere behaves wildly due to sunset. Apart from rapid phase winding, which is worse for the longer baseline, one can also see short term amplitude effects: scintillation. Station CS302 shows an as yet un-understood phase offset between the XX and YY signals. This will be investigated and fixed shortly. Around t=215 minutes there is a bit of interference. After that, the ionosphere calms down. The long timescale amplitude variations are due to a combination of the source structure and the antenna beam pattern. After 370 minutes, the phases run around like crazy on the short baselines, while the signal completely decorrelates on the long baseline. This was due to a software problem in the signal processing in Groningen. This problem was quickly identified and resolved, and did not bother us anymore for the HBA observation that was recorded the next night. |
| Copyright: | ASTRON |
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