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difx:difx2.0multipc [2009/10/29 09:06]
adamdeller
difx:difx2.0multipc [2015/10/21 10:08] (current)
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 ====== Multiple phase centres in DiFX2.0 ====== ====== Multiple phase centres in DiFX2.0 ======
  
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 The image below illustrates what is happening.  The red circle indicates the primary beam of the antenna - something like 30 arcmin for a 25m dish at 1400 MHz, for example.  However, imaging that entire primary beam at VLBI resolution is prohibitive - it requires 10s or 100s of TB of visibilities to be produced, and the end result would be an image with 100s of Gpixels, 99.9999999% of which would be noise.  Since we don't want that, your typical VLBI correlator produces visibilities with time resolution of ~seconds, and frequency resolution of ~100s kHz.  This means that the visibility data set is much smaller - only GBs.  Also, time and frequency smearing limits the useful field of view to a few arcseconds, meaning the final images are much smaller (and much better matched in size to the objects typically studied).  A single correlator field of view (a "pencil beam") is represented on the image (not to scale) as a green circle. The image below illustrates what is happening.  The red circle indicates the primary beam of the antenna - something like 30 arcmin for a 25m dish at 1400 MHz, for example.  However, imaging that entire primary beam at VLBI resolution is prohibitive - it requires 10s or 100s of TB of visibilities to be produced, and the end result would be an image with 100s of Gpixels, 99.9999999% of which would be noise.  Since we don't want that, your typical VLBI correlator produces visibilities with time resolution of ~seconds, and frequency resolution of ~100s kHz.  This means that the visibility data set is much smaller - only GBs.  Also, time and frequency smearing limits the useful field of view to a few arcseconds, meaning the final images are much smaller (and much better matched in size to the objects typically studied).  A single correlator field of view (a "pencil beam") is represented on the image (not to scale) as a green circle.
  
-{{primarybeam_pencilbeam.png?800}}+{{primarybeam_pencilbeam.png?250}}
  
 Now, there are a few ways to get dozens of these pencil beams.  In any old correlator, including any version of DiFX, you could run dozens of correlator passes, each with a different phase centre.  That obviously gets prohibitive in time pretty quickly.  Alternatively, you could write out the data at high time and frequency resolution (avoiding the smearing effects) and uvshift post-correlation.  Not many correlators can produce such fine-grained data - DiFX and the JIVE correlator are the only two I know of.  Still, to do shifts to the edge of the primary beam, you need to turn 100s of TB of baseband data into many 10s of TB of visibilities, then repeatedly read that dataset into memory, shift and average, and write it out again.  Again, this rapidly becomes prohibitive in time (and disk space!) Now, there are a few ways to get dozens of these pencil beams.  In any old correlator, including any version of DiFX, you could run dozens of correlator passes, each with a different phase centre.  That obviously gets prohibitive in time pretty quickly.  Alternatively, you could write out the data at high time and frequency resolution (avoiding the smearing effects) and uvshift post-correlation.  Not many correlators can produce such fine-grained data - DiFX and the JIVE correlator are the only two I know of.  Still, to do shifts to the edge of the primary beam, you need to turn 100s of TB of baseband data into many 10s of TB of visibilities, then repeatedly read that dataset into memory, shift and average, and write it out again.  Again, this rapidly becomes prohibitive in time (and disk space!)
difx/difx2.0multipc.txt · Last modified: 2015/10/21 10:08 (external edit)