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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)