[Difx-users] complex support

[Jayce Dowell]

Hi Adam, Chris, and Cormac,

I'm working the data from the Long Wavelength Array and I am trying to 
see if we can move from our homegrown correlator to DiFX.  The two modes 
I need to support are a LWA-only mode with homogeneous data and an 
"eLWA" mode with heterogeneous data that combines the LWA stations with 
the VLA 4-band system.  For the LWA-only mode I have 4+4-bit complex 
voltage data from two 19.6 MHz wide spectral windows.  I am currently 
converting these data from the LWA format into two VDIF files, one per 
window, for correlation with DiFX.  My .input file describes the data as:

DATA FORMAT:        INTERLACEDVDIF/0:1
QUANTISATION BITS:  4
DATA FRAME SIZE:    7872
DATA SAMPLING:      COMPLEX_DSB
DATA SOURCE:        FILE

and I can at least run mpifxcorr without any fatal errors at this point. 
  The auto-correlations suggest that something in my VDIF conversion is 
off.  I need to look at that a little more closely before I try the 
suggestion of using trunk instead of my current 2.6.2 installation.

I am less sure about DiFX supporting our eLWA mode.  For this we have 
the same 4+4-bit complex voltage data from the LWA stations but with a 
bandwidth of 9.8 MHz.  We are combining this with 4-bit real voltages 
from the VLA with 8 MHz of bandwidth and a slightly different frequency 
setup.  We currently process these data by shifting the center frequency 
of the LWA data to match that of the VLA, running two F-engines with 
different channelization to arrive at the same channel width for both 
data sources, and then cross correlate what overlaps.  Is this mode 
something that DiFX supports?

Jayce
--
On 2/3/21 8:09 PM, Adam Deller wrote:
> *  [EXTERNAL]*
> 
> **
> Complex double sideband has now been much more extensively tested in 
> DiFX trunk but mainly for one specific case, which is data from ASKAP 
> (so that is CODIF format, and lower sideband.)  So it depends a little 
> on your setup is.  If you provide some more details on the data source 
> and the configuration, we might be able to provide some advice (or a 
> short data file is always helpful if that is possible.)
> 
> Cheers,
> Adam
> 
> On Thu, 4 Feb 2021 at 07:41, Jayce Dowell via Difx-users 
> <difx-users at listmgr.nrao.edu <mailto:difx-users at listmgr.nrao.edu>> wrote:
> 
>     Hi,
> 
>     I was wondering what the status of complex double sideband support
>     is in
>     DiFX?  I had found something that suggests that it should work but that
>     it was largely untested.  The reason I ask is that I have been
>     attempting to use it but I keep ending up with results don't make
>     sense.
>        I am trying to figure out if it is the data, my configuration, or
>     something within DiFX itself.
> 
>     Thanks,
>     Jayce
>     --
> 
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> -- 
> !=============================================================!
> A/Prof. Adam Deller
> ARC Future Fellow
> Centre for Astrophysics & Supercomputing
> Swinburne University of Technology
> John St, Hawthorn VIC 3122 Australia
> phone: +61 3 9214 5307
> fax: +61 3 9214 8797
> 
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