[Difx-users] structure of fits-idi files

[Franz Kirsten]

Hi all,

I am currently looking at the fits-idi files as they are spit out by 
DiFX -- and noticed a difference in the structure of the visibility 
array compared to that produced by SFXC**. I'd mostly like to know how 
those visibilities are structured/ordered in the 'FLUX' field of the 
UV_DATA table.

Just so you know what I look at (in python):
import astropy.io.fits as fits
hdulist=fits.open('./difx-file.idifits')
vis=hdulist['UV_DATA'].data['FLUX']
vis.shape
 > (n_times * n_baselines, some_number)

Now, in my case, some_number = 4096. I have 8 IFs, 128 channels/IF and 2 
polarisations. Assuming I have a pair of numbers (amp + phase, or 
real+imaginary?) for each visibility, 8 * 128 * 2 * 2 = 4096. But how 
are the data ordered? Are the, say, first 256 entries amp+phase for IF1, 
chan1-chan128, pol 1? In this logic the next 256 entries would be that 
of IF1, chan1-128, pol2. And then I'd jump to IF2 and repeat. Is this 
correct?

Many thanks for your help!

Regards,
Franz

**In fits-idi files that come out of SFXC:
vis.shape = (n_times * n_baselines, 1, 1, n_IF, n_chan, n_pol, 3), where 
the three entries in the last index are amp, phase, weight.