[Difx-users] structure of fits-idi files

[harro verkouter]

Hi Franz,

FITS dates from the era of FORTRAN and thus the first axis should go
fastest, then the 2nd etc.
But it depends a bit on what the (implied) order of the axes is in the difx
FITS-IDI array. You may want to take a closer look at the full header for
the 'UV_DATA' table - there may be hints in there what the order of the
axes is within the 'FLUX' column. The logical choice would indeed be IF *
CHANNELS * POL * 2 values / complex number (or 3, in sfxc's case; we record
a weight per spectral point too: (re, im, wgt)), thus IF goes first, then
channel etc.

In sfxc those axes have been made explicit - as you can tell, so there's no
ambiguity of how to interpret them.

Cheers,
h


On 25 October 2016 at 06:28, Franz Kirsten <franz.kirsten at curtin.edu.au>
wrote:

> 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.
>
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