[Difx-users] structure of fits-idi files

[Adam Deller]

Hi Franz,

I looked up the FITS-IDI standard, and I think it describes how to extract
this information.  See Section 4.1 of http://www.aoc.nrao.edu/~
egreisen/AIPSMEM114.PDF.  Basically, you need to look at the CTYPE#
keywords, where # runs from 1 through to 5 or 6.  CTYPE1 describes the
fastest varying parameter, CTYPE2 the next fastest, and so on.
Corresponding keywords MAXIS# give the number of entries along that axis.
CTYPE1 is constrained to be of type COMPLEX (i.e., the re/im[/weight]
values) but the other ones can come in whatever order, you need to look at
the keywords.  They are STOKES for polarisation, FREQ for frequency
channel, and BAND for the subband.

These should all be accessible via astropy.

Cheers,
Adam

On 25 October 2016 at 17:57, harro verkouter <hverkouter at gmail.com> wrote:

>
> 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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>
>
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-- 
!=============================================================!
Dr. Adam Deller
ARC Future Fellow, Senior Lecturer
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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