[Difx-users] zoom/recband and ACCOR for DiFX 2.6.1

[Adam Deller]

Hi Jan,

What if you plot the raw (uncorrected) autocorrelation and cross
correlation amplitudes using POSSM? Based on what you've written, I expect
it will show that the cross correlation amplitudes are identical in the two
correlations and the autocorrelation amplitudes are different.  This of
course should not be, but that would be consistent with what ACCOR is
doing.  But it would be good to isolate that first, because then that tells
us we need to go find how the autocorrelation amplitudes could possibly be
different with a simple flip.

The one thing I could think of is that there is a large DC spike and it has
been zeroed out in the zoom band case (because the DC channel became the
upper edge channel and gets forced to zero) and that has affected the
band-average autocorrelation amplitude.  POSSUM spectra of the raw
amplitudes should confirm one way or the other.

Cheers,
Adam

On Tue, 26 Nov 2019 at 02:13, Jan Wagner via Difx-users <
difx-users at listmgr.nrao.edu> wrote:

> Noted one strange issue in DiFX 2.6.1. Has anyone noticed this and
> can confirm it?
>
> The scaling of DiFX data converted with difx2fits and loaded into
> AIPS appears to be different when using zoom band correlation,
> versus correlating natively recorded bands:
>
>
> Case: Stations recoded 32 MHz LSB at 86348.00 MHz. Two correlation
> runs with DiFX 2.6.1.
>
> Correlation (1): 32 MHz LSB recorded bands flipped to USB via
> addZoomFreq freq at 86348.00/bw at 32.0/noparent at true
>
> Correlation (2): 32 MHz LSB recorded bands correlated as-is
>
>
> Processing: difx2fits, AIPS FITLD digicor=1, ACCOR solint=2/60,
> CLCAL #2 = CL#1 + SN#Accor, POSSM cross power spectra docalib=1
> gainuse=CL#2.
>
>
> Result: compared "flux density" on three baselines and 4 IFs
>
> Data from zoom correlation (1): amplitudes ~0.5 Jy, accor gains
> ~0.65 or thereabouts
>
> Data from rec band correlation (2): amplitudes ~1.0 Jy, accor gains
> ~1.00
>
>
> It is strange that 32 MHz zoom (identical to 32 MHz recoderd except
> for sideband flip to USB) would produce a factor 2 difference in the
> scaling of cross power spectra. Has anyone noticed this? Is there a
> workaround? The experiments in question were already correlated some
> months ago and not all raw data are available anymore. In case
> someone has already ran into this issue, is simply a "manual"
> rescaling by factor 2 in AIPS a permissible solution?
>
> regards,
> Jan
>
>
> _______________________________________________
> Difx-users mailing list
> Difx-users at listmgr.nrao.edu
> https://listmgr.nrao.edu/mailman/listinfo/difx-users
>


-- 
!=============================================================!
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

office days (usually): Mon-Thu
!=============================================================!
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://listmgr.nrao.edu/pipermail/difx-users/attachments/20191127/21b2c5a5/attachment.html>