7mm receiver polarisation problems

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7mm receiver polarisation problems

Postby ste616 » Mon Feb 08, 2010 9:18 am

It was brought to our attention that the 7mm receiver was producing measurements of polarisation that were 90 degrees away from the expected values.

It was determined that this could be caused if:
  • the probes are mislabelled and one of them has reverse polarity
  • the probes are rotated by 90 degrees

Both these possibilities were investigated, although due to the design of the receiver it is not possible to have a reverse-polarity probe. It now appears as though the receiver is oriented incorrectly, although this has yet to be confirmed.
cheers
Jamie Stevens
ATCA Senior System Scientist
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Re: 7mm receiver polarisation problems

Postby ste616 » Wed Feb 17, 2010 2:19 pm

The receiver engineers, led by Jock and Mark B., have determined that for the 7mm system (and probably for the 3mm system), X and Y have been swapped. Mark W. and I have gone through the maths, and we agree that this would be able to cause the problems that have been observed during polarimetry experiments using these receivers.

Mark W. is attempting to make a software fix, and it is possible that a hardware fix will be made in the future. More information will be available as we test the potential fixes.
cheers
Jamie Stevens
ATCA Senior System Scientist
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Re: 7mm receiver polarisation problems

Postby Mark.Wieringa » Thu Feb 18, 2010 1:12 pm

A fix has now been made to atlod which reverses the sign of the XY and YX correlations for 7mm data. This results in Q and U (and V) having the opposite sign. The polarization angle changes by 90 degrees. The new atlod should be available from tomorrow.
If the problem is fixed in hardware at some future time, the code will be adjusted to take this into account for subsequent data.
At this stage 3mm data is left unchanged. Please post your results here if you have suspicions about the 3mm polarization angles miriad produces.
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Re: 7mm receiver polarisation problems

Postby Jess Broderick » Thu Mar 04, 2010 1:01 am

Hi,

I used two versions of miriad (20100204 and 20100218) to confirm if the 7 mm fix gives a sensible result (the latter has the updated version of atlod). The CABB dataset tested is from project C956; the observations took place on 2009 Aug 13/14 at 43 and 45 GHz. The target was the CSS source B1221-423, which is resolved into a number of components in lower-resolution data at 3 cm (Safouris et al. 2003, PASA, 20, 1). The primary, secondary, and bandpass calibrators were 1934-638, 1144-379, and 0420-014, respectively.

I followed exactly the same standard data reduction procedure each time (i.e. the procedure outlined in the ATCA users' guide). In atlod I used the options birdie, xycorr, rfiflag, opcorr and noauto. The data were manually flagged with blflag.

Here are some of my findings at 43 GHz; the output at 45 GHz is very similar. All the plots are in the file http://www.astro.soton.ac.uk/~jb34g09/7mm_plots.tar.gz


1. The bandpass calibration is nearly identical in both versions of miriad (plots 0420_X/Y_feb04/18). A quick check with uvflux after using mfcal:

--Feb 04--

UvFlux: version 1.0 24-Jun-99
Applying bandpass corrections to 0420-014.43000
Applying gain corrections to 0420-014.43000
--------------------------------------------------------------------------------
Source Pol Theoretic Vector Average RMS Average RMS Amp Number
RMS (real,imag) Scatter Amp Scatter Corrs
------ --- -------- -------------------- ------- --------- -------- ------
0420-014 I 1.8E-01 5.849E+00 -4.694E-04 1.5E-01 5.851E+00 1.45E-01 779875
Q 1.8E-01 2.079E-03 6.338E-04 1.4E-01 1.771E-01 9.73E-02 779875
U 1.8E-01 -5.133E-02 -9.015E-03 1.5E-01 1.885E-01 1.02E-01 779875
V 1.8E-01 -1.406E-02 -2.941E-02 1.7E-01 2.140E-01 1.12E-01 779875
--------------------------------------------------------------------------------

--Feb 18--

UvFlux: version 1.0 24-Jun-99
Applying bandpass corrections to 0420-014.43000
Applying gain corrections to 0420-014.43000
--------------------------------------------------------------------------------
Source Pol Theoretic Vector Average RMS Average RMS Amp Number
RMS (real,imag) Scatter Amp Scatter Corrs
------ --- -------- -------------------- ------- --------- -------- ------
0420-014 I 1.8E-01 5.849E+00 -4.344E-04 1.5E-01 5.851E+00 1.45E-01 776205
Q 1.8E-01 -2.099E-03 -7.817E-05 1.4E-01 1.770E-01 9.72E-02 776205
U 1.8E-01 5.146E-02 9.124E-03 1.5E-01 1.884E-01 1.02E-01 776205
V 1.8E-01 1.399E-02 2.921E-02 1.7E-01 2.141E-01 1.12E-01 776205
--------------------------------------------------------------------------------

so nothing has really changed except that the signs for Q/U/V have been flipped, as expected.


2. Here is some output for the secondary calibrator 1144-379. Again, nothing has changed significantly except that the signs have flipped for Q/U and the leakages.

--Feb 04 version of Miriad--

MFCAL: MfCal: version 1.1 17-Dec-09
MFCAL: Executed on: 10MAR02:10:04:36.0
MFCAL: Command line inputs follow:
MFCAL: vis=1144-379.43000
MFCAL: interval=0.1
MFCAL: options=nopassol
GPCAL: Gpcal: version 1.0 30-Apr-09
GPCAL: Executed on: 10MAR02:10:06:21.0
GPCAL: Command line inputs follow:
GPCAL: vis=1144-379.43000
GPCAL: options=qusolve,xyvary
GPCAL: interval=0.1
GPCAL: I flux density: 0.808
GPCAL: Percent Q: 5.846
GPCAL: Percent U: -3.893
GPCAL: Leakage terms:
GPCAL: Ant 1:Dx,Dy = ( 0.008,-0.001),
(-0.008, 0.001)
GPCAL: Ant 2:Dx,Dy = (-0.009,-0.005),
( 0.003,-0.003)
GPCAL: Ant 3:Dx,Dy = (-0.024,-0.002),
( 0.018, 0.001)
GPCAL: Ant 4:Dx,Dy = (-0.008, 0.002),
( 0.005, 0.004)
GPCAL: Ant 5:Dx,Dy = ( 0.022, 0.002),
(-0.026, 0.004)
GPCAL: Ant 6:Dx,Dy = (-0.001,-0.003),
(-0.004, 0.000)
GPBOOT: Miriad GpBoot: version 21-Jan-01
GPBOOT: Executed on: 10MAR02:10:06:23.0
GPBOOT: Command line inputs follow:
GPBOOT: cal=1934-638.43000
GPBOOT: vis=1144-379.43000
GPBOOT: select=+time(6:50,7:40)
GPBOOT: Secondary flux density scaled by: 1.051


UvFlux: version 1.0 24-Jun-99
Applying bandpass corrections to 1144-379.43000
Applying gain corrections to 1144-379.43000
Applying polarization leakage corrections to 1144-379.43000
--------------------------------------------------------------------------------
Source Pol Theoretic Vector Average RMS Average RMS Amp Number
RMS (real,imag) Scatter Amp Scatter Corrs
------ --- -------- -------------------- ------- --------- -------- ------
1144-379 I 2.0E-01 8.489E-01 2.226E-05 1.7E-01 8.663E-01 1.61E-0114039585
Q 2.0E-01 4.962E-02 -1.751E-05 1.6E-01 2.009E-01 1.24E-0114039585
U 2.0E-01 -3.304E-02 3.166E-05 1.6E-01 1.978E-01 1.23E-0114039585
V 2.0E-01 -4.240E-05 -2.351E-05 1.6E-01 1.955E-01 1.22E-0114039585
--------------------------------------------------------------------------------


--Feb 18 version of Miriad--

MFCAL: MfCal: version 1.1 17-Dec-09
MFCAL: Executed on: 10MAR02:11:09:04.0
MFCAL: Command line inputs follow:
MFCAL: vis=1144-379.43000
MFCAL: interval=0.1
MFCAL: options=nopassol
GPCAL: Gpcal: version 1.0 30-Apr-09
GPCAL: Executed on: 10MAR02:11:10:49.0
GPCAL: Command line inputs follow:
GPCAL: vis=1144-379.43000
GPCAL: options=qusolve,xyvary
GPCAL: interval=0.1
GPCAL: I flux density: 0.811
GPCAL: Percent Q: -5.845
GPCAL: Percent U: 3.891
GPCAL: Leakage terms:
GPCAL: Ant 1:Dx,Dy = (-0.007, 0.001),
( 0.009,-0.001)
GPCAL: Ant 2:Dx,Dy = ( 0.009, 0.005),
(-0.003, 0.003)
GPCAL: Ant 3:Dx,Dy = ( 0.023, 0.001),
(-0.020,-0.001)
GPCAL: Ant 4:Dx,Dy = ( 0.007,-0.002),
(-0.005,-0.005)
GPCAL: Ant 5:Dx,Dy = (-0.020,-0.001),
( 0.027,-0.004)
GPCAL: Ant 6:Dx,Dy = ( 0.001, 0.003),
( 0.004, 0.001)
GPBOOT: Miriad GpBoot: version 21-Jan-01
GPBOOT: Executed on: 10MAR02:11:10:50.0
GPBOOT: Command line inputs follow:
GPBOOT: cal=1934-638.43000
GPBOOT: vis=1144-379.43000
GPBOOT: select=+time(6:50,7:40)
GPBOOT: Secondary flux density scaled by: 1.047


UvFlux: version 1.0 24-Jun-99
Applying bandpass corrections to 1144-379.43000
Applying gain corrections to 1144-379.43000
Applying polarization leakage corrections to 1144-379.43000
--------------------------------------------------------------------------------
Source Pol Theoretic Vector Average RMS Average RMS Amp Number
RMS (real,imag) Scatter Amp Scatter Corrs
------ --- -------- -------------------- ------- --------- -------- ------
1144-379 I 2.0E-01 8.482E-01 1.410E-04 1.7E-01 8.661E-01 1.63E-0114129500
Q 2.0E-01 -4.957E-02 1.182E-05 1.6E-01 2.010E-01 1.24E-0114129500
U 2.0E-01 3.299E-02 1.703E-05 1.6E-01 1.990E-01 1.24E-0114129500
V 2.0E-01 -4.328E-05 2.766E-05 1.6E-01 1.951E-01 1.22E-0114129500
--------------------------------------------------------------------------------


In the Feb 18 version of miriad, the polarisation PA for 1144-379 is consistent with the value at 12 mm, which is encouraging. What is strange, however, is that the gain amplitudes are significantly different around the time 04:00 - 06:00 when the source transits (plots 1144_X/Y_feb04/18). The phase plots are identical, though.


3. After copying over the calibration tables to the target, the Q/U/V data are a lot noisier on some baselines in the later version of miriad (plots b1221_quv_feb04/18). The image quality is also significantly poorer for both Q and U (plots b1221-423_q/u_feb04/18). Apart from the flipped signs, the Q and U images obtained with the Feb 04 version of miriad look sensible given the images I already have of the source at 12 mm and 3 cm. This is not the case in the Feb 18 version. On the other hand, I cannot see any obvious problems for Stokes I; both versions of miriad give similar final images and flux densities.


Any ideas about what is going on?

Thanks,
Jess
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Posts: 5
Joined: Thu Mar 04, 2010 12:12 am
Location: University of Southampton

Re: 7mm receiver polarisation problems

Postby Mark.Wieringa » Fri Mar 05, 2010 9:27 am

Hi Jess,

thanks for investigating this. It looks like the fix I came up with was not what we wanted. I've checked in a new fix, this time I'm changing the feed receptor angle by 90 degrees. I reprocessed your data with the new atlod and the strange gain behaviour is gone, but the sign of Q and U are still as required for consistency with lower freqs.
Tomorrow's miriad update should have the new code in place.
Let me know if your images make more sense with this version. My images seem to have only one or two components in Q and U now (I flagged all data with seeing >300 to get sensible gain solutions).

Cheers,

Mark
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Posts: 257
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Re: 7mm receiver polarisation problems

Postby Jess Broderick » Tue Mar 09, 2010 3:45 am

Hi Mark,

Thanks for implementing the new fix. My images make much more sense now.


Cheers,
Jess
Jess Broderick
 
Posts: 5
Joined: Thu Mar 04, 2010 12:12 am
Location: University of Southampton


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