atlod options=notsys and calibration
Posted: Fri Nov 24, 2017 7:22 pm
I am having a bit of trouble calibrating some C/X band data, and I was hoping for a bit of advice.
My data has the two IFs set at central frequencies of 7.7 and 9.5GHz, giving roughly 50MHz of overlap (after flagging out edge channels). At these frequencies 1934-638 is strong enough to perform a bandpass calibration and establish a flux density scale.
My steps of the reduction are as follows:
Inspecting the spectrum of 1934-638 across both IFs with uvfmeas, it is as expected. However, when plotting the spectrum of 0327-241 with uvfmeas, the two bands are clearly discontinuous. The small overlapping region of ~50MHz gets average together.
When it comes to the target source, it I plan on imaging the data together, so to me it makes sense to ensure that the spectrum of the secondary calibrator, which is roughly 1Jy, is also consistent. I kept this small amount of overlap as a way to 'QA' things before imaging, particularly with the phase calibrator 0327-241. If you refer to the norml_reduction.png attachment you can see the behavior.
I have tried a couple thing - mostly mostly four nfbins and different combinations of flagging with no effect.
I have managed to recover something that is more consistent by loading the data in with the notsys keyword
A normal calibration after loading the data in with this option gives a much more consistent spectrum. If you refer to the option_notsys.png attachment you can see the difference.
I was hoping someone could explain what exactly is going on, and how can I ensure the most robust calibration strategy? My understanding of the system temperature was that it is an additional weight for each visibility, so it wouldn't change its amplitude, just reduced its weight when imaging or fitting. Have I got a more fundamental misunderstanding?
This was meant to be a quick look, as I got the data early this morning and am observing more over the coming weeks. I would like to be certain this it was not a mistake I made during observing.
My data has the two IFs set at central frequencies of 7.7 and 9.5GHz, giving roughly 50MHz of overlap (after flagging out edge channels). At these frequencies 1934-638 is strong enough to perform a bandpass calibration and establish a flux density scale.
My steps of the reduction are as follows:
Code: Select all
mfcal vis=1934-638.9500 interval=0.1
gpcal vis=1934-638.9500 options=xyvary interval=0.1 nfbin=2
gpcopy vis=1934-638.9500 out=0327-241.9500
gpcal vis=0327-241.9500 options=xyvary,qusolve interval-0.1 nfbin=2
gpboot vis=0327-241.9500 cal=1934-638.9500
mfcal vis=1934-638.7700 interval=0.1
gpcal vis=1934-638.7700 options=xyvary interval=0.1 nfbin=2
gpcopy vis=1934-638.7700 out=0327-241.7700
gpcal vis=0327-241.7700 options=xyvary,qusolve interval-0.1 nfbin=2
gpboot vis=0327-241.7700 cal=1934-638.7700
When it comes to the target source, it I plan on imaging the data together, so to me it makes sense to ensure that the spectrum of the secondary calibrator, which is roughly 1Jy, is also consistent. I kept this small amount of overlap as a way to 'QA' things before imaging, particularly with the phase calibrator 0327-241. If you refer to the norml_reduction.png attachment you can see the behavior.
I have tried a couple thing - mostly mostly four nfbins and different combinations of flagging with no effect.
I have managed to recover something that is more consistent by loading the data in with the notsys keyword
Code: Select all
atlod out=uv_data_notsys.uv options=rfiflag,birdie,xycorr,noauto,notsys
I was hoping someone could explain what exactly is going on, and how can I ensure the most robust calibration strategy? My understanding of the system temperature was that it is an additional weight for each visibility, so it wouldn't change its amplitude, just reduced its weight when imaging or fitting. Have I got a more fundamental misunderstanding?
This was meant to be a quick look, as I got the data early this morning and am observing more over the coming weeks. I would like to be certain this it was not a mistake I made during observing.