Amplitude self-calibration
Posted: Sun Aug 31, 2014 6:03 pm
I have encountered a problem with amplitude self-calibration. I have been applying this to individual pointings at 1.3-1.8 GHz. I am finding that amplitude selfcal is affecting the fluxes of faint (SNR < 50 or S < 1 mJy) sources – see plot below which compares the flux after amplitude selfcal with the flux before amplitude selfcal (in both cases I applied phase selfcal). I used all CLEAN components above 5 sigma as input model for amplitude selfcal. Could this occur if there is a significant amount of flux which is missing in the model? I did not set options=noscale, i.e. I am using the default which is to scale the gains so that average solution amplitudes is 1.0. Am I right in thinking that scaling the gains in this way is supposed to avoid this problem?
https://www.dropbox.com/s/650yx79mi2ngu ... arison.pdf
Amplitude selfcal does a good job at removing artefacts around bright sources but for some pointings also introduces stripes across the image because of anomalously high amplitude solutions. I have found that a ‘solution’ to this problem is to flag the data where the difference between the visibilities and model is greater than 5 sigma. I did this using uvmodel, setting options=flag and sigma=5. This can flag up to 0.5% of the data and seems to be quite effective at removing bad selfcal solutions. I am mentioning this in case it might be related to the above problem.
https://www.dropbox.com/s/650yx79mi2ngu ... arison.pdf
Amplitude selfcal does a good job at removing artefacts around bright sources but for some pointings also introduces stripes across the image because of anomalously high amplitude solutions. I have found that a ‘solution’ to this problem is to flag the data where the difference between the visibilities and model is greater than 5 sigma. I did this using uvmodel, setting options=flag and sigma=5. This can flag up to 0.5% of the data and seems to be quite effective at removing bad selfcal solutions. I am mentioning this in case it might be related to the above problem.