WorkshopSelfcal
This page describes the hands-on self calibration portion of the ALMA data reduction workshop. In this session you will carry out self-calibration on ALMA Science Verification data to improve the image quality. This page describes the data and introduces the scripts. It is not a full-fledged CASA guide, only background for the hands on portion of the workshop. For the imaging session in the morning go here: WorkshopImaging
Overview
Data Description
You will use the now-familiar NGC 3256 Band 3 or TW Hydra Band 7 Science Verification data. You have seen both already this morning. Both contain several lines, including CO emission (1-0 for NGC 3256 and 3-2 for TW Hydra), and continuum emission. We will begin by imaging the continuum emission and using the resulting model to self calibrate the data. We will then see how to apply that self-calibration to improve the line images.
Note that it is perfectly possible to self-calibrate the other TW Hydra data or the Antennae Band 7 data (see AntennaeBand7). If you finish quickly, you might try to write your own script to do this.
Recommended Approach
Review the basics of how to use these scripts in WorkshopImaging. Assuming that you are comfortable with imaging, we suggest to grab the script above. Use it to image NGC 3256 or TW Hydra (just uncomment/comment the relevant portions to apply the script to either data set), making sure that the "calready" parameter is set to True. Then use gaincal to derive the complex gain corrections to each antenna that best match the data to the model. When you are satisfied with these corrections, apply them to the data to create a new corrected data set. Image those data.
The script takes you through one iteration of this self-calibration but the approach can be iterative, improving the model of the source as the corrected data become more and more accurate. Once you are comfortable with the procedure, try iterating and perhaps adding an amplitude calibration to the selfcal. Your degrees of freedom in this process are the solution interval and how you otherwise combine the data (averaging polarizations, spectral windows or channels, using a specific field in a mosaic). Try experimenting with these settings to see how your results are affected. Your basic diagnostic is the plotting of solutions using plotcal but the amount of noise and the structure of the residuals in the final image can also be useful indicators.
Continuum Self Calibration
This script can be configured to step you through a first phase self-calibration of either the TW Hydra Band 7 or NGC 3256 Band 3 data. Adjust the comments to use the data set that you like. The script will create a first, basic image following the same procedure that you used to image in the previous session. You will then self-calibrate to create a better image. Some comments to bear in mind:
- Remember that each CASA Measurement Set has three columns: data, corrected, and model. Conceptually data are the raw u-v data, corrected data are the data corrected by some complex gain solution, and model represents the model visibility that should be matched by the data in the absence of noise, antenna-dependent complex gain terms, or other corruptions. Data are required for every measurement set, corrected data appear as a result of applying a calibration via applycal (or copying the data column in a few cases), and model data are either read in or initialized via setjy or copied back into the measurement set during deconvolution by clean with the calready option turned on.
- You fill the model column with clean. This is done during the deconvolution, so you want to clean down to the point where you believe that the model you have built is still real but no further. gaincal will try derive solutions to match your data to whatever model you have in. So be sure that the model is real! (This is a potential danger of self-calibration.)
Continuum-to-Line Self Calibration
Self-calibration can also be used to improve your spectral line imaging.
