Self-Calibration Hands-on CASA introduction

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1 Self-Calibration Hands-on CASA introduction Adam North American ALMA Science Center Atacama Large Millimeter/submillimeter Array Expanded Very Large Array Robert C. Byrd Green Bank Telescope Very Long Baseline Array

2 Outline Basic Idea and Workflow Selfcal for High-z Quasar BR1202

3 Self-Calibration in CASA Self-calibration is just regular calibration. With a model of your source, you can calibrate on your source. You solve for the calibrations that best match your data to your model. You can get a model by carrying out an initial imaging of the source. You can iterate the self calibration as model improves. USUALLY PHASE-ONLY SELFCAL FIRST, AMPLITUDE SELFCAL LATER (IF AT ALL). AMPLITUDE LESS RECOVERABLE THAN PHASE IT WILL MATCH THE MODEL. 3

Calibrate to match data to model gaincal! Apply the new calibration applycal!

4 Self-Calibration in CASA Image your source, deconvolve build a model, place model in MS clean! Initial Image Measurement Set Now has associated model. Calibrate to match data to model gaincal! Apply the new calibration applycal! Re-image the better-calibrated data clean! Phase Calibration Table Amplitude Calibration Table Measurement Set Improved corrected column. Improved Image 4

5 Self-Calibration in CASA Image your source, deconvolve build a model, place model in MS clean! Initial Image Measurement Set Now has associated model. Calibrate to match data to model gaincal! Apply the new calibration applycal! Re-image the better-calibrated data clean! Phase Calibration Table Amplitude Calibration Table Measurement Set Improved corrected column. Improved Image 5

6 Self-Calibration in Practice The game in selfcal is signal-to-noise and the model. Requires that your source is bright enough. Even for bright sources, some degree of averaging is usually needed. Time averaging (often longer than a scan). Average together several SPWs, trust that you have calibrated any offset. Average together polarizations, trust that you have calibrated any offset. For lines and mosaics, calibrate on a bright field or line channels. 6

7 gaincal Averaging: solint sets solution interval combine allows combination across scan, field, spw

8 gaincal Averaging: gaintype T averages polarizations

9 Self-Calibration in Practice Initial round of cleaning CAREFUL NOT TO OVERDO IT: THE SELF CALIBRATION CAN LOCK IN ARTIFACTS Experiment with solution interval (solint) and averaging pols/spws Inspect resulting solutions LOOK FOR SMOOTH TRENDS OF PHASE, AMP. WITH TIME, NOT TOO MANY FAILURES May take multiple iterations MODEL WILL SUCCESSIVELY IMPROVE, START WITH PHASE, THEN TRY AMPLITUDE 9

10 Self-Calibration Iteration Several options: Apply solutions you like, split corrected data, keep going. EASY TO STEP BACK AND ASSESS IMPROVEMENT, LOTS OF FILES. Keep applying, imaging (to improve the model) and resolving. GIVES YOU THE ONE BEST SOLUTION AT A GIVEN TIME. Make a series of incremental tables using gaintable. SOMEWHERE IN BETWEEN. 10

11 Outline Basic Idea and Workflow Selfcal for High-z Quasar BR1202

12 Initial Image This gets you a model. 12

13 First Phase Calibration Use the model to calibrate the data on the source. 13

14 Improved Image With the self calibration applied. 14

15 Second Round of selfcal Re-calibrate with the improved image as model. 15

16 Improved Image (Round 2) With two phase self calibrations applied. 16

17 An Amplitude Selfcal After verifying that remaining phase improvements are marginal. 17

18 Improved Image (Phase+Amp) Substantial improvement over original. 18

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