Imaging and Self-Calibration Hands-on CASA introduction
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Transcript of Imaging and Self-Calibration Hands-on CASA introduction
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Atacama Large Millimeter/submillimeter ArrayExpanded Very Large Array
Robert C. Byrd Green Bank TelescopeVery Long Baseline Array
WhoeverNorth American ALMA Science Center
Imaging and Self-CalibrationHands-on CASA introduction
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Imaging in CASA
• CASA exposes imaging and deconvolution via the clean taskSTARTING POINT: CALIBRATED MS (“CORRECTED” COLUMN, IF PRESENT)
• Can be run interactively (using the viewer) or automaticallyINTERACTIVE ALLOWS ON-THE-FLY CLEAN BOXING AND STOPPING
• Key decisions:o How to grid the data (image, cell size)o How to handle the frequency axiso What (if any) deconvolution to carry outo Selection and weighting of visibility data
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• (Visibility) data selection
• Treatment of spectral axis
• Basic image (gridding) parameters
• Deconvolution (actual CLEANing)
• Weighting
clean
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clean: Imaging
• image sizeTYPICALLY ~ PRIMARY BEAM AREA UNLESS IN SPECIAL CASE
• cell sizeSET THIS TO PLACE ~4-5 PIXELS ACROSS YOUR PSF CORE
• Weighting (“uniform”, “robust”, “natural”)USED TO ASSEMBLE VISIBILITIES INTO IMAGE, AFFECT PSF/SENSITIVITY
• Optionally “taper” (smooth) the data to target resolution
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clean: Imaging • Handling of spectral axis for cube:
START, STOP, WIDTH OF PLANE
o Define planes by channelo Define planes by velocityo Define planes by frequency
• Handling of spectral axis for image:
o “multifrequency synthesis” accounts for u-v position vs. frequency
o (Optional) Deconvolution components can have spectral indexI.E., INTENSITY DEPENDENT ON FREQUENCY
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clean: Deconvolution• Image reconstruction to account for imperfect u-v
coverage
• Basic Procedure:
o Identify brightest spot in image
o Subtract a point source with some fraction of that intensity
o Add a corresponding point source to a “model” image
o Proceed until no signal left in image
o Convolve model with “clean beam” and add to residuals
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• Find brightest points in “dirty” image
Deconvolution Illustrated
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• Find brightest points in “dirty” image• Create model image containing a fraction of those flux points
Deconvolution Illustrated
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• Find brightest points in dirty image• Create model image containing a fraction of those
flux points• Subtract model from data, leaving a residual
Deconvolution Illustrated
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• Find brightest points in dirty image
• Create model image containing a fraction of those flux points
• Subtract model from data, leaving a residual
• Final product = residual + model
(convolved with restoring Gaussian beam)
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residual (log scale)
model convolved w/ restorimg beam (log scale)
cleaned image (log scale)residual (linear scale)
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residual (log scale)
model convolved w/ restoring beam (log scale)
cleaned image (log scale)residual (linear scale)
restrict where the algorithm can search for clean components, with a mask
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residual (log scale)
model convolved w/ restoring beam
cleaned image (log scale)residual (linear scale) 10
iterations
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residual (log scale)
model convolved w/ restoring beam
cleaned image (log scale)residual (linear scale) 20
iterations
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residual (log scale)
model convolved w/ restoring beam
cleaned image (log scale)residual (linear scale) 30
iterations
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residual (log scale)
model convolved w/ restoring beam
cleaned image (log scale)residual (linear scale) 40
iterations
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residual (log scale)
model convolved w/ restoring beam
cleaned image (log scale)residual (linear scale) 50
iterations
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residual (log scale)
model convolved w/ restoring beam
cleaned image (log scale)residual (linear scale) 60
iterations
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residual (log scale)
model convolved w/ restoring beam
cleaned image (log scale)residual (linear scale) 70
iterations
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residual (log scale)
model convolved w/ restoring beam
cleaned image (log scale)residual (linear scale) 80
iterations
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residual (log scale)
model convolved w/ restoring beam
cleaned image (log scale)residual (linear scale) 90
iterations
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residual (log scale)
model convolved w/ restoring beam
cleaned image (log scale)residual (linear scale) 100
iterations
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residual (log scale)
model convolved w/ restoring beam
cleaned image (log scale)residual (linear scale) 125
iterations
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residual (log scale)
model convolved w/ restoring beam
cleaned image (log scale)residual (linear scale) 150
iterations
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residual (log scale)
model convolved w/ restoring beam
cleaned image (log scale)residual (linear scale) 200
iterations
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residual (log scale)
model convolved w/ restoring beam
cleaned image (log scale)residual (linear scale) 300
iterations
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residual (log scale)
model convolved w/ restoring beam
cleaned image (log scale)residual (linear scale) 400
iterations
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residual (log scale)
model convolved w/ restoring beam
cleaned image (log scale)residual (linear scale) 500
iterations
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residual (log scale)
model convolved w/ restoring beam
cleaned image (log scale)residual (linear scale) 1000
iterations
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residual (log scale)
model convolved w/ restoring beam
cleaned image (log scale)residual (linear scale) 1500
iterations
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CLEANED IMAGE (LOG SCALE)
DIRTY IMAGE (LOG SCALE)
clean: Deconvolution
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clean: Deconvolution• Key decisions:
o Constraining where the signal can be (clean boxing)MANUALLY USING THE VIEWER OR INPUT AS AN IMAGE OR REGION
o Setting stopping thresholdTYPICALLY A SMALL NUMBER TIMES THE RMS NOISE
o Number of iterations allowedNOT USUALLY A GOOD CRITERIA TO STOP
o Deconvolution algorithmBALANCE OF MAJOR/MINOR CYCLES, ETC.
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• residual image in viewer
• define a mask with R-click on shape type
• define the same mask for all channels
• or iterate through the channels with the tape deck and define separate masks
Interactive clean
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• perform N iterations
• and return – every time the residual is displayed is a major cycle
• continue until #cycles or threshold reached, or user stop
Interactive clean
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• clean restarts from existing filesWILL FIRST RECOMPUTE RESIDUALS FROM MODEL
• The mask image, in particular, can be reusedBE CAREFUL OF IMSIZE – MASK MUST MATCH IMAGE
• don’t hit ^C while imaging – this can do bad things to your MS
clean: Notes
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Self-Calibration in CASA
• “Self-calibration” is just regular calibration
• With a model of your source, you can calibrate on your source
• Requires that your source is bright enoughNEEDED TO GET SUFFICIENT S/N; GET SOME S/N BACK TIME AVERAGING.
• Can be iterated as model improvesUSUALLY PHASE-ONLY SELFCAL FIRST, AMPLITUDE SELFCAL LATER (IF AT ALL)
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Self-Calibration in CASAImage your source, deconvolve build a model, place model
in MSclean
Calibrate to match data to modelgaincal
Apply the new calibrationapplycal
Re-image the better-calibrated dataclean
Phase Calibration TableAmplitude Calibration
Table
Measurement SetNow has associated
model.
Measurement SetImproved corrected
column.
Improved Image
Initial Image
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Self-Calibration in practice
• Initial round of cleaningCAREFUL NOT TO OVERDO IT: THE SELF CALIBRATION CAN “LOCK IN” ARTIFACTS
• Experiment with solution interval (solint)S/N USUALLY LIMITING CONCERN, TRY POL. COMBINATION (GAINTYPE=‘T’)
• Inspect resulting solutionsLOOK FOR SMOOTH TRENDS OF PHASE, AMP. WITH TIME
• May take multiple iterationsMODEL WILL SUCCESSIVELY IMPROVE, START WITH PHASE, THEN TRY AMPLITUDE
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Your Turn• Follow the imaging CASA guide
http://casaguides.nrao.edu/index.php?title=TWHydraBand7_Im_SS12
• We have provided the full calibrated data setNO NEED TO USE THIS MORNING’S DATA, BUT YOU CAN IF YOU LIKE
• Try:o CONTINUUM IMAGINGo LINE IMAGINGo SELF-CALIBRATION AND RE-IMAGINGo MOMENT MAP CREATIONo IMAGING YOUR CALIBRATORS
ASK IF YOU NEED HELP!