widebandpbcor
- widebandpbcor(vis, imagename='', nterms=2, threshold='', action='pbcor', reffreq='', pbmin=0.2, field='', spwlist=[''], chanlist=[''], weightlist=[''])[source]
Wideband PB-correction on the output of the MS-MFS algorithm
[Description] [Examples] [Development] [Details]
- Parameters
vis (path) - Name of measurement set.
imagename (string=’’) - Name-prefix of multi-termimages to operate on.
nterms (int=2) - Number of taylor terms to use
threshold (string=’’) - Intensity above which to re-calculate spectral index
action (string=’pbcor’) - PB-correction (pbcor) or only calc spectral-index (calcalpha)
action = pbcor
reffreq (string=’’) - Reference frequency (if specified in clean)
pbmin (double=0.2) - PB threshold below which to not correct
field (string=’’) - Fields to include in the PB calculation
spwlist (intVec=[‘’]) - List of N spw ids
chanlist (intVec=[‘’]) - List of N channel ids
weightlist (doubleVec=[‘’]) - List of N weights (relative)
- Description
The task widebandpbcor performs a WideBand Primary-beam correction. It computes a set of PBs at the specified frequencies, calculates Taylor-coefficient images that represent the PB spectrum, performs a polynomial division to PB-correct the output Taylor-coefficient images from clean or tclean (with nterms>1), and recomputes the spectral index (and curvature) using the PB-corrected Taylor-coefficient images. Optionally, it is possible to skip the PB-correction and only recalculate the spectral index with a different threshold.
This is a temporary task, meant for use until a widebandpbcor option is enabled from within the tclean task.
When running widebandpbcor, an output directory named ‘imagename.pbcor.workdirectory’ is created, and it is filled with an image-cube of the evaluated primary beams at all specified frequencies, Taylor-coefficients, and a ‘spectral index’ due to the primary beam. Note that for the actual PB-correction, only the Taylor-coefficient images are used.
For more information about the widebandpbcor task, please see the Wide Band Imaging page in the Synthesis Imaging section of CASAdocs.
Choosing spwlist, chanlist, and weightlist
The basic principles at work here are:
Imaging = fitting a polynomial to a noisy spectrum (with weights). The polynomial represents \(I_{\nu}\) x \(P_{\nu}\), where \(I_{\nu}\) is the intensity and \(P_{\nu}\) the primary beam (PB) at frequency \(\nu\).
PB model = fitting a polynomial to a collection of PBs at different frequencies (with weights). The polynomial represents the primary beam \(P_{\nu}\).
Dividing the two polynomials via their coefficients.
Steps (1) and (2) need to be consistent with each other (with respect to frequencies used and their weights) to produce FITS that when divided give exactly only the sky parameters. Unless you use the same math (and code) for both, they won’t be exactly consistent. The way to minimize differences is to choose a list of frequencies (via spws/chans) and weights for widebandpbcor that resemble the frequency structure of the data you have used for imaging. For example, if you have 3 spws in your data and the middle spw has a factor of 10 less weight in the data, then using just one channel each from the two outer spws for the PB modeling may be close enough to using all 3 spws. Or, you could also pick the middle channel of all 3 spws, and assign weights as [1.0, 0.1, 1.0].
Parameter descriptions
vis
Name of input visibility file. Default: none. Examples: vis=’ngc5921.ms’. Only one MS can be specified here, and it must contain at-least one timestep of data at all frequencies required to calculate the PB spectrum.
Note
NOTE1: If the imaging was done using a list of MSs, and any one MS covers the entire frequency range, then it will suffice to supply only that one MS. This MS is used only to extract frequencies at which to compute primary beams before fitting Taylor polynomials.
Note
NOTE2: In case of multiple MSs that cover different frequency ranges, please split/concat a small fraction of the data from each MS to form one single MS that contains the full frequency range. This task uses the MS only for frequency meta-data.
imagename
Pre-name of input and output images. Same as in the clean or tclean task. Examples: imagename = ‘run1’; Restored-images (run1.image.tt0, etc.) and residual images (run1.residual.tt0, etc… ) must be available on disk.
nterms
Number of Taylor terms to be used to model the frequency-dependence of the primary beam. Examples: nterms = 2; nterms must be less than or equal to the number of frequencies specified via spwlist, chanlist and weightlist. nterms=1 will do a standard division by the average PB computed over all specified frequencies.
threshold
Flux level in the restored intensity map, below which to not recalculate spectral index. Examples: threshold = ‘0.1Jy’
action
Choice of PB-correction with spectral-index recalculation or only spectral-index recalculation (using the specified threshold). Examples: action=’pbcor’ or action=’calcalpha’. With action=’pbcor’, the following output images are created/overwritten:
imagename.pbcor.workdirectory: This directory contains an image cube with PBs at the list of specified frequencies, and Taylor-coefficient images that describe the PB spectrum.
imagename.pb.cube: Concatenated cube of PBs
imagename.pb.tt0, tt1, …: Taylor coefficients describing the PB spectrum
imagename.pb.alpha: Spectral index of the PB (for information only)
imagename.image.pbcor.tt0,tt1,…: Corrected Taylor coefficients
imagename.pbcor.image.alpha: Corrected Spectral Index
imagename.pbcor.image.alpha.error: New error map.
With action=’calcalpha’, the following output images are created/overwritten
imagename.image.alpha: Corrected Spectral Index
imagename.image.alpha.error: New error map.
action=’pbcor’ expandable parameters
reffreq
Reference frequency about which the Taylor-expansion is defined. Examples: reffreq = ‘1.5GHz’. If left unspecified, it is picked from the input restored image.
Note
NOTE: If reffreq was specified during task clean to produce the images it must be specified here.
pbmin
PB gain level below which to not compute Taylor-coefficients or apply PB-corrections. Examples: pbmin = 0.1
field
Field selection for the Primary Beam calculation. Examples: field = ‘3C291’. This field selection must be identical to that used in clean or tclean.
spwlist
List of SPW ids for which to make separate Primary Beam.
chanlist
List of channel ids, within the above SPW ids, at which to make PBs. Examples: spwlist=[0,1,2] chanlist=[32,32,32], make PBs at frequencies corresponding to channel 32 of spws 0,1 and 2; spwlist=[0,0,0] chanlist=[0,10,20], make PBs at frequencies corresponding to channels 0, 10, 20 of spw 0.
Primary beams are computed at these specified frequencies and for pointings selected by field. Taylor-coefficients that represent the PB spectrum are computed from these images.
weightlist
List of relative weights to apply to the PBs selected via the spwlist and chanlist parameters. Weights should approximately represent the sum-of-weights applicable during imaging each of these frequencies. Examples: weightlist=[0.5,1.0,1.0]
The first frequency had less usable data due to flagged RFI, but the other two had relatively equal weight. These weights are applied to the PB spectrum while computing PB Taylor-coefficients. Setting weights to anything other than 1.0 makes a difference only with very lop-sided weights.
- Examples
Run the MTMFS deconvolver to generate wideband Taylor coefficient solutions from spectral windows 0, 1 and 2 of a dataset:
tclean(vis='xxx.ms', imagename='try', spw='0~3', imsize=200, cell='10.0arcsec', deconvolver='mtmfs', nterms=2, niter=20)
Apply wideband PB correction using the middle channel (for example, channel number 32) from each spectral window to compute a primary beam cube to which Taylor coefficients are fit:
widebandpbcor(vis='xxx.ms', imagename='try', nterms=2, threshold='0.1Jy', action='pbcor', spwlist=[0,1,2], chanlist=[32,32,32], weightlist=[1.0,1.0,1.0])
Use the ‘calcalpha’ mode to recalculate spectral index with a different threshold at which to apply the True/False mask in the ouput image:
widebandpbcor(vis='xxx.ms', imagename='try', nterms=2, threshold='0.05Jy', action='calcalpha')
- Development
No additional development details
- Parameter Details
Detailed descriptions of each function parameter
vis (path)
- Name of measurement set.imagename (string='')
- Name-prefix of multi-termimages to operate on.nterms (int=2)
- Number of taylor terms to usethreshold (string='')
- Intensity above which to re-calculate spectral indexaction (string='pbcor')
- PB-correction (pbcor) or only calc spectral-index (calcalpha)reffreq (string='')
- Reference frequency (if specified in clean)pbmin (double=0.2)
- PB threshold below which to not correctfield (string='')
- Fields to include in the PB calculationspwlist (intVec=[''])
- List of N spw idschanlist (intVec=[''])
- List of N channel idsweightlist (doubleVec=[''])
- List of N weights (relative)