ft(vis, field='', spw='', model='', nterms=1, reffreq='', complist='', incremental=False, usescratch=False)¶
Insert a source model as a visibility set
vis (string) - Name of input visibility file
field (string=’’) - Select field using field id(s) or field name(s)
spw (string=’’) - Select spectral window/channels
model (variant=’’) - Name of input model image(s)
nterms (int=1) - Number of terms used to model the sky frequency dependence
nterms != 1
reffreq (string=’’) - Reference frequency (e.g. '1.5e+9' or '1.5GHz')
complist (string=’’) - Name of component list
incremental (bool=False) - Add to the existing model visibility?
usescratch (bool=False) - If True, predicted visibility is stored in MODEL_DATA column
ft converts a source model or a components list into model visibilities that is inserted into the MODEL_DATA column of the MS. Alternatively (usescratch=False) it can be stored in the header of the MS to be served on the fly when requested. The function of ft is needed to use more complicated sources, or to attach models to be subtracted later from the visibilities with uvsub.
The source model should be in units of Jy/pixel, usually a *.model image that tclean saves as a result of deconvolution.
ft only uses a standard gridder (a prolate spheroidal) but supports multi-term images (see tclean help). Other gridders, like mosaic or widefield, are currently not supported. If you need one of the non-standard gridders, attach the model to the MS with the task **tclean, ** setting ** ** savemodel=True, and gridder to the requested algorithm.
The default is to replace any current MODEL in the MS by the model specified in the model or complist parameters. The parameter incremental=True will add the specified model to the current MODEL in the MS. This can be useful, e.g., when new point sources are identified that need to be added to the model in the MS.
When both, a model and a component list (in complist) are specified, ft will only use the model image in the model parameter.
ft is strict in its frequency interpretation. Only frequencies that the model covers are inserted into the MS. If the model has a different frequency range, it needs to be modified to span the MS spws that are to be associated. Otherwise, the frequency ranges that are not covered by the model will be unaltered in the model column, or filled with zeros in the case that a MODEL_DATA column is added because none previously existed.
In addition to tclean, another alternative to ft is setjy, which attaches a model and also scales the flux of the model to a flux value calculated from a standard or to a provided value.
NOTE: Model images for standard VLA calibrators 3C48, 3C138, 3C147, and 3C286 for the VLA bands are provided with the release. The location inside the package is dependent on the operating system; setjy (listmodels=True) will show all available models.
Add a multi-term model (image.model from clean) to an MS (‘myMS.ms’), spectral window 3, writing directly to the MODEL column:
ft(vis='myMS.ms', spw=3, nterms=2, model=['image.model.tt0','image.model.tt1'], usescratch=True)
Create a component list with a point source at position J2000 12h33m45.3s -23d01m11.2s with a spectral index of -0.8 and a flux of 13.4 Jy at a reference frequency of 1.25GHz; insert it into ‘myMS.ms’ as a virtual model for field 3:
# first create the component list cl.addcomponent(shape='point', flux='13.4Jy', spectrum type='spectral index', index=-0.8, freq'1.25GHz', dir='J2000 12h33m45.3s -23d01m11.2s') # save the component list under the name 'mycomplist.cl' cl.rename('mycomplist.cl') cl.close() # now insert the component list into myMS.ms as a virtual MODEL ft(vis='myMS.ms', field='3', complist='mycomplist.cl', usescratch=True)
No additional development details
- Parameter Details
Detailed descriptions of each function parameter
vis (string)- Name of input visibility fileDefault: noneExample: vis=’ngc5921.ms’
field (string='')- Select field using field id(s) or field name(s)Default: ‘’ (all fields)BUT, only one source can be specified in amulti-source vis.Examples:field = ‘1328+307’ specifies source‘1328+307’field = ‘4’ specified field with index 4
spw (string='')- Select spectral window/channelsDefault: ‘’ (all spectral windows and channels)
model (variant='')- Name of input model image(s)Default: ‘’ (none)Example:model=’/usr/lib/casapy/data/nrao/VLA/CalModels/3C286_X.im’NOTE: The model visibilities are scaled from themodel frequency to the observed frequency of thedata.
nterms (int=1)- Number of terms used to model the sky frequencydependenceDefault: 1 (one model image is required)Example: nterms=3 - represents a 2nd orderTaylor-polynomial in frequency and should beused in conjuction with coefficient modelimages asmodel=[‘xxx.model.tt0’,’xxx.model.tt1’,‘xxx.model.tt2’]
reffreq (string='')- Reference-frequency about which this Taylor-expansion isdefined.Default: ‘’ (reads the reference frequency fromthe model image)Example: reffreq = ‘1.5GHz’
complist (string='')- Name of component listDefault: noneExample: complist=’test.cl’WARNING: component lists are difficult to make
incremental (bool=False)- Add model visibility to the existing model visibiltiesstored in the MSDefault: FalseOptions: False|True
usescratch (bool=False)- Story visibilities in MODEL_DATA column?Default: FalseOptions: False|TrueIf True, model visibilities will be stored in thescratch column MODEL_DATA; if False, the modelvisibilities will be generated on the fly (thismode may save some disk space equivalent to thevolume of the observed data).