Two types of point-source deconvolution, as well as multi-scale deconvolution, are available. A continuum image (mfs) is produced by gridding together all spectral data. Individual channels or groups of channels can also be images and then placed in an output image cube. The cleaning regions can be specified by an input mask image, from a file containing rectangular regions, or interactively as the deconvolution progresses. The mosaic task only uses the CORRECTED datacolumn which is made from the DATA datacolumn using applycal with the appropriate calibration tables. Many Stokes combinations are available.

WARNING: The task mosaic will be deprecated in CASA 5.4.


Parameter descriptions


Name of input visibility file. Default: none. Examples: vis=''


Pre-name of output images. Default: none. Examples: imagename='m2'; output images are: m2.image, cleaned and restored image; m2.flux, relative sky sensitivity over field; m2.model, image of clean components; m2.residual, image of residuals; m2.interactive.mask, image containing clean regions


Frequency Specification. Default: 'mfs'. Examples: mode = 'mfs', means produce one image from all specified data; mode = 'channel', use with nchan, start, width to specify output image cube; mode = 'velocity', means channels are specified in velocity; mode = 'frequency', means channels are specified in frequency.

mode expandable parameters (for modes other than 'mfs')

start, width are given in units of channels, frequency or velocity as indicated by mode, but only channel is complete.


Number of channels (planes) in output image. Default: 1. Examples: nchan=3


Start input channel (index-0). Default=0. Examples: start=5


Output channel width (>1 indicates channel averaging). Default=1. Example: width=4

mode examples: spw = '0,1' mode = 'mfs', will produce one image made from all channels in spw 0 and 1; spw='0:5~28^2' mode = 'mfs', will produce one image made with channels (5,7,9,...,25,27); spw = '0' mode = 'channel' nchan=3; start=5; width=4, will produce an image with 3 output planes: plane 1 contains data from channels (5+6+7+8), plane 2 contains data from channels (9+10+11+12), plane 3 contains data from channels (13+14+15+16); spw = '0:0~63^3' mode='channel' nchan=21 start = 0 width = 1, will produce an image with 20 output planes: plane 1 contains data from channel 0, plane 2 contains date from channel 2, plane 21 contains data from channel 61; spw = '0:0~40^2' mode = 'channel' nchan = 3 start = 5 width = 4, will produce an image with three output planes: plane 1 contains channels (5,7), plane 2 contains channels (13,15), plane 3 contains channels (21,23)



Algorithm to use (expandable). Default: 'clark': Options: 'clark', 'hogbom', 'multiscale', 'entropy'. 'hogbom' cleans from the images only. Only inner quarter of image is cleaned. 'clark' cleans from gridded us data. Only inner quarter of image is cleaned. 'multiscale' cleans with several resolutions using hobgom clean. Currently much slower than single resolution. For extended sources, try single resolution with interactive and 'entropy' maximum entropy algorithm is still experimental and not recommended for general use.

'multiscale' expandable parameter


In pixel numbers; the size of component to deconvolve. Default = [0,3,10] recommended sizes are 0 (point), 3 (points per clean beam), and 10 (about a factor of three lower resolution).


Stop component search when the largest scale has found this number of negative components; -1 means continue component search even if the largest component is negative. Default: 2. Examples: negcomponent=-1


entropy (MEM) expandable parameters (experimental)


Target image sigma. Default: '0.001Jy'. Examples: sigma='0.1Jy'


Target flux for final image. Default: '1.0Jy'. Examples: targetflux='200Jy'


Constrain image to match target flux, otherwise targetflux is used to initialize model only. Default: False. Examples: constrainflux=True


Name of MEM prior images. Default: ['']. Examples: prior='source_mem.image'



Image pixel size (x,y). Default = [256,256]. Example: imsize=[350,350], imsize = 500 is equivalent to [500,500]


Cell size (x,y). Default= none. Examples: cell=['0.5arcsec,'0.5arcsec'] or cell=['1arcmin', '1arcmin']; cell = '1arcsec' is equivalent to ['1arcsec','1arcsec']

NOTE: cell = '2' makes default cell size of 2 radians!



Direction measure or field ID for the mosaic center. Default: 0 (imply field=0 as center). Examples: phasecenter=6 or phasecenter='J2000 19h30m00 -40d00m00'


Stokes parameters to image. Default='I'. Examples: stokes='IQUV'; options: 'I','IV''QU','IQUV','RR','LL','XX','YY','RRLL','XXYY'


Maximum number iterations, set to zero for no CLEANing. Default: 500. Examples: niter=500


Loop gain for CLEANing. Default: 0.1. Examples: gain=0.5


Flux level at which to stop CLEANing (units=mJy). Default: 0.0. Examples: threshold=0.0


Name of mask image used for CLEANing. Default '' means no mask. Examples: mask='orion.mask'. It is useful to use a mask from a previous interactive mosaic session for a new execution. The mask image shape must be the same as the new mosaic.


Cleaning region. Default: [ ] defaults to inner quarter of image. Three specification types:

  1. 'interactive' allows the user to build the cleaning mask interactively using the viewer. The viewer will appear every npercycle interation, but modify as needed. The final interactive maks is saved in the file imagename_interactive.mask.
  2. Explicit pixel ranges. Examples: cleanbox=[110,110,150,145] clean region with blc=110,100; trc=150,145 (pixel values). Only one clean region can be given this way.
  3. Filename with pixel values with ascii format: on each line
1 45 66 123 124
2 23 100 300 340


'interactive' expandable parameter


This is the number of iterations between each clean to update mask interactively. Set to about niter/5, can also be changed interactively.


Select fields in mosaic. Use field id(s) or field name(s). [go listobs to obtain the list id's or names]. Default: ''=all fields. If field string is a non-negative integer, it is assumed to be a field index otherwise, it is assumed to be a field name. Examples: field='0~2', field ids 0,1,2; field='0,4,5~7', field ids 0,4,5,6,7; field='3C286,3C295', field named 3C286 and 3C295; field = '3,4C*', field id 3, all names starting with 4C


Select spectral window/channels.

NOTE: This selects the data passed as the INPUT to mode.

Default: ''=all spectral windows and channels. Examples: spw='0~2,4', spectral windows 0,1,2,4 (all channels); spw='<2', spectral windows less than 2 (i.e. 0,1); spw='0:5~61', spw 0, channels 5 to 61; spw='0,10,3:3~45', spw 0,10 all channels, spw 3, channels 3 to 45; spw='0~2:2~6', spw 0,1,2 with channels 2 through 6 in each; spw='0:0~10;15~60', spectral window 0 with channels 0-10,15-60; spw='0:0~10,1:20~30,2:1;2;3', spw 0, channels 0-10, spw 1, channels 20-30, and spw 2, channels, 1,2 and 3


Time range. Default = '' (all). Examples: timerange = 'YYYY/MM/DD/hh:mm:ss~YYYY/MM/DD/hh:mm:ss'

NOTE: If YYYY/MM/DD is missing date defaults to first day in dataset.

timerange='09:14:0~09:54:0' picks 40 min on first day; timerange= '25:00:00~27:30:00' picks 1 hr to 3 hr 30min on next day; timerange='09:44:00' data within one integration of time; timerange='>10:24:00' data after this time


Specify rest frequency to use for image. Default=''. Occasionally it is necessary to set this (for example some VLA spectral line data). For example, for NH_3 (1,1) put restfreq='23.694496GHz'.


Input Single Dish image to use for model. Default='' (no image). Examples: sdimage=''


Name of output(/input) model image. Default='' (none=imagename.model). Examples: modelimage='orion.model'

NOTE: This specifies the output model if a single dish image is input or the output model name from the imaging.


Weighting to apply to visibilities. Default='natural'. Examples: weighting='uniform'; options: 'natural', 'uniform', 'briggs', 'radial', 'superuniform'

Weighting expandable parameters

For weighting='briggs':


Robustness mode (see help mosaic). Default='norm'. Examples: rmode='abs'; options: 'norm', 'abs', 'none'


Brigg's robustness parameter. Default=0.0. Examples: robust=0.5; options: -2.0 to 2.0; -2 (uniform)/+2 (natural)


Noise parameter to use for rmode='abs' in 'briggs' weighting. Example noise='1.0mJy'

For superuniform/briggs weighting:


Number of pixels to determine uv-cell size for weight calculation. Examples: npixels=7



Individually weight the fields of the mosaic. Default: False. Examples: mosweight=True; This can be useful if some of your fields are more sensitive than others (i.e. due to time spent on-source); this parameter will give more weight to higher sensitivity fields in the overlap regions.


Gridding method for the image. Options: ft (standard interferometric gridding), sd (standard single dish) both (ft and sd as appropriate), mosaic (gridding use PB as convolution function). Default: 'mosaic'. Examples: ftmachine='ft'


Change the threshold at which the deconvolution cycle will stop, degrid and subtract from the visibilities. For poor PSFs, reconcile often (cyclefactor=4 or 5); For good PSFs, use cyclefactor 1.5 to 2.0. Default: 1.5. Examples: cyclefactor=4; cycle threshold = cyclefactor * max sidelobe * max residual


Cycle threshold doubles in this number of iterations. Default: -1. Examples: cyclespeedup=500


Controls scaling of pixels in the image plane. Default='SAULT'. Examples: scaletype='PBCOR'; options: 'PBCOR','SAULT'. 'SAULT' scale makes an output image where the noise is constant across the whole mosaic. However, the image is NOT corrected for the PB pattern, and therefore is not "flux correct". Division of the  SAULT image_name.image image by the image_name.flux image will produce a "flux correct image". The 'PBCOR' option uses the SAULT scaling scheme for deconvolution, but when interactively cleaning shows the primary beam corrected image; the final PBCOR image is "flux correct".


Minimum PB level to use. Default=0.1. Examples: minpb=0.01


Run asynchronously. Default = False. Do not run asychronously.