Release Notes CASA 5.5

This is the documentation for CASA 5.5.0. CASA 5.5.0 is intended for general use. Changes compared to the CASA 5.4 release are listed below.


  • Several new tasks have been introduced: apparentsens, polfromgain, and sdpolaverage.
  • The tasks fringefit, gaincal, polcal, immath, and plotms have a number new capabilities, as described below.
  • In tclean, new weighting options are added, and improved noise calculation was implemented for "auto-multithresh".
  • rerefant now supports rereferencing solutions produced by the fringefit task.
  • importfitsidi now imports flagging information and correctly concatenates WEATHER tables.
  • statwt has the excludechangs parameter re-introduced, and can now also use data selection by scan.
  • The simulator has been updated to include the spectral variation of each component in a componentlist.
  • The CASA Docs documentation has a new look and layout


  • CASA Docs now has a more modern look and layout, with easier navigation
  • CASA Docs has a section added that collects CASA memos and knowledgebase
  • Inline help and parameter description on the CASA Docs task pages are being improved, to provide a more complete documentation on task-parameters. This is work in progress that will continue for CASA 5.6

Data Import/Export

  • importfitsidi now imports flagging information.
  • importfitsidi now correctly concatenates WEATHER tables if multiple input files are given.

MS Viewing, Editing, Manipulation

  • In plotms, a new interactive flagging mode has been added, "Flag all/Unflag all". Contrary to the existing mode that applies flagging operation to selected regions within plot panels, the new mode offers per-panel flagging operation to the user.
  • plotms now has the capability to plot antennas pointing directions. Refer to Ant-Ra / Ant-Dec axes, and to the Antenna Pointing Direction Parameters section in plotms documentation. Averaging is currently not supported for these axes. This feature has only been implemented for ALMA, ASTE, and NRO data.
  • In plotms, a feature has been added to allow connections between plotted visibilities in calibration tables (MeasurementSets are not yet supported). Connected points will have the same metadata (including flag) with only the x-axis value changing. Points will be colorized based on their connection metadata. No connectors are drawn between flagged and unflagged points, even when flagged points are not displayed.


  • The task fringefit has several new capabilities:
    • fringefit now allows specification of multiple reference antennas and automatically chooses an alternative reference antenna if no data for the initial reference antenna is available.
    • fringfit now allows specification of search windows for delay and rate.
    • fringefit now has the option to skip the globalization step.
    • fringefit uses a new convergence criterion that matches classic AIPS more closely. This speeds up the task in most cases as it requires fewer iterations in solving.
  • rerefant now supports rereferencing solutions produced by the fringefit task.
  • The task polcal has two new parameters, Xfparang+QU and PosAng. These are considered experimental in CASA 5.5.  
    • Xfparang+QU uses parallactic angle coverage on a significantly linearly polarized calibrator to estimate the cross-hand phase spectrum prior to instrumental polarization calibration (for either basis). It solves for the apparent cross-hand phase spectrum (channelized) and integrated (per-spw) fractional linear polarization, Q and U. Xfparang+QU is part of "standard" polarization calibration for ALMA (linear feed basis), and is under study as an option for the VLA (circular basis).
    • PosAng solves directly for the absolute position angle offset spectrum, for either the linear or circular basis. PosAng is redundant with traditional cross-hand phase spectrum calibration (poltype='Xf') for the circular feed basis (but permits a semantic distinction in heuristics); for the linear basis, an external calibration reference more accurate than the presumed feed registration is required (e.g., a polarized beacon such as ALMA is constructing).
  • A new task polfromgain is added, which can estimate calibrator linear polarization from gain solutions (which assumed zero linear polarization in the model) in the linear basis. This is part of the "standard" ALMA polarization calibration. Task polfromgain is considered experimental in CASA 5.5.
  • A new task apparentsens is added, which will estimate nominal imaging sensitivity from the visibility weights as a function of data selection and imaging weight parameters. Also reports relative sensitivity compared to pure natural weighting. Supports only continuum (specmode='mfs') imaging at this time.  
  • A new single-dish task sdpolaverage is added to export data averaged over different polarizations.
  • In task gaincal, the new parameters solmode and rmsthresh have been added to support robust solving modes: L1 and/or on-the-fly outlier rejection while solving. This feature is experimental in CASA 5.5.
  • statwt can now use data selection by scan (under selectdata).
  • In statwt, the excludechans parameter has been re-implemented in statwt. It now takes a Boolean setting such that, when True, the fitspw selection is inverted and the specified channels are excluded from the computation of the weights.


  • The capabilities of clean are now in tclean. The clean task will be deprecated in the near future! Please use tclean instead. All major functionality from clean is present in tclean via a modified interface along with additional algorithmic options.
  • A new visibility weighting option, weighting='briggsabs', is now available in tclean. The advantage of this weighting option is that it is much more aggressive with respect to changes in npixels, the uv-cell size. The Briggsabs option is experimental in CASA 5.5 - please use at own discretion! 
  • A new parameter perchanweightdensity has been implemented in tclean, which when set to perchanweightdensity=True provides a per channel way to calculate Briggs weight density for image cubes. This results in a more uniform sensitivity per channel for cubes, but with generally larger psfs than the default setting of perchanweightdensity=False (prior behavior).
  • In tclean, the ability is added to do full AW-Projection corrections, using frequency and time dependent primary beams, suitable for VLA widefield and wideband imaging. The use AW-Projection is experimental in CASA 5.5 - please use at own discretion! The corrections are azimuthally asymmetric to account for feed leg structures and sidelobes of the primary beam. They also include beam squint, which is corrected during gridding as a function of time, band, and frequency within a given band. The frequency dependence of the primary beam within the data being imaged is included in the calculations and can optionally also be corrected for during gridding. In addition, w-corrections to account for sky curvature and non-coplanar baselines are now possible. The AW-Projection algorithm is currently undergoing commissioning for the VLA Sky Survey pipeline and is being tested for a variety of other use cases. We aim to officially endorse it for general use in CASA 5.6.
  • In tclean, a new improved noise calculation was implemented for "auto-multithresh" automask and n-sigma threshold stopping. It allows more robust noise estimate in the presence of extended source structures and/or absorption. Set a newly introduced parameter, fastnoise, to False to active the new noise calculation. When the default, fastnoise=True, is chosen a simple median absolute deviation (MAD) based noise calculation will be used as in the previous releases. In addition, the sidelobe threshold is now being correctly chosen over the noise threshold for the cases where the peak residual is negative and both automask and n-sigma threshold stopping are used together.
  • The ia.decimate tool now ensures that the beam information is flushed to disk even if the image tool is not closed properly.


  • immath now employs two new modes, lpoli and tpoli, for linear and total polarization images. The poli mode, which was previously available (and for backwards compatibility has not been changed), is flexible and will fall back to tpoli when Stokes V is provided and lpoli when Stokes V is not available.
  • A new parameter prec has been added to the task immath and tool ia.imagecalc(), which will be used to explicitly indicate what the precision of the output image should be, float or double.

Single Dish

  • a new SD task sdpolaverage is added to export data averaged over different polarizations.


  • The simulator has been updated to include the spectral variation of each component in a componentlist. For a component with a flat spectrum, the predicted visibilities now show the frequency variation of the beam.
  • CASA 5.5 introduces a great improvement in memory use for large mosaics when using mosweight=True in tclean. As downside of the memory improvement, mosaics with large number of fields and MSs may suffer from an "too manby open files" error if the limit for open files is set too low (see Known Issues for details).

Bug-fixes (selected)

  • A bug in tclean when using deconvolver='multiscale', such that the model column was being written incorrectly when tclean finished by threshold (the behavior was fine when finishing by niter), has been fixed.
  • A bug in tclean was fixed where parallel processing with parallel=True failed when mosweight=True