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Release Information

Information concerning this release of CASA

Release Notes CASA 6.2

These are the release notes for CASA 5.8/6.2. Changes compared to the CASA 5.7/6.1 release are listed below.

Highlights (details below)

  • Several missing tasks, as well as the interactive flagdata GUI, were added to CASA 6.

  • An extensive refactor of cube imaging in tclean was implemented to enhance reliability, flexibility, and peformance.

  • A new task sdatmcor for the atmospheric correction of single dish data was added.

  • The accor task now supports the corrdepflags parameter.

  • tclean has a new option ‘briggsbwtaper’ for image weighting.

  • In tclean, the algorithm that fits a Gaussian to the PSF has been improved.

  • In tclean, the normalization factor used to calculate the imaging weights for perchanweightdensity=True in Briggs weighting now conforms to the definition used in perchanweightdensity=False.

  • The chanchunks parameter has been removed from tclean after a refactor of the code made it redundant.

  • The GAIN_CURVE subtable is now available to the gencal task with caltype=’gc’

  • Task listobs has been extended to return MeasurementSet metadata in a dictionary.

  • plotms now has averaging types: time, field, scan, channel, baseline, antenna, spw, and scalar.

  • plotms has added support for M Mueller and Fringe Jones calibration tables.

  • plotms has channel selection implemented for all types of calibration tables, and completed implementation of averaging types.

  • sdbaseline has new parameters ‘updateweight’ and ‘sigmavalue’.

  • The task simalma was updated to produce the expected output.

  • Error handling has been made consistent among tasks.

  • The model for Mars was updated.

  • The tec_maps script for ionospheric calibration has been ported to CASA 6

  • All CASA tasks now follow a consistent error handling approach.

  • A number of bugs were fixed.

Full Release Notes

CASA 6 transition

  • The following missing tasks were added to CASA 6: apparentsens, msuvbin, imview, testconcat, uvcontsub3.

  • Flagdata: The interactive flagdata GUI is now available in CASA 6

Installation and operation

  • Error handling: All CASA tasks now follow a consistent error handling approach. In CASA 6, when imported from casatasks, all tasks will raise exceptions in case of a severe errror. New documentation pages have been added to explain how errors and exceptions work in CASA tasks, for CASA 5 and CASA 6.

MS viewing, editing, manipulation

  • Plotms: plotms has added the following features:

  • support was added for M Mueller and Fringe Jones calibration tables. The new axes “Delay Rate” (or “rate”) and “Disp Delay” (or “disp”, for dispersive delay), along with the existing “Delay” axis, are available for Fringe Jones tables.

  • Support for BPOLY tables has been refactored and tested.

  • support for all averaging types for calibration tables except BPOLY and GSPLINE tables. Averaging may not be combined with channel selection but is supported with all other selection including spw.

  • channel selection has been implemented for all types of calibration tables.

  • Antenna selection was fixed for the three types of calibration tables: baseline-based (selects on ANT1 and ANT2 as described in MSSelection), reference antenna-based (selects on ANT1 unless ‘&’ syntax is used for the refant), and pure antenna-based (selects on ANT1 only).

  • mstransform: In CASA 5.8/6.2, a bug in mstransform has been fixed such that the SIGMA and WEIGHT columns (and their _SPECTRUM variants) are correctly calculated for datacolumn settings other than “CORRECTED” and when time averaging is not selected. Previously, only these two cases (datacolumn=”CORRECTED” and/or when time averaging was being used) resulted in correct settings for the SIGMA and WEIGHT columns. Further work remains to address some remaining inconsistencies in these columns, as described in the Known Issues list.

  • ms tool: the ms tool has been fixed such that when channel averaging has been enabled (via ms.selectchannel), the WEIGHT column will now have the correct array size. If you have existing code where the WEIGHT column was retrieved in a separate call to ms.getdata from other columns to avoid this bug, then these calls should now be consolidated into a single call (i.e., with all columns) because the ordering of the rows returned by separate calls is no longer guaranteed to be the same. Also, the avgtime option of ms.getdata will now correctly retrieve all qualifying rows, regardless of the time sorting of the measurement set.

  • listobs: Task listobs has been extended to return MeasurementSet metadata in a dictionary.


  • accor: The accor task now supports the corrdepflags parameter. This makes it possible to use accor on (parts of) data-sets where one polarizations is flagged.

  • Calibration models (Mars): The model for Mars from January 1, 2021 to December 31, 2030 was corrected. Users of earlier versions that 6.2 should update their local data repository manually. (This correction of the Mars model was included also in CASA 6.1.1, but not CASA 6.1.0 or 6.1.2.)

  • Gain curves: The GAIN_CURVE subtable, when present in the MS, is now available to the gencal task with caltype=’gc’ and creates an appropriate gain curve calibration table from the information provided in the GAIN_CURVE table.

  • setjy: A correction to the log message has been made in task setjy when standard=’Butler-JPL-Horizons 2012’ and listmodels=True; the correction reflects the correct starting date for the availability of the time dependent asteroid models (2014.01.01).

  • interpolation schemes: The lanczos interpolation method was removed from imdev/ia.deviation().

  • TEC map: The tec_maps script for ionospheric “Total Electron Content” calibration has been ported to CASA 6 (python3). To follow new server conventions at NASA’s Crustal Dynamics Data Information System (CDDIS), the TEC data retrieval mechanism within the tec_maps script has been updated (the out-dated version should have been inoperable since 2020 Nov 1 due to security-related changes in the NASA CDDIS server). The script has been removed from the CASAaddons repository in favor of a better-maintained version which exists within casatasks.

  • A frequency-dependent version of the generic Jones matrix calibration class has been added to the calibration infrastructure. Like its frequency-independent counterpart, this functionality is not used by any of the CASA tasks and will not be visible for most users, but it can be used to generate fully generic calibration tables thought the createcaltable() method of the calibrater tool.


  • Cube refactor: A refactor of cube imaging in tclean was implemented in CASA 6.2, with the goals of ensuring identical results between serial and parallel runs (to within numerical precision), and enhancing flexibility and peformance of cube imaging, especially in parallel runs. As part of this extensive cube refactor, the following features were implemented in 6.2:

    • Parallel and serial runs now use the same code and that has fixed the differences which used to be found between serial and different n-parallel runs. Serial and parallel runs now give identical results to within numerical precision.

    • Refconcat cubes are no longer produced (also the directory workingdir is no longer made for cube runs)

    • tclean can be restarted in parallel or serial independent of how the first run was achieved.

    • For cubes, model visibilities can now be saved in parallel.

    • In parallel run, a common beam can now be given (e.g restoringbeam=’common’). Previously, tclean had to be re-run in serial to restore to a single restoring beam for all channels.

    • Tracking a moving source (with ephemeris tables) now works in parallel with specmode=’cubesource’.

    • The chanchunks parameter has been removed, as the refactor code made it redundant.

    • parallel=True or parallel=False is not considered for cube imaging. If casa is launched via the mpicasa call then cube imaging is in parallel else if it is invoked via the casa call then it is run in serial.

    • Using a psf to reset to another psfphasecenter for mosaic now works for serial and parallel.

    • The major and minor cycles states have been dissociated when a major cycle is happening, the minor cycle code does not hold any state in memory and vice versa. This is the reason why a selectvis message will be seen  at every major cycle. This should reduce the amount of memory used.

More details can be found in this CASA Knowledgebase article

  • tclean: A new imaging weighting option, ‘briggsbwtaper’, has been added to tclean which modifies the per-channel cube imaging weights (i.e., when perchanweightdensity = True) to have a similar density to that of the continuum imaging weights, improving alignment between cube and mfs beams for a given dataset. This is currently an experimental weighting scheme, but has been verified for ALMA use-cases.

  • tclean: Briggs style weighting with perchanweight=True no longer creates many temporary files, but only one. Previously, when doing Briggs style weighting, a temporary file was created to keep the weight density grid that is used in calculating the imaging weight. That temporary file has 1 grid when perchanweight=False, but it contained nchan grids when perchanweight=True. When mosweight=True, then a temporary file was kept for each field in the mosaic and also for each MS, so a mosaic with a large number of pointing could create hundreds of temporary files. In CASA 6.2, when perchanweight=True, only one temporary file is made which contains the imaging weights for every rows and channels of the MSs used.

  • tclean: The chanchunks parameter has been removed following a cube imaging refactor of the tclean code in 5.8/6.2 which rendered the parameter unnecessary. Please see the updated tclean documentation describing current parallel processing for imaging for more information.

  • tclean: The algorithm that fits a Gaussian to the PSF in tclean has been improved for cases where the number of pixels across the main PSF lobe are less than 5 and larger than 10. The fit Gaussian beam size is now stable as a function of the number of pixels in these regimes.

  • tclean: The normalization factor used to calculate the imaging weights for tclean(…, perchanweightdensity=True, weighting=’briggs’, …) has been updated to conform to the definition used in perchanweightdensity=False (which is derived from Briggs D., 1995 (pdf version). This will cause the beams to be smaller in the region of robust = -1 to 1.5, however the beam achieved at robust = -2 (uniform) and robust = 2 (natural) should not change significantly. The same set of beam sizes previously achievable are still available, albeit at different robust values.

Single Dish

  • sdatmcor: A new task sdatmcor for the atmospheric correction of single dish data is released in CASA 5.8/6.2. The sdatmcor task mitigates the residual atmospheric lines due to the elevation difference between the target and reference positions, utilizing the Atmospheric Transmission at Microwaves (ATM) library in CASA.

  • sdbaseline: Task sdbaseline has new parameters ‘updateweight’ and ‘sigmavalue’. When updateweight is True, weight of each baseline-subtracted spectrum is calculated based on its standard deviation or rms and stored in the WEIGHT column of the output MS.

  • tsdimaging: Task tsdimaging does no longer creates a .subwt image.

  • atmostphere tool: Performance of atmosphere tool has been improved. After the improvement, atmospheric model calculation using atmosphere tool is twice as fast as the previous versions of CASA. Performance of the tsdimaging task was also slightly improved.


  • simalma: The task simalma was updated to produce the expected output. A bug was fixed where the task simalma failed to return properly in due to import dependency problems. In addition, a problem calculating the image size for the default imsize=’[]’ was fixed, and an extraneous primary beam scaling before combining interferometric and single dish data has also been removed. These fixes were also included in CASA 6.1.1, but not in CASA 6.1.0 and 6.1.2.


  • CASA Viewer: The viewer task is deprecated in lieu of imview and msview, which contain the exact same functionality. Please invoke the imview (msview) task for visualizing images or image cubes (visibility data). For the time being, typing ‘viewer’ in CASA invokes the imview task under the hood.

Bug fixes

  • Data respository: A bug in the way that tclean calls the data repository in CASA 6 has been fixed. Previously tclean was looking at .casa/ for the location of the distro data but only considering the root data directory existence and not checking existence of the data (more specifically, antenna response data) actually used by it. These have now been fixed to look at the correct casadata distro location, which varies depending on whether it is CASA 5 or 6 and is a tarball or built from source.

  • importfitsidi: A bug in importfitsidi that affected polarization visibility data, such that the cross-hands may previously have been spuriously mis-ordered on some baselines, has been fixed. This issue has been seen in e-MERLIN data; EVN and VLBA data are unaffected.

  • mstransform: A bug in mstransform has been fixed such that the SIGMA and WEIGHT columns (and their _SPECTRUM variants) are correctly calculated for datacolumn settings other than “CORRECTED” and when time averaging is not selected. Previously, only these two cases (datacolumn=”CORRECTED” and/or when time averaging was being used) resulted in correct settings for the SIGMA and WEIGHT columns. Further work remains to address some remaining inconsistencies in these columns, as described in the Known Issues.

  • applycal: A bug was fixed where applycal imported from the casatasks module in CASA 6.1 gave the wrong default for the calwt parameter, namely calwt=False instead of the intended calwt=True.

  • Cal library: An error was corrected in the Cal Library Syntax documentation, to the effect that calwt settings are controlled by the first entry (per caltable, in multi-entry per caltable cases), and NOT that if any entry specifies calwt=True, all entries will use True. Nevertheless, if users wish to specify calwt explicitly in their scripts.

  • initweights: A bug in the initweights task which was causing it to fail in mpicasa has been fixed (this fix was implemented in the non-public CASA 6.1.3 release version).

  • tclean: A bug in the way that the awproject gridder in tclean calls the data repository in CASA 6 has been fixed. Previously tclean was looking at .casa/ for the location of the distro data, but only considering the root data directory existence and not checking existence of the data (more specifically, antenna response data) actually used by it. These have now been fixed to look at the correct casadata distro location, which varies depending on whether it is CASA 5 or 6 and is a tarball or built from source.

  • tclean: A minor bug in how the imaging weights are calculated for tclean(…, perchanweightdensity=True, weighting=’briggs’, …) has been fixed. This bug causes a faint vertical sidelobe structure in the PSF when the uv plane is poorly sampled. It did not have a significant impact on most images, and had a marginal ($<$3%) impact on high dynamic range (SNR >300) imaging cases tested for ALMA.

  • tclean: A bug was fixed that has the default value for imsize missing in tclean and apparentsens in CASA 6.

  • tclean: A bug was solved for imaging ephemeris objects in tclean, for which tracking solar system objects was failing when imaging continuum in parallel. Parallel continuum imaging is partitioned over time, and each process was using a different initial time reference. The bug was resolved by forcing all of the processes to use the epoch recorded in the image as the reference.

  • tclean: A bug in the nsigma parameter in tclean has been fixed; values expressed as integers are now being recognized.

  • tclean: A bug has been fixed in that deconvolve=’multiscale’ now correctly uses the per-channel PSF, and the convergence-health of the deconvolution runs has demonstrably improved. Some situations that earlier diverged now converge successfully.

  • feather: In previous versions of CASA, the feather task was incompatible with input images that were created in parallel. This bug has been fixed by the cube refactor in CASA 5.8/6.2.

  • imdev: The imdev interpolation parameter was not being recognized and linear interpolation was being used no matter what its value. That bug has been fixed and now imdev correctly performs the specified interpolation algorithm.

  • sdbaseline: A bug in sdbaseline was fixed, in which rerun failed if ms_blparam.txt file exists. With the fix, ms_blparam.txt will be overwritten if it exists.

  • sdintimaging: several bugs were fixed in sdintimaging, and this task has now been enabled in parallel mode.

  • A bug was fixed where terminal control, such as the use of keyboard arrows, did not work in CASA 6.1.

Known Issues

Summary Most Important Issues

  • The GUI-based application plotcal has not been migrated into the CASA 6 series. Instead, all of the plotcal functionality is now available in plotms.

  • There are generic problems putting multiple MSs into tclean that have mismatches in their shape

  • The task clean is no longer being actively maintained; instead, tclean is now the recommended task for imaging.

  • Wideband and widefield imaging in tclean are only partially validated - please use at own risk and read wideband and widefield documentation.

  • In tclean, uvtaper does not work with natural weighting.

  • When imaging large mosaics with mosweight in tclean, an error “too many open files” may occur that may require to increase the limit for open files.

  • stawt may fail when the correlator integration time changes within an MS and statwt is run with timebin set to an integer value.

  • CASA is not using LD_LIBRARY_PATH anymore but CASALD_LIBRARY_PATH to avoid confusion.

  • cvel is calculating the velocity incorrectly for ephemeris objects. We recommend to use mstransform or its offspring cvel2, although the latter should be used with care as it is not fully commissioned yet.

  • When running CASA with the pipeline option ‘casa –pipeline’, plotcal may not show all plots and comes up with an error message. Use CASA without ‘–pipeline’ when performing interactive data reduction.

  • fixvis uses the small angle approximation and may be incorrect for large phase shifts. Use tclean for phase center shifts during imaging when applicable.

  • With parallel calibration on MMS files, fixvis does not write out the the new MMS specified in outputvis correctly, hence fixvis solutions are not applied when writing to a new MMS.

  • In fringefit, calibration tables created with CASA 5.5 and before cannot be used with CASA 5.6 and later.

  • In tclean, defining image cubes in optical velocity in some cases is known not to work.

  • In tclean, using the mosaic gridder with the default nchan=-1 is in some cases known to produce errors.

  • Ionospheric TEC corrections are currently validated in CASA only for VLA data.

  • ephemeris objects are not correctly supported by virtual model columns.

  • In tclean, the combination of specmode=’cube’ and gridder=”awproject” has not been commissioned for use and may result in errors

  • Setting pblimit to a negative value will no longer unmask the full image, but only the area within primary beam is visible.


Several issues have been encountered for CASA 6:

  • The GUI-based application plotcal has not been migrated into the CASA 6 series. Instead, all of the plotcal functionality is now available in plotms.

  • inp/go does not work for the following tasks: msuvbin, browsetable, imview, msview, deconvolve, uvcontsub3, testconcat. Please invoke the arguments directly when running these tasks in CASA 6.


  • For Mac OSX, the default behavior when downloading multiple versions of CASA is to call it “CASA” (i.e., including a space). However, CASA is unable to find the viewer when a space exists in the application name. The workaround is to rename the application excluding the space.

  • If you use a version of RHEL6 with a kernel version that is older than 2.6 you may encounter an error like:

E0324 17:24:18.576966686   27444]     check for SO_REUSEPORT:
{"created":"@1585038258.576951288","description":"OS Error","errno":92,"file":
not available","syscall":"setsockopt(SO_REUSEPORT)"}
  • NFS mounted disks

    • It is not recommended that you run CASA (e.g. have your data) on disks that are NFS mounted. It can be done, but in some cases the files will be NFS locked and this can crash CASA or its tasks. In this case, you have to restart CASA.

    • If you receive messages like xvfb timeout you may try to clean out your /tmp folder, then restart CASA.

  • Python:

    • Environment variables set for personal use may be incompatible with CASA 6, given that the CASA comes with a Python version that may be different from the one installed for regular use. It is still unclear which specific errors can occur, but one workaround solution for these types of errors is to unset PYTHONSTARTUP before starting casa. We are looking into possible solutions for CASA 6.3.

    • Files in the current directory with the same name as ipython files will cause errors like this error that occurs when a file exists in the current directory: ```python

AttributeError                            Traceback (most recent call last)
/lib/python2.7/site-packages/IPython/core/interactiveshell.pyc in enable_matplotlib(self, gui)
   2945                 gui, backend = pt.find_gui_and_backend(self.pylab_gui_select)
-> 2947         pt.activate_matplotlib(backend)
   2948         pt.configure_inline_support(self, backend)

/lib/python2.7/site-packages/IPython/core/pylabtools.pyc in activate_matplotlib(backend)
    292     matplotlib.rcParams['backend'] = backend
--> 294     import matplotlib.pyplot
    295     matplotlib.pyplot.switch_backend(backend)

/lib/python2.7/site-packages/matplotlib/ in <module>()
     21 from matplotlib.cbook import dedent, silent_list, is_string_like, is_numlike
     22 from matplotlib import docstring
---> 23 from matplotlib.figure import Figure, figaspect
     24 from matplotlib.backend_bases import FigureCanvasBase
     25 from matplotlib.image import imread as _imread

/lib/python2.7/site-packages/matplotlib/ in <module>()
     16 import artist
     17 from artist import Artist, allow_rasterization
---> 18 from axes import Axes, SubplotBase, subplot_class_factory
     19 from cbook import flatten, allequal, Stack, iterable, is_string_like
     20 import _image

/lib/python2.7/site-packages/matplotlib/ in <module>()
   8453 #This is provided for backward compatibility
-> 8454 Subplot = subplot_class_factory()
   8456 docstring.interpd.update(Axes=martist.kwdoc(Axes))


  • Starting CASA 6: For “execfile” calls within a script which itself is run via “execfile”, it is necessary to add globals() as the second argument to those “execfile” calls in order for the nested script to know about the global variables of the calling script. For example, within a script ‘’, calls to another script ‘’ should be written as follows: execfile(‘’, globals()).

  • There are cases where two scripts call on each other (i.e., where script “a” uses script “b” and vice versa). In casa 6, the only way to execute these scripts is by running the first script twice:


MS viewing, editing, manipulation

  • statwt

    • In some circumstances when an MS data selection is specified, chanbin is not equal to the default value of spw, and the WEIGHT_SPECTRUM or SIGMA_SPECTRUM columns don’t exist, the statwt task may need to be run twice in order to complete successfully due to a known issue with initializing the WEIGHT_SPECTRUM and/or SIGMA_SPECTRUM columns in the code. In these circumstances, an exception will be raised with instructions to restart the task. If you are using the tool method, first close the ms tool, then reopen it using the same data set, apply the same selection, and then run ms.statwt().

  • mstransform

    • SPW combination (combinespws=True) requires that all the SPWs selected have the same number of channels.

    • Some inconsistencies are present in CASA in the SIGMA/WEIGHT columns (and their _SPECTRUM variants) when splitting on datacolumn=’data’, such as:

    • For an MS with WEIGHT_SPECTRUM but no SIGMA_SPECTRUM (as obtained from initweights), SIGMA_SPECTRUM is created and initialized to SIGMA. While split/mstransform correctly initializes the output WEIGHT to 1/SIGMA^2, it does not initialize the output WEIGHT_SPECTRUM to 1/SIGMA_SPECTRUM^2 (instead it copies the original WEIGHT_SPECTRUM).

    • For an MS with both WEIGHT_SPECTRUM and SIGMA_SPECTRUM, the output WEIGHT_SPECTRUM is again a copy of the input instead of being initialized to 1/SIGMA_SPECTRUM^2.

    Future work in CASA is planned to address such inconsistencies.

  • cvel

    • cvel is calculating the velocity incorrectly for ephemeris objects. We recommend to use mstransform or its offspring cvel2, although the latter should be used with care as it is not fully commissioned yet.

    • cvel fails on MMS files used for parallel processing. We recommend to use mstransform or its offspring cvel2, although the latter should be used with care as it is not fully commissioned yet.

  • flagdata

    • In CASA 6.2, running flagdata can take much longer than in CASA 6.1. In the worst case (summary mode in flagdata), a factor of 4 increase in run time has been reported for certain settings. This issue will be addressed in a next CASA release.


  • bandpass

    • Currently, bandpass will not find good solutions if any correlation (including cross-correlation) in the data is completely flagged. As an interim solution one may split the unflagged data in a separate file and then perform bandpass

  • polcal

    • Polarization position angle calibration poltype=’X’ or ‘Xf’ will be sensitive to any unmodelled position shift in the specified calibrator linear polarization model relative to the centroid location of total intensity (typically the phase center). Excess phase due to the position shift will introduce a bias in the cross-hand phase calibration (which is the same as position angle calibration in the circular feed basis). For this reason, it is best to use truly point-like (in all polarizations) calibrators, if possible, or accurate resolved models.

  • setjy

    • Sometimes setjy does not properly overwrite a current model in the header of the ms (virtual scratch column). It is recommended to use delmod if a model exists and shall be overwritten.

    • virtual model columns in the MS do not correctly support ephemeris objects, although they will run without generating errors or warnings. If any of your calibrators exhibit significant celestial motion on the timescale of your observation (e.g. , any solar system object), you must set ‘usescratch=True’ in calls to setjy().

  • plotcal (also check the matplotlib section for plotcal issues)

    • When plotcal does not release a calibration table properly but keeps it locked, try to hit the quit button in the plotcal GUI first. Then redisplay the table.

    • If you use plotcal on a caltable, it will then be put into the cache of the table system. If you try to re-run a solve (e.g. gaincal, bandpass, blcal, polcal) with the same caltable name and append=F to overwrite, then when it tries to delete the original caltable it cannot due to the cache. You will see an error like:

    SEVERE  gaincal::Calibrater::solve
    Caught exception: Invalid Table operation: SetupNewTable
    ngc5921.demo.gcal is already opened
    (is in the table cache)
    • Either use a different output caltable name, or restart your CASA session (exit and start again) to free up the cache. You can use the rmtables task to delete tables.

    • BPOLY solutions from bandpass must be plotted versus frequency and not channel. BPOLY and B solutions can only be overlaid if xaxis = ‘freq’.

    • GSPLINE and G solutions from gaincal can be overlaid, though this has not been extensively tested.

    • Currently, plotcal needs to know the MS from which the caltable was derived to get indexing information. It does this using the name stored inside the table, which does not include the full path, but assumes the MS is in the same directory as the cal-table and has the same name it had when the caltable was created.

    • If there are flagged channels at the lower edge of the data, say, 0~4, and you select a channel and ‘locate’ it from plotcal, it will give the incorrect channel number (will count from the first unflagged channel, not zero).

  • uvcontsub

    • fitorder should be kept low (<= 1) unless the line band is relatively narrow compared to the fit bands. If the image rms markedly rises in the middle line channels after uvcontsub, fitorder should probably be lowered.

    • fitorder > 0 does not work with solint > ‘int’

  • CASA cal library

    • The CASA cal library (docallib=True in applycal, gaincal, bandpass, etc.) may exhibit problems when calibration is unavailable for a subset of MS spectral windows. Use of spwmap to (transparently, harmless) supply nominal calibration for these spectral windows may help avoid this problem. For antenna position corrections, try spwmap=[0] to relieve a variety of this problem.

  • VLA Switched Power

    • In CASA v4.2.2 and higher, the weight calibration for EVLA switched power/Tsys corrections is still being investigated. Visibility corrections are ok. Since switched power calibration is not used by the EVLA pipeline (except for requantizer gain corrections, for which this problem is irrelevant), and since calwt=F remains the general recommendation, users should rely on statwt to generate appropriate data weights.

  • fringefit

    • For task fringefit, calibration tables created with CASA 5.5 and before cannot be applied with CASA 5.6 and later. Attempting to do so will fail with an error about non-confirming array sizes. Additionally, plotcal in CASA 5.6 and up will not plot calibration tables generated by CASA 5.5 or earlier; older versions of plotcal will also not plot CASA 5.6 fringefit tables correctly.

  • fixvis

    • fixvis uses the small angle approximation and may be incorrect for large phase shifts. This may result in sources shifting position if large phase shifts are being applied (shifts up to a few beam sizes have been reported). Please use tclean for phase center shifts during imaging when applicable.

    • With parallel calibration on multi-MS (MMS) files, fixvis does not write out the outputvis correctly, hence fixvis solutions are not applied when writing to a new MMS. The recommended work-around solution is to over-write the input MMS by leaving the outputvis parameter empty. This will change the input MMS, so if you are concerned about that, we recommend to make a copy before running fixvis in parallel mode. Writing output MS files in serial mode is not affected by this bug.

  • tec_map

    • Ionospheric TEC corrections in CASA are currently validated only for the VLA. TEC corrections for other observatories are experimental and should be done at your own discretion.

    • Do not use CASA 6.1.0 for tec_map corrections.

  • fixplanets

    • To supply JPL-Horizons ephemeris data, a query function, which construct an email query request, can be used but it is only available to CASA 5.8. Constructing a query request manually via email as described in the fixplanets task section should still work for CASA 6.3 as long as the data file is saved in MIME format. An alternative function to query for ephemeris data via JPL-Horizons web interface and CASA readable format directly is planned for the future releases.

Synthesis and Single Dish Imaging

  • clean

    • The task clean is no longer being actively maintained; instead, tclean is now the recommended task for imaging. For Known Issues on clean, see previous CASA Docs versions.

  • tclean

    • In tclean, the uvtaper parameter does not work with natural weighting in CASA 5.8/6.2 and later. As a workaround solution, please approach the desired outcome by using uvtaper with briggs weighting and robust=+2, or use CASA 5.6.

    • Generic problems putting multiple MSs into tclean that have mismatches in their shape: Recently, generic problems have been found with putting multiple MSs into tclean when there are mismatches in shape across the data set. For example, certain data columns may cause a segment fault if they are present in only some of the input data sets. And for mosaics, please specify the phasecenter explicitly, otherwise tclean will select the first pointing from the first MS. Other mismatches in shape across multiple input MSs may cause similar problem in tclean. The CASA team is in the process of coherently addressing these issues for CASA 5.8/6.2. Please contact the Helpdesk if you experience related issues that you cannot otherwise solve.

    • The gridder=’awproject’ has not been fully commisisoned for use with specmode=’cube’ in tclean. The following message appears: The gridder=’awproject’ has not been fully tested for ‘cube’ imaging (parallel=True or False). Formal commissioning of this mode is expected in a subsequent release, where ‘awproject’ will be aligned with recent framework changes. Until then, please report errors/crashes if seen.

    • In tclean, if gridder=’awproject’ is run with psterm=True, the output Primary Beam currently still includes the Prolate Spheroidal function. In order to do a primary beam correction, a separate PB needs to be made with psterm=False. See the CASA pages on AWproject for more information.

    • For widefield imaging in tclean, the following features still need to implemented and commissioned (for usepointing=True, with full heterogenous pointing support):

      • gridder=’mosaic’ : Enable accurate pointing corrections for baselines with antennas pointing in different directions

    • In tclean, the gridders ‘mosaic’ and ‘awproject’ include aperture illumination functions in the gridding convolution functions used for the PSF generation. Although this strictly follows the math, it has several undesirable features especially in the situation where data are not uniform across all axes across which they are being combined (i.e. if the mosaic pattern is not relatively flat, if the center of the image has no mosaic pointing, if different pointings have drastically different uv-coverages or frequency coverages). All such variations cause the PSFs to be position-dependent and could relate to potential instabilities during deconvolution, either requiring many major cycles to converge or diverging. For spectral-cube imaging, the effects are lower because PSFs are normalized to peak 1 no matter what their raw peak values are. For multi-term imaging, the ratios between the spectral PSF central values matter and the effect/error is enhanved. When all these uv-coverage variations are avoided (in careful simulations), both algorithms perform as expected for joint wideband mosaics (both with conjbeams=True or False). For CASA 5.7/6.1, the guidelines are: - Full-field, single pointing imaging (spectral cube as well as multi-term) will be accurate as long as the image phase center matches the PB pointing center.

    • For multi-term wideband joint mosaics, we recommend the use of gridder=’awproject’ with conjbeams=True as that is the only combination that has demonstrated accurate wideband pb-correction (at niter=0) especially in the presence of position-dependent uv-coverage. All other options will need monitoring and several major cycles to ensure convergence. The image should ideally be centered on a PB center.

    • For spectral cube joint mosaics, both gridders are usable. The effects of PSF position dependence are limited to shape differences (i.e. the peak values and fluxes will still be correct). The most validated mode is for ALMA mosaics (via the pipeline) that uses gridder=’mosaic’ with spectral cubes.

    • In tclean, the mosweight parameter for multi-field imaging has a new default value of mosweight=True as of CASA 5.4. The new default setting of mosweight=True in tclean optimizes noise characteristics for Briggs/uniform weighting, but one should be aware that it has the following disadvantages:

      • it may potentially cause memory issues for large VLA mosaics

      • the major and minor axis of the synthesized beam may be up to ~10% larger than with mosweight=False

    • Please change to mosweight=False to get around these issues.

    • When imaging mosaics with a large number of fields and many MSs in tclean, an error can occur that specifies too many open files. This can happen for both manual and pipeline imaging when using the mosweight=True parameter. The reason is that in CASA 5.5, a trade-off was made to reduce memory demands in tclean when using mosweight, by placing the weights on disk using multiple files. Unfortunately, this memory fix may cause open file problems for data sets consisting of many MSs and fields. The problem has been characterized based on the number of MSs and fields: with respect to earlier CASA releases, the imager code now uses #MSs x #fields x 2 additional files. In CASA 5.8/6.2, the number of open files required for the default perchanweightdensity=True case has been dramatically reduced, though perchanweightdensity=False imaging runs are more limited. While the CASA team is working on a permanent solution for future CASA versions, the recommended work-around solution is to manually increase the limit for the number of open files, e.g.: ulimit -Sn 8000 or ulimit -Sn 16000. In some cases, increasing the hard-limit on number of open files may be necessary, which requires admin/root permissions. As a rule of thumb, each MS requires ~54 simultaneously open files. With a 16k limit and a maximum of 150 fields per MS, for a use case with perchanweightdensity=False, mosweight=True tclean will encounter too many open files with ~46 MSs or more, while with perchanweightdensity=True, mosweight=True tclean will encounter too many open files with ~300 MSs or more.

    • There are small, systematic offsets known to occur when using tclean. Our initial tests show that the offset in dec is of the order ~50 milli-arcsec, while the offset in RA is a function of declination, but also amounting to ~50mas. This issue is currectly being investigated.

    • Currently the parameter type of niter is defined as an integer, therefore the integer value larger than 2147483647 will not be set properly as it causes an overflow.

    • Using deconvolver=’mtmfs’, nterms=1 and specmode=cube does not yet work in parallel imaging mode. Use specmode=’mfs’ instead.

    • In tclean, defining image cubes in optical velocity in some cases is known not to work. This problem is under investigation.

    • The awproject gridder in tclean does not support the virtual model scheme.

    • When using interactive tclean when running tclean, please do not use the option “Zoom Out To Entire Screen” in the CASA Viewer. This will close the interactive tclean and will hang your CASA session.

    • Interactive tclean only works when a region or mask is selected in the CASA Viewer. There is a known bug that when a region is first selected, and then de-selected to produce an empty mask (filled with zeros), the CASA Viewer that runs interactive tclean will still allow you to proceed, and tclean will detect an empty mask and stop. Please always mark a region/mask to continue interactive tclean (if the entire image should be cleaned, draw a box around the entire image), and do not forget to double-click inside the green contours to select the region.

    • When using interactive tclean, hand-edited cyclethresholds do not change back to the auto-calculated values in the GUI until two major cycles later. However, the logger contains the most accurate information about what was used, and the expected behaviour (of hand-edited cyclethresholds applying to only the current minor cycles) is seen and has been tested. Therefore, iteration control and imaging will proceed as expected. This known issue affects CASA versions 5.6 and 5.7/6.1

    • For imaging of cubes using the mosaic gridder, a seg-fault can be triggered if the default nchan=-1 is used and channel selection is set with spw. This can result in either a seg-fault, or image planes with NaN values. It is not yet clear what is the root cause of this issue. The problem was found in VLA data, but it is not clear whether it can affect data from other telescopes. We expect this problem to be fixed in CASA 6.3. As a work-around solution, we recommend to set the nchan parameter to anything different than -1.

    • In CASA 6.2, setting pblimit to a negative value will no longer unmask the full image. Instead, only the area within the value for the limit of the primary beam is visible in the viewer (i.e. the default behavior). A partial work-around is to set pblimit to something very small (.e.g, pblimit = 0.00001) to get most of the field of view. If that does not recover outlier sources, then please use the tclean in CASA version 6.1 or earlier in order to uncover outlier sources outside the pblimit.

    • In the makemask task, region files using the minus sign ( - ) to create cutouts are known not to work.

    • The combined imaging of different large MSs where differences in the topo-centric frequencies are large compared to the channel width can fail in tclean as a result of memory issues, in particular when imaging in parallel mode. A potential workaround solution is to use the task concat to first combined the MSs before running tclean, although further testing is needed to ensure that this solution is valid for all cases. This issue will be fixed in a next CASA release.

  • imregrid

    • Position-velocity (PV) images are not supported by imregrid, because their coordinate systems are nonstandard, lacking a direction coordinate and having a linear coordinate.

    • When converting from between coordinate system that require rotation (e.g., from celestial to galactic coordinates), CASA is known to introduce deviations in position from other software packages that can be several tenths of an arcsec. This could be because the rotation of the rectangular grid in a non-cartesian coordinate system is imperfect, possibly due to internal inconsistencies in the conversion matrices. The conversion between one frame and another in general becomes less accurate as distance from the output image’s reference pixel increases. The imregid task and Measures tool suffer from this Known Issue (see imregrid task page).


  • viewer

    • The CASA viewer does not yet support the entire region shapes and parameters at this stage.

    • For equatorial cubes, i.e. data cubes that include dec=0 (exact), the viewer only gives spectra for sources at dec > 0. No spectra are produced for any points with dec <0.

    • Viewer may not properly open saved region files.

    • With the new region panel being used now, It may be advisable to rename the $HOME/.casa/viewer/rc file that stores previous configurations of the viewer.

    • Viewer - labels are not shown - this can be caused by a conflict between an installed version of PGPLOT and the version of PGPLOT that comes with the non-root version of CASA. If you do have PGPLOT installed in a standard location (e.g. /usr/lib), you may try moving it aside and see if it resolves the problems. If you do encounter this problem, please report it to the CASA team.

    • Some X11 settings can make the viewer unstable. We identified that the line Load “glx” in /etc/X11/xorg.conf is such a setting. If you don’t need this line for aother applications, it would be better to have it removed.

    • The viewer can only load MeasurementSets (MS) for which all spectral windows have the same channel width. If this is not the case, an ArrayColumn error will appear. To get around this, use SPLIT`` to place the spectral windows of interest in a separate MS, or try the table browser tool.

    • When exiting CASA after using the viewer, a message similar to the following may appear: proc vtool_1EziEss1P2tH0PxJbGHzzQ is being killed. This is a cosmetic issue and can be ignored.

    • For some OSs and window managers, parts of the display may be eclipsing interactive elements. We recommend to change the window manager styles for these cases.

    • When multiple animators are open, it can happen that it is not possible to make them active, when the ‘Images’ animator is inactive. Active the ‘Images’ animator first to enable the other animators.

    • MeasurementSet with sizes of tens of Gb may not visualize the full data set properly on all machines, which can give the appearance that part of the data is flagged.

    • The line tool in the MAC viewer plots unreadable hex numbers.

  • plotms

    • In plotms, PDF and PS exports have been reported not to work with some older sub-versions of RH7 (e.g., RH 7.6). While the problem has not been completely characterized, upgrading to a newer RH version (e.g., RH 7.9) has shown to solve this issue.

    • In RedHat 7 we found that in some circumstances the vertical tab of the viewer appears on the right hand side instead of the left hand side. This eclipses the scrollbar and makes it difficult to use. To fix, add the following to the top of ~/.config/Trolltech.conf

    • When plotting pointing axes in plotms on RHEL6, the tick-values of minutes and seconds on the axes are not multiples of 5

    • For concatenated data sets, plotms can create an output error if certain data columns were present in some of the concat input MSs, but missing in others (making concat inset zero values). A practical workaround is to either handle the MSs separately, or delete those columns using the tb.removecols tool (but in case of the latter one has to take care that the columns are not crucial).

    • Channel averaging for a caltable in plotms can be very slow when no antenna selection is set, and crashes may be encountered when using channel averaging and avgantenna=True in the same plotms call for a caltable. These issues will be fixed in a next CASA version.


  • uvmodelfit

    • When running uvmodelfit, the output componentlist does not contain the uncertainty in flux that the task calculates (and displays at the end of the fitting process).

  • imstat

    • The use of the “centerbox” parameter when specifying a region in imstat has a known issue that under very specific circumstances, less pixels are taken into account for the statistics than what is expected. This only occurs when all of the following are true: (1) values are specified in pixels; (2) the width of the box is an even number of pixels (e.g, 4pix, 16pix, or 100pix); and (3) the box is located away from the image center in Right Ascension (progressively more pixels are dropped when moving away from the image center, but only in RA). The issue is a combination of machine rounding errors (when the boundary of the centerbox is exactly at the center of a pixel), and the fact that centerbox has to converts pixel coordinates to sky coordinates to allow all possible combinations of regions. Note that the “box” parameter is not affected by this, because it can be more strict in only using flat pixel coordinates. As a simple work-around solution, we recommend to always give the width of the centerbox in “odd” number of pixels. Please note that because centerbox places the center of a box in the middle of a pixel and CASA only includes full pixels, the width of a centerbox always has an odd number of pixels anyway. For example, centerbox=[[1000pix,4000pix],[4pix,4pix]] for an 8000x8000 pixel image should give npts=25, but due to the above issue will result in npts <25. Instead, centerbox=[[1000pix,4000pix],[5pix,5pix]] will always give npts=25.

  • tb.statistics

    • The table tool’s statistics function tb.statistics currently ignores the useflags parameter, so statistics are calculated for all values in the specified column, and flagged values cannot be avoided.


  • simobserve / simanalyze

    • CASA simulations do not yet fully support all spectral types of components (i.e., ability to include spectral lines or spectral indices)

    • When cleaning with a simulated MS, it should be considered best practice to declare the phasecenter parameter using the ‘J2000’ notation to account for possible rounding errors that can create an offset in the image.

    • corruption of simulated MS by an atmospheric phase screen is only available from the toolkit. simobserve and sm: Under some circumstances, running sm.setnoise and sm.corrupt, or simobserve with thermal noise, twice using the same project name, the noise can be applied a second time, doubling the noise level. Be sure to use different project names for creating different simulations with noise. See for the latest simulation information

Single Dish

  • general

    • Difficulty in allocating memory to import/processing of Band 9 (fast-mapped, double-circle) data. Use high-performance machines as workaround.

    • Please avoid using spectral window and channel selection by frequency range. It may occasionally fail. So far, this has only been reported on Mac OS but it may happen on Linux, too.

  • sdimaging

    • Frequencies and frequency increments in the weight image generated by sdimaging could be slightly different from those in the science image. The difference in frequency resolution (CDELT3 in .fits) could be 0.1% at maximum, which is negligible in most cases. If the difference matters, please use tsdimaging instead although it takes more time to process than sdimaging.

    • sdimaging task may fail when more than several MSes are chosen as inputs (infiles) to create single output image. It is because the file descriptor opened by the task exceeds the limit defined by OSes. You can relax the limit of the number of open file descriptors by the command, e.g., ulimit -n 4096 . Note the typical number of file descriptors opened by the task is 35/MS.

  • sdintimaging

    • For task sdintimaging, gridder=’awproject’ is not yet available. It will be enabled in a subsequent release.

  • plotprofilemap

    • The task intermittently seg faults on Mac OS.

The following issues in the previous releases are fixed

  • The sdbaseline task fails with blmode = ‘apply’ and selected data contains SPWs with different number of channels.

  • Spectra with only one polarization cannot be calibrated in sdcal and sdgaincal.

  • The importnro task fails when data contains a disabled array.


  • doc(): The various versions of the doc() command, to open task and tool documentation, do not work in CASA 5.8. Please use CASA 6.2, or look up the CASA Docs documentation manually at