This page provides details about simager, the spectral-line imaging program. The purpose of this program is to provide a distributed way of creating a large image cube, making use of a parallel processing system.

This program is a prototype, designed to show a scalable solution to distributed spectral-line processing. It is meant to be an interim solution, available until a spectral-line imager within an improved software framework is made available.

Running the program

It can be run with the following command, where “” is a file containing the configuration parameters described in the next section.

$ <MPI wrapper> simager -c

Parallel/Distributed Execution

The program is distributed and uses a master/worker pattern to distribute and manage work. The master process (note that only one of the processes is designated the master) is responsible for creating and writing the output image cubes. It does no imaging itself.

The worker processes get assigned individual channels by the master, and these channels are imaged independently. Once all the imaging is completed on a channel, the worker sends the various arrays back to the master for writing to the cube.

Any number of processes can be allocated to an simager job. The available channels are allocated to workers as the workers become available, until all channels have been imaged.

On the Cray XC30 (galaxy) platform executing with the MPI wrapper takes the form:

$ aprun -n 4000 -N 20 simager -c

The -n and -N parameters to the aprun application launcher specify 4000 MPI processes will be used (3999 workers and one master) and each node will host 20 MPI processes. This job then requires 200 compute nodes.

Configuration parameters

The parameter set interface for simager differs a little from the cimager interface. The interface to defining the image cube properties is a bit more streamlined, without the requirement to include the image name in the parameter name, so that many of them start simply with Simager.Images.. The simager-specific parameters are summarised in the following table.

Unlike cimager, simager does not have the advise capability enabled, which would allow the user to leave out certain parameters (such as direction, cellsize etc). The cost of filling these in by reading the measurement set is likely to be much larger for simager, so all parameters must be provided.

The different modules, for gridding, cleaning, restoring, have the same interface, and are referenced in the same manner as for cimager - that is, via Simager.gridder.XXX, Simager.solver.XXX, Simager.restore.XXX and so forth.

Note, however, that on-the-fly application of calibration parameters is not (yet) enabled (that is, there is no equivalent of the Cimager.calibrate=true option). Users are recommended to use ccalapply (Calibration Applicator) to apply the calibration solution directly to the measurement set prior to imaging.

There are a few spectral-line-specific parameters that have been introduced to simager.

  • The user can define the rest frequency to be stored in the image cube metadata, either by giving a frequency or using the shortcut ‘HI’ (= 1420.405751786 MHz).
  • When restoring, each channel is treated separately, so if Simager.restore.beam=fit, then each channel will have a different restoring beam. Since only a single beam can be recorded in the image, the user can select which channel is used as the reference, by using the Simager.restore.beamReference parameter. This can be a channel number (0-based), or one of ‘first’, ‘last’ or ‘mid’.
  • To record the individual channel beams, simager will produce an ASCII text file listing the beam parameters for each channel. This is known as the “beam log”. If the image cube name is “image.i.blah”, then the beam log will be called “beamlog.image.i.blah.txt”. The file has columns: index | major axis [arcsec] | minor axis [arcsec] | position angle [deg] Should the imaging of a channel fail for some reason, the beam for that channel will be recorded as having zero for all three parameters. This beam log is compatible with other askapsoft tasks, specfically the spectral extraction in Selavy (see Extraction of Spectra, Images and Cubelets).

Here is an example of the start of a beam log:

#Channel BMAJ[arcsec] BMIN[arcsec] BPA[deg]
0 64.4269 59.2985 -70.8055
1 64.4313 59.299 -70.8831
2 64.4333 59.3018 -70.9345
3 64.4338 59.2996 -70.9256
4 64.4349 59.2982 -70.9108
Parameter Type Default Description
dataset string or vector<string> None Measurement set file name(s) to read. If the parameter is given as a vector of strings all measurement sets given by this vector are treated as being concatenated together (each worker will only get a single frequency channel at a time anyway). string None The base name of the image cubes to be produced. Only a single name is accepted, and it must (as for cimager) start with ‘image’.
Images.direction vector<string> None Direction to the centre of the required image (or tangent point for facets). This vector should contain a 3-element direction quantity containing right ascension, declination and epoch, e.g. [12h30m00.00, -, J2000]. Note that a casa style of declination delimiters (dots rather than colons) is essential. Only J2000 directions are currently supported. Note also that this must be provided, as the advise capability is not enabled.
Images.shape vector<int> None Shape in pixels of the image cube’s spatial axes.
Images.cellsize vector<string> None A two-element vector of quantity strings, indicating the size of pixels in the spatial axes, e.g. [6.0arcsec, 6.0arcsec]
Images.polarisation vector<string> [“I”] Polarisation planes to be produced for the image (should have at least one). Polarisation conversion is done on-the-fly, so the output polarisation frame may differ from that of the dataset. An exception is thrown if there is insufficient information to obtain the requested polarisation (e.g. there are no cross-pols and full stokes cube is requested). Note, ASKAPsoft uses the correct definition of stokes parameters, i.e. I=XX+YY, which is different from casa and miriad (which imply I=(XX+YY)/2).The code parsing the value of this parameter is quite flexible and allows many ways to define stokes axis, e.g. [“XX YY”] or [“XX”,”YY”] or “XX,YY” are all acceptable
Images.restFrequency string None A string indicating the rest frequency to be written to the image cube header (for the restored, model and residual cubes only). The string can be a quantity string (e.g. 1234.567MHz) or the special string ‘HI’, which resovles to 1420.405751786 MHz. If not given, no rest frequency is written to the cubes.
restore bool false If true, the image will be restored (by convolving with the given 2D gaussian), in the same manner as for cimager. The restoration is done separately for each channel.
restore.beam vector<string> None Either a single word ‘fit’ or a quantity string describing the shape of the clean beam (to convolve the model image with). If quantity is given it must have exactly 3 elements, e.g. [30arcsec, 10arcsec, 40deg]. Otherwise an exception is thrown. This parameter is only used if restore is set to True. If restore.beam=fit, the code will fit a 2D gaussian to the PSF image and use the results of this fit. In this case, each channel with have an independently-fitted beam.
restore.beamReference string mid The channel to use as the reference for the beam - this channel’s beam is written to the cube header. Values can be an integer indicating the channel number (0-based), or one of ‘mid’, ‘first’, or ‘last’.
restore.beam.cutoff double 0.05 Cutoff for the support search prior to beam fitting, as a fraction of the PSF peak. This parameter is only used if restore.beam=fit. The code does fitting on a limited support (to speed things up and to avoid sidelobes influencing the fit). The extent of this support is controlled by this parameter representing the level of the PSF which should be included into support. This value should be above the first sidelobe level for meaningful results.

Example parset

Simager.dataset                                =
#                            = image.i.cube
Simager.Images.shape                           = [2048,2048]
Simager.Images.cellsize                        = [10arcsec,10arcsec]
Simager.Images.direction                       = [17h44m25.4506, -, J2000]
Simager.Images.restFrequency                   = HI
Simager.gridder.snapshotimaging                = true
Simager.gridder.snapshotimaging.wtolerance     = 2600
Simager.gridder                                = WProject
Simager.gridder.WProject.wmax                  = 2600
Simager.gridder.WProject.nwplanes              = 99
Simager.gridder.WProject.oversample            = 4
Simager.gridder.WProject.diameter              = 12m
Simager.gridder.WProject.blockage              = 2m
Simager.gridder.WProject.maxfeeds              = 36
Simager.gridder.WProject.maxsupport            = 512
Simager.gridder.WProject.variablesupport       = true
Simager.gridder.WProject.offsetsupport         = true
Simager.gridder.WProject.frequencydependent    = true
Simager.solver                                 = Clean
Simager.solver.Clean.algorithm                 = Basisfunction
Simager.solver.Clean.niter                     = 500
Simager.solver.Clean.gain                      = 0.3
Simager.solver.Clean.scales                    = [0,3,10]
Simager.solver.Clean.verbose                   = False
Simager.solver.Clean.tolerance                 = 0.01
Simager.solver.Clean.weightcutoff              = zero
Simager.solver.Clean.weightcutoff.clean        = false
Simager.solver.Clean.psfwidth                  = 512
Simager.solver.Clean.logevery                  = 50
Simager.threshold.minorcycle                   = [30%, 15mJy]
Simager.threshold.majorcycle                   = 20mJy
Simager.ncycles                                = 3
Simager.Images.writeAtMajorCycle               = false
Simager.restore                                = true
Simager.restore.beam                           = fit
Simager.restore.beamReference                  = first
Simager.preconditioner.Names                   = [Wiener, GaussianTaper]
Simager.preconditioner.GaussianTaper           = [50arcsec, 50arcsec, 0deg]
Simager.preconditioner.Wiener.robustness       = 0.25

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