imager

This page provides some details about the imager program. The purpose of this software is to perform imaging in a parallel/distributed environment or on a single computer system. The software leverages MPI, however can be run on a simple laptop or a large supercomputer. It provides all the functionality of the cimager program but permits a more flexible distribution of data and has a smaller aggregated memory footprint.

By default the imager processes each channel independently (grid/FFT) and in continuum mode all channels are combined in image space. Although beneficial for spectral line data-sets, especially those with single channel tiling. It is not optimal for continuum imaging with many-channel tiling as it performs much more IO and nchannel x more FFTs than are required. Therefore if imager is processing in continuum mode it is suggested that the following option be used:

Cimager.combinechannels = true

This switches on a mode where the multiple channels of a machine allocation are accessed and gridded together before the FFT. This improves IO performance and reduces the FFT count roughly by the number of channels in each allocation (see nchanpercore). Note this is <OFF> by default.

Running the program

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

$ <MPI wrapper> imager -c config.in

The config file is the same format as that required by the cimager this has been done to accentuate the similarity between the two imagers. Both the Cimager or imager prefix are accepted, but you can’t mix the two in one config file.

Parallel/Distributed Execution

The program is distributed and used a master/worker pattern to distribute and manage work. The program requires at least two processes to execute, and failure to either execute imager as an MPI process or specifying only one MPI process will result in an error.

On the Cray EX platform executing with MPI takes the form:

$ srun -n 289 -N 4 imager -c config.in

The -n and -N parameters to the srun application launcher specify 289 MPI processes will be used (288 workers and one master) distributed over 4 compute nodes, each node will host 72 or 73 MPI processes.

As with cimager the imager has no implicit parallelism, rather the configuration parameters allow the user to specify what subset of the total task each worker will carry out. This is controlled by the nchanpercore keyword.

Allocation of work and temporary files

It should be noted that imager can be run in exactly the manner as cimager if required, but its primary benefit is that you do not have to. The caveat is that imager always needs at least two processes to run.

Example 1: Multiple Channels Per Core

Typically, if we are processing a single measurement set with 288 spectral channels, using 289 workers the following configuration parameter can be used to restrict each worker to a single spectral channel:

Cimager.nchanpercore    = 1

You will need 289 cores to run. However if you decide that a core will process 4 channels. Then you would use:

Cimager.nchanpercore    = 4

Now you would only need to invoke:

$ srun -n 73 -N 1 imager -c config.in

This would only require 1 node.

The Channels and Frequencies keywords:

The Channels keyword behaves in a similar way to cimager:

Cimager.Channels = [<num>,<start>]

You can specify the number of channels and the start channel. This will be taken from the complete list of channels as determined by the input measurement sets.

The Frequencies keyword that allows you to specify the dimensions of the spectral cube in frequency space:

Cimager.Frequencies = [<num>,<start>,<width>] - all in Hz

(This keyword has not been tested extensively with the freqframe options)

Update Specifying the Frequency frame

Cimager.freqframe = topo | bary | lsrk

You can choose a default frequency frame for the spectral cube as either, topocentric (the default), barycentric or the kinematic local standard of rest. Other frames can be added on request.

Example 2: Beam Selection

This imager has built in splitting of the input measurement set. At the moment only a single beam is split out but it can be any one and is determined by the first entry int the following vector:

Cimager.beams       =[0]

Note: The Local Merge

The minor cycle of clean is currently still performed on a single master core. But the individual channel allocations are merged locally to the worker core.

Spectral-line processing

Imager can also be used for spectral line processing. Each channel is processed independently and written to one or more shared output cubes. To enable this mode use:

Cimager.solverpercore = true

Example 3: Multiple writers

When writing CASA images this imager can write to mode than one output image cube to improve disk throughput. This has been implemented to remove a serious bottle neck in the spectral line processing. For FITS imagetypes a single cube can be written - but multiple writers are used - once again to improve write performance:

Cimager.nwriters = 16
Cimager.singleoutputfile = true

Example 4: Joint cleaning mode / per beam processing

In spectral line mode only, imager can be instructed to process each measurement set independently and to merge the subimage into a larger image for the minor cycles. Note this is different to faceting - which processes the sub images entirely independently. This scheme grids and images the fields individually but cleans them jointly as in Cornwell 1989. You can specify the shape of the mosaicked image and the subshape of the individual images:

Cimager.updatedirection = true
Cimager.solverpercore = true
Cimager.Images.shape = [4096,4096]
Cimager.Images.subshape = [2048,2048]

Each subimage will be centred on its tangent point as determined by the MeasurementSet and selection, but the centre of the overall mosaic image can be set using the image specific direction parameter:

Cimager.Images.*Imagename*.direction = [12h30m00.00, -45.00.00.00, J2000]

Nyquist gridding

To decrease memory usage and runtime, visibilities can be gridded onto the smallest uv grid that can fit them all. This grid size is used for all uv grids, for the merge and main image FFT operations, and for preconditioning. FFT padding is then used before cleaning and image output to set the desired cellsize and shape. This feature is enabled using either of the two relevant parameters defined below: griddingcellsize or nyquistgridding.

Example 5: Memory reduction

For SB11816 the largest uvw coordinates are 28822.3, 29641.3 and 24250.9 wavelengths. So an ideal cellsize that oversamples the synthesised beam by a factor of six is around 1.16 arcseconds. But that resolution is not needed until cleaning. Before then grids can retain all of the information with 2x oversampling. The nyquistgridding parameter determines a griddingcellsize of 3.25arcsec:

Cimager.Images.shape           = [6144, 6144]
Cimager.Images.cellsize        = [1.16arcsec, 1.16arcsec]
Cimager.Images.nyquistgridding = true

Or alternatively the coarse resolution can be entered directly:

Cimager.Images.griddingcellsize = [3.25arcsec, 3.25arcsec]

Using the BasisfunctionMFS solver with four scales and two Taylor terms, the memory usage reported when running on nine worker nodes is:

PeakVM (MB):

Rank

0

1

2

3

4

5

6

7

8

9

Normal

14432

6397

5326

5324

5724

4727

4715

4681

4392

4392

Nyquist

9166

3132

2826

2815

2907

2739

2724

2724

2632

2632

PeakRSS (MB):

Rank

0

1

2

3

4

5

6

7

8

9

Normal

11976

3044

3048

3041

2447

2455

2446

2132

2132

2131

Nyquist

6997

922

922

910

850

849

849

795

758

758

Configuration Parameters

Parset parameters understood by imager are given in the following table (all parameters must have either Cimager or imager prefix, i.e. Cimager.dataset). For a number of parameters certain keywords are substituted, i.e. %w is replaced by the worker number (rank-1, if there is only one pool of workers) and %n by the number of nodes in the parallel case. In the serial case, these special strings are substituted by 0 and 1, respectively. This substitution allows to reuse the same parameter file on all nodes of the cluster if the difference between jobs assigned to individual nodes can be coded by using these keywords (e.g. using specially crafted file names). Note, if there is more than 1 group of workers (e.g. parallel calculation of Taylor terms), %w index spans the workers in one group rather than the global pool of workers. This is done to allow the same file name to be used for corresponding worker in different groups (i.e. all Taylor terms are build from the same file). If a parameter supports substitution, it is clearly stated in the description.

A number of other parameters allowing to narrow down the data selection are understood. They are given in a separate table (see Data Selection) and should also have the Cimager prefix. Note that option Cimager.CorrelationType will be ignored and default to “cross”.

To record the individual channel beams when run in spectral-line mode, the imager can 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

Similar logs can be produced for per channel weights and image statistics. Alternatively, these data can be written to a subtable (casa format) or binary table (FITS format) in the image file.

Parameter

Type

Default

Description

imagetype

string

“casa”

Type of the image handler (determines the format of the images, both which are written to or read from the disk). The default is to create casa images but “fits” can also be chosen.

imagealloc

string

“fast”

Set to “fast” to use the fast file allocation scheme This can save a lot of time when creating large cubes. This is the default, use “” to turn off.

dataset

string or vector<string>

None

Measurement set file name to read from. Usual substitution rules apply if the parameter is a single string. If the parameter is given as a vector then the sets can be different frequencies of the same observation or different epochs

nworkergroups

int

1

Number of worker groups. This option can only be used in the parallel mode. If it is greater than 1, the model parameters are distributed (as evenly as possible) between the given number of groups of workers (e.g. if one calculates a Taylor term expansion of the order of 1 for one image, setting this parameter to 3 will allow parallel computation of the Taylor terms for this image). This is on top of the normal parallelism within the group (the %w index spans from 0 to the number of workers per group - 1). Essentially, this option allows to throw several workers on the same problem if the model allows partitioning.Taylor terms, faceting and multiple images in the model are the typical use cases.

nchanpercore

int

1

Number of channels allocated to each worker core

combinechannels

bool

false

When true combine all nchanpercore channels into a single grid before the FFT. Generally optimal for continuum (Taylor term) processing.

Channels

vector<int>

[]

Channels to be selected from the measurement set. Syntax is [<number>,<start>]. Defaults to all the channels

Frequencies

vector<double>

[]

Dimensions of the output cube in freuency space Syntax is [<number>,<start>,<width] all in Hz. Defaults to the same as the input MS

beams

vector<int>

[0]

Beam number to be selected from the measurement set

nwriters

int

1

The number of output cubes to generate in spectral cube mode.

freqframe

string

topo

Generate output cubes in the given frame options are topocentric (topo), barycentric (bary) and kinematic local standard of rest (lsrk)

dopplertracking

bool

false

For use with bary and lsrk frame: track the change in frequency during the observation, selecting the nearest channel at each time. Note that by default this uses the field direction (antenna pointing) as direction reference, not the tangent point or beam pointing

dopplertracking.direction

vector<string>

None

Specify the reference direction to use for the velocity calculations, e.g., [12:34:56.7,-23.45.56.7,J2000]

singleoutputfile

bool

false

Single output cube. Useful in the case of multiple writers

solverpercore

bool

false

Turn on distributed solver (spectral imaging) mode

updatedirection

bool

false

Turn on joint cleaning mode (see above)

datacolumn

string

“DATA”

The name of the data column in the measurement set which will be the source of visibilities.This can be useful to process real telescope data which were passed through casapy at some stage (e.g. to image calibrated data which are stored in the CORRECTED_DATA column). In the measurement set convention, the DATA column which is used by default contains raw uncalibrated data as received directly from the telescope. Calibration tasks in casapy make a copy when calibration is applied creating a new data column.

sphfuncforpsf

bool

false

If true, the default spheroidal function gridder is used to compute PSF regardless of the gridder selected for model degridding and residual gridding. This has a potential to produce better behaving PSF by taking out two major factors of position dependence. Note, this doesn’t make the PSF correct or otherwise,it is just a different approximation

calibrate

bool

false

If true, calibration of visibilities will be performed before imaging. See Access to calibrator solutions for details on calibration parameters used during this application process.

calibrate.scalenoise

bool

false

If true, the noise estimate will be scaled in accordance with the applied calibrator factor to achieve proper weighting.

calibrate.allowflag

bool

false

If true, corresponding visibilities are flagged if the inversion of Mueller matrix fails. Otherwise, an exception is thrown should the matrix inversion fail

calibrate.ignorebeam

bool

false

If true, the calibration solution corresponding to beam 0 will be applied to all beams

calibrate.interpolatetime

bool

false

If true, do linear interpolation of gain solutions

gainsfile

string

“”

This is an obsolete parameter, which is still supported for backwards compatibility defining the file with antenna gains (a parset format, keywords look like gain.g11.0, where g11 or g22 in the middle correspond to different polarisations and the trailing number is the zero-based antenna number. The default value (empty string) means no gain correction is performed. The gain file format is the same as produced by Ccalibrator.

restore

bool

false

If true, the image will be restored (by convolving with the given 2D gaussian). This is an additional step to normal imaging, which, by default, ends with just a model image. The restored image is written into a separate image file (with the .restore suffix). The convolution is done with the restore solver (see also Solvers) which reuses the same parameters used to setup the image solver (and therefore ensuring the same preconditioning is done). The only additional parameter of the restore solver is the shape of the gaussian representing clean beam (or flag to determine the shape). It is given by the restore.beam parameter, which must be present if restore is set to True

residuals

bool

false

If true write out the residual image. (deprecated)

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 (first encountered if multiple images are solved for) and use the results of this fit.

restore.beam.cutoff

double

0.5

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.

restore.beam.maxsupport

int

101

Maximum support size for beam fitting - this avoids spending excessive CPU time trying to fit very poor beams

restore.preconditioner.xxx

vector<string>

None

Specify an additional preconditioner to be used when restoring. This will result in a second set of restored files with the .alt.restored suffix. See preconditioner below. When using multiple restore solvers, one should use the restore.beam=fit option to allow for different beam shapes.

restore.preconditioner .suffix

string

“alt”

Change the suffix for the alternative preconditioner products

restore.equalise

bool

false

If true, the final residual is multiplied by the square root of the truncated normalised weight (i.e. additional weight described by Sault et al. (1996), which gives a flat noise). Note, that the source flux densities are likely to have position-dependent errors if this option is used because not all flux is recovered during the clean process. However, the images look aesthetically pleasing with this option.

restore.updateresiduals

bool

true

The residual image written out by the restore solver can be updated using the latest model. This is now the default behviour. Note the majorcycle outputs do not pass through the restore solver so are not updated so therefore correspond to the residuals at the beginning of the last minor cycle.

Images.xxx

various

A number of parameters given in this form define the images one wants to produce (shapes, positions, etc). The details are given in a separate section (see below)

nUVWMachines

int32

number of beams

Size of uvw-machines cache. uvw-machines are used to convert uvw from a given phase centre to a common tangent point. To reduce the cost to set the machine up (calculation of the transformation matrix), a number of these machines is cached. The key to the cache is a pair of two directions: the current phase centre and the tangent centre. If the required pair is within the tolerances of that used to setup one of the machines in the cache, this machine is reused. If none of the cache items matches the least accessed one is replaced by the new machine which is set up with the new pair of directions. The code would work faster if this parameter is set to the number of phase centres encountered during imaging. In non-faceting case, the optimal setting would be the number of synthetic beams times the number of fields. For faceting (btw, the performance gain is quite significant in this case), it should be further multiplied by the number of facets. Direction tolerances are given as a separate parameter.

uvwMachineDirTolerance

quantity string

“1e-6rad”

Direction tolerance for the management of the uvw-machine cache (see nUVWMachines for details). The value should be an angular quantity. The default value corresponds roughly to 0.2 arcsec and seems sufficient for all practical applications within the scope of ASKAPsoft.

gridder

string

None

Name of the gridder, further parameters are given by gridder.something. See Gridders for details.

includeflagged

bool

true

By default the flags are ignored when scanning through the data for ranges of frequencies, uvw values and other parameters because this is faster. Set to false to exclude flagged data and make the advice more accurate. Flagged data are always excluded during imaging.

rankstoringcf

int

1

In the parallel mode, only this rank will attempt to export convolution functions if this operation is requested (see tablename option in the Gridders) This option is ignored in the serial mode.

visweights

string

“MFS” if any nterms>1, “” otherwise

If this parameter is set to “MFS” gridders are setup to grid/degrid with the weight required for multi-frequency synthesis. At the moment, this parameter is decoupled from the image setup, which has to be done separately in a consistent way to use MSMFS (nterms should be set to something greater than 1).

visweights.MFS.reffreq

double

ave freq (see frequency above)

Reference frequency in Hz for MFS processing (see above)

solver

string

None

Name of the solver, further parameters are given by solver.something. See Solvers for details

thershold.xxx

various

Thresholds for the minor and major cycle (cycle termination criterion), see Solvers for details.

preconditioner.xxx

various

Preconditioners applied to the normal equations before the solver is called, see Solvers for details.

ncycles

int32

0

Number of major cycles (and iterations over the dataset)

sensitivityimage

bool

false

If true, an image with theoretical sensitivity will be created in addition to weights image (deprecated)

sensitivityimage.cutoff

float

0.01

Desired cutoff in the sensitivity image

freqframe

string

topo

Frequency frame to work in (the frame is converted when the dataset is read). Either lsrk or topo is supported.

channeltolerance

double

0

Whether to use the floating-point tolerance in comparing frequencies from different datasets, allowing for small differences in the frequency settings. Default is to require the frequencies to match exactly.

calcstats

bool

false

If true, calculate the image statistics and write it to the image table.

clearcache

bool

false

If true, clear the table cache for the DATA and FLAG after processing each channel. This reduces memory use for the spectral line case

Parameters of images

This section describes parameters used to define images, i.e. what area of the sky one wants to image and how. All parameters given in the following table have Cimager.Images.* prefix, e.g. Cimager.Images.reuse = false. Parameters are global, affecting all images in the Cimager.Images.Names vector, unless the prefix includes one of those names, i.e. Cimager.Images.ImageName.*. In continuum mode you can specify more than one set of image parameters (like in Cimager) and all the images will be cleaned together with separate minor cycles but a combined major cycle. This can be used, e.g., to model and subtract sources outside the main image. The offset images can be made smaller than the main image to reduce memory usage. If Nyquist gridding is used all image shapes need to square and be integer multiples of the smallest image - they will be adjusted if this is not the case. In spectral mode (solverpercore=true) only a single image parameter is accepted and you should specify the default image parameters instead of the image specific ones - the exception is the joint cleaning mode (per beam processing) with updatedirection=true where the image specific parameter is used to specify the centre of the overall image.

Parameter

Type

Default

Description

reuse

bool

false

If true, the model images will be read from the disk (from the image files they are normally written to according to the parset) before the first major cycle. If false (the default), a new empty model image will be initialised for every image solved for. Setting this parameter to true allows to continue cleaning the same image if more major cycles are required after inspection of the image. Note, there is little cross check that the image given as an input is actually a result of the previous run of cimager with the same Image parameters. So the user is responsible to ensure that the projection, shape, etc matches.

shape

vector<int>

1.7 * pb FWHM (~1st null) + 2 * max(pb offset)

pb FWHM = 1.2*lambda/12

Optional parameter to define the default shape for all images. If an individual shape parameter is specified separately for one of the images, this default value of the shape is overridden. Individual shape parameters (see below) must be given for all images if this parameter is not defined. Must be a two-element vector.

cellsize

vector<string>

1/max(u,v) / 6 rad

Optional parameter to define the default pixel (or cell) size for all images. If an individual cellsize parameter is specified separately for one of the images, this default value is overridden. Individual cellsize parameters (see below) must be given for all images, if this parameter is omitted. If defined, a 2-element quantity string vector is expected, e.g. [6.0arcsec, 6.0arcsec].

griddingcellsize

vector<string>

None

Optional parameter to define the pixel size used during gridding, merging and preconditioning. Before cleaning or output, images are oversampled to the desired cellsize. If defined, a 2-element quantity string vector is expected, e.g. [6.0arcsec, 6.0arcsec].

nyquistgridding

bool

false

Optional parameter to determine the griddingcellsize parameter automatically using uvw metadata. If griddingcellsize is also defined, that value will be used. Setting nyquistgridding=false will not disable griddingcellsize if it is defined.

writeAtMajorCycle

bool

false

If true, the current images are written to disk after each major cycle (.cycle suffix is added to the name to reflect which major cycle the image corresponds to). By default, the images are only written after ncycles major cycles are completed.

Names

vector<string>

None

List of image names which this imager will produce. If more than one image is given, a superposition is assumed (i.e. visibilities are fitted with a combined effect of two images; two measurement equations are simply added). Parameters of each image defined in this list must be given in the same parset using ImageName.something keywords (with usual prefix). Note, all image names must start with word image (this is how parameters representing images are distinguished from other type of free parameters in ASKAPsoft), otherwise an exception is thrown. Example of valid names are: image.10uJy, image, imagecena In spectral mode only the first name is used.

ImageName.nchan

int32

1

Number of spectral planes in the image cube to produce. Set it to 1 if just a 2D image is required

ImageName.frequency

vector<double>

[min freq,max freq] if nchan>1, [ave freq,ave freq] if nchan=1

ave freq = (min+max)/2

Note: these are the min and max frequencies being processed, which may be a subset of the full frequency range.

Frequencies in Hz of the first and the last spectral channels to produce in the cube. The range is binned into nchan channels and the data are gridded (with MFS) into a nearest image channel (therefore, the number of image channels given by the nchan keyword may be less than the number of spectral channels in the data. If nchan is 1 all data are MFS’ed into a single image (however the image will have a degenerate spectral axis with the frequency defined by the average of the first and the last element of this vector; it is practical to make both elements identical, when nchan is 1). The vector should contain 2 elements at all times, otherwise an exception is thrown

ImageName.direction

vector<string>

phase centre of the visibilities

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, -45.00.00.00, J2000]. Note that a casa style of declination delimiters (dots rather than colons) is essential. Only J2000 directions are currently supported.

ImageName.tangent

vector<string>

“”

Direction to the user-defined tangent point, if different from the centre of the image. This vector should contain a 3-element direction quantity containing right ascension, declination and epoch, e.g. [12h30m00.00, -45.00.00.00, J2000] or be empty (in this case the tangent point will be in the image centre). Note that a casa style of declination delimiters (dots rather than colons) is essential. Only J2000 directions are currently supported. This option doesn’t work with faceting.

ImageName.ewprojection

bool

false

If true, the image will be set up with the NCP or SCP projection appropriate for East-West arrays (w-term is equivalent to this coordinate transfer for East-West arrays)

ImageName.shape

vector<int>

None

Optional parameter if the default shape (without image name prefix) is defined. This value will override the default shape for this particular image. Must be a 2-element vector.

ImageName.cellsize

vector<string>

None

Optional parameter if the default cell size (without image name prefix) is defined. This value will override the default cell size for this particular image. A two-element vector of quantity strings is expected, e.g. [6.0arcsec, 6.0arcsec]

ImageName.nfacets

int32

1

Number of facets for the given image. If greater than one, the image centre is treated as a tangent point and nfacets facets are created for this given image (parameters/output model images will have names like ImageName.facet.x.y, where x and y are 0-based facet indices varying from 0 to nfacet-1). The facets are merged together into a single image in the restore solver (i.e. it would happen only if restore is true).

ImageName.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

ImageName.nterms

int32

1

Number of Taylor terms for the given image. If greater than one, a given number of Taylor terms is generated for the given image which are named ImageName.taylor.x, where x is the 0-based Taylor order (note, it can be combined with faceting causing the names to be more complex). This name substitution happens behind the scene (as for faceting) and a number of images (representing Taylor terms) is created instead of a single one. This option should be used in conjunction with visweights (see above) to utilize multi-scale multi-frequency algorithm. With visweights=”MFS” the code recognizes different Taylor terms (using _taylor.x_ name suffix) and applies the appropriate order-dependent weight.

ImageName.facetstep

int32

min(shape(0),shape(1))

Offset in tangent plane pixels between facet centres (assumed the same for both dimensions). The default value is the image size, which means no overlap between facets (no overlap on the shortest axis for rectangular images). Overlap may be required to achieve a reasonable dynamic range with faceting (aliasing from the sources located beyond the facet edge). The alternative way to address the same problem is the padding option of the gridder (see Gridders for details).

Output parameters

This section describes parameters used to select which of the many possible outputs to write to disk. Note that the restored image is always written if restore=true is specified. All parameters given in the following table have Cimager.write* prefix, e.g., Cimager.write.residualimage = false Please leave parameter Cimager.solver.Clean.saveintermediate = true (the default) if specifying write.psfimage=true or write.residualimage=true otherwise an exception is thrown.

In addition you can specify a number of keywords to add to the header of the output images using parameters with Cimager.header. prefix, e.g., Cimager.header.OBSERVER = [“Joe Bloggs”,”name of observer”] Cimager.header.SBID = [“32385”,”Scheduling Block Identifier”,”INT”] Note that you can specify an optional comment in the second entry of the string vector and the type of the keyword (“INT”,”DOUBLE” or “STRING”) as optional 3rd entry. The default type is string.

Parameter

Type

Default

Description

write.residualimage

bool

false

Write the residual image(s) if true (replaces residuals parameter)

write.psfrawimage

bool

false

Write the ‘raw’ (unnormalised, non-preconditoned) psf if true

write.psfimage

bool

true

Write the preconditioned psf if true

write.weightsimage

bool

false

Write the weights image(s) if true If both weightsimage and weightslog are false write a header keyword or table

write.modelimage

bool

false

Write the model image(s) if true. Default is true if restore is false or writeAtMajorCycle is true

write.maskimage

bool

false

Write the mask image(s) if true (cont. only)

write.sensitivityimage

bool

false
Write the sensitivity image(s) if true.

(continuum only)

write.firstrestore

bool

false

Write the image(s) produced in the first restore iteration if true (continuum only)

write.grids

bool

false

Write the complex uv grids (gridded vis, psf, and pcf) if true (spectral imaging only)

write.grids.uvcoord

bool

false

Label FITS output grid with UV coordinates instead of (incorrect) RA/Dec coordinates. UV Grids in casa format are always labeled UV. FITS readers (wcslib) don’t support UV well.

write.grids.fft

bool

false

Write the FFT of the grids, real part only. These are the dirty image, dirty psf and pcf Coordinates will be RA/Dec.

write.weightslog

bool

false

Write the weights to a log file. Alternative to weights images if they are constant.

write.beamlog

bool

true

Write the channel dependent restoring beam sizes to a log file. If false write them to the image as an attached table.

write.multiple

bool

false

Write out all the images selected above for multiple image parameters. By default, just write the outputs for the first image.

write.multiplemodels

bool

true

Write out all the model images, handy if you do not want all the outputs, but need the models for use in e.g., ccontsubtract

header.<keyword>

vector<string>

[“”,””,”STRING”]

Header keyword to write to the image header. Replace <keyword> with up to 8 character name of the keyword. Second string is the keyword description or comment (default no comment). Third string is the optional type (INT,DOUBLE, or STRING) with STRING the default.

Traditional weighting (experimental)

The imager supports traditional weighting approach (i.e. weighting before gridding) in addition to preconditioning mentioned above. Both weighting schemes can be used together if desired, but the general intention is to use one or the other in the long term. The parameters configuring traditional weighting all have Cimager.uvweight prefix, e.g., Cimager.uvweight.robustness = 0.1. The feature itself is enabled if the list of effects passed via Cimager.uvweight parset parameter is not empty (by default it is, so no traditional weighting is done). In addition, the code takes some general gridder parameters such as MaxPointingSeparation, padding and oversample (see Gridders) into account to ensure that the grid used to accumulate sample density, handling of oversampling and the sample rejection criterion are matching those used for imaging. Note, the weight grid itself is not oversampled but the knowledge of the oversampling factor used by the actual imaging gridder is required to fill the weight grid identically to the image grid.

Parameter

Type

Default

Description

uvweight

vector<string>

[]

Main parameter enabling computation and application of traditional weights. If the list is not empty, sample density is computed (in a distributed fashion if applicable) and passed to the effects (procedures) in the order they are listed in this parameter. The result is used as the weight grid for all subsequent imaging. Same weights apply to all polarisation products and Taylor terms. Currently, the following effects (weight calculators) are supported: ConjugatesAdderFFT, Robust and Reciprocal. The first one ensures conjugate symmetry of the sample density via a relatively brute force approach using the FFT. This is necessary for robust weighting to work correctly with its current implementation. The second option (Robust) performs calculation of the robust weights. And the last option, Reciprocal, exists largely for experiments. It simply computes an inverse for every weight value above the threshold. All weight values below the threshold are replaced by 0.

uvweight.robustness

float

None

The value of robustness, only used if there is Robust among the list of effects given in the uvweight parameter. The value is supposed to be between -2. and 2., inclusive

uvweight.recipthreshold

float

1e-5

Threshold for Reciprocal weight calculator

uvweight.perbeam

bool

false

If true, the weight is accumulated and applied per beam. Otherwise, the beam index is ignored

uvweight.maxbeams

int

36

This parameter is only used if perbeam parameter is true and represents the maximum number of beams which can be encountered in the measurement set (beam indices are expected to range from 0 to (maxbeams - 1), inclusive)

uvweight.alldatawt

bool

value of perbeam

If true, all samples are included to populate the initial sample density grid. Otherwise, only data from a representative beam and field are accepted. This option is equivalent to alldatapsf in the case of preconditioning, and may be handy if the measurement set contains interleaved pointings or multiple beams.

Example

#
# Input measurement set
#
Cimager.dataset                                 = 10uJy_stdtest.ms

#
# Define the image(s) to write
#
Cimager.Images.Names                            = [image.i.10uJy_clean_stdtest]
Cimager.Images.shape                            = [2048,2048]
Cimager.Images.cellsize                         = [6.0arcsec, 6.0arcsec]
Cimager.Images.image.i.10uJy_clean_stdtest.frequency    = [1.420e9,1.420e9]
Cimager.Images.image.i.10uJy_clean_stdtest.nchan        = 1
Cimager.Images.image.i.10uJy_clean_stdtest.direction    = [12h30m00.00, -45.00.00.00, J2000]

#
# Use a multiscale Clean solver
#
Cimager.solver                                  = Clean
Cimager.solver.Clean.algorithm                  = MultiScale
Cimager.solver.Clean.scales                     = [0, 3, 10, 30]
Cimager.solver.Clean.niter                      = 10000
Cimager.solver.Clean.gain                       = 0.1
Cimager.solver.Clean.tolerance                  = 0.1
Cimager.solver.Clean.verbose                    = True

Cimager.threshold.minorcycle                    = [0.27mJy, 10%]
Cimager.threshold.majorcycle                    = 0.3mJy

Cimager.ncycles                                 = 10

#
# Restore the image at the end
#
Cimager.restore                                 = True
Cimager.restore.beam                            = [30arcsec, 30arcsec, 0deg]

#
# Use preconditioning for deconvolution
#
Cimager.preconditioner.Names                    = [Wiener, GaussianTaper]
Cimager.preconditioner.Wiener.noisepower        = 100.0
Cimager.preconditioner.GaussianTaper            = [20arcsec, 20arcsec, 0deg]