Discrete Sky Operator (DiSkO) Synthesis Imaging

Author: Tim Molteno tim@elec.ac.nz

Its so cool its POINTLESS. Image by using the telescope operator keeping track of the telescope null-space and range-space. DiSkO uses a discrete representation of the field of view (as a healpix grid, or unstructured mesh) in the sky space. This means we can image arbitratily wide fields of view (including the full sphere), as well as arbitrarily shaped fields (circles are preferred).

DiSkO can perform sparsity reduction by regularization and controls the volume of the sky solution. The result is an imaging algorithm that is sensitive to diffuse broad sources, and does not require restoration like CLEAN. Publications to appear :)

Howto

disko --display --show-sources

To load a data from a measurement set

disko --ms test_data/test.ms --tikhonov --nside 32 --PDF

VLA imaging with DiSkO

Download the VLA 5GHZ continuum survey measurement set. AG733_A061209.xp1 from the NRAO site.

Calibrate and then split the measurement set, following the CASA tutorial [https://casaguides.nrao.edu/index.php/VLA_5_GHz_continuum_survey_of_Seyfert_galaxies]

disko --fov 0.01 --ms NGC1194.split.ms --SVG --arcmin 0.0025 --tikhonov

More challenging

This tutorial should generate a file with lots of diffuse radiation. [https://casaguides.nrao.edu/index.php/VLA_Continuum_Tutorial_3C391-CASA5.5.0]

wget http://casa.nrao.edu/Data/EVLA/3C391/3c391_ctm_mosaic_10s_spw0.ms.tgz
wget https://github.com/jaredcrossley/CASA-Guides-Script-Extractor/blob/master/extractCASAscript.py

python extractCASAscript.py 'https://casaguides.nrao.edu/index.php?title=VLA_Continuum_Tutorial_3C391-CASA5.5.0'

Then in CASA

execfile("VLAContinuumTutorial3C391-CASA5.5.0.py")

This should generate a suitable measurement set to image.

TODO

• Add a --full-sphere option which fixes the sphere in celestial coordinates, and then points the phase center of an observation correctly. Requires a beam pattern to be specified (or at least a hemispherical beam). A beam is a sky vector mask. I.e., should fall to zero 'outside' the beam.
• Deal with flagging, output residuals in a way that can be used to flag in the measurement set. This means some casa expression that changes the MS.

Changelog

0.9.5b4 Add a timestamp to images (or a title if specified) in SVG mode
0.9.5b3 Use gmsh rather than optimesh... (WIP)
0.9.5b2 Fix RA direction in generated FITS files (thanks Ben Hugo)
0.9.5b1 Add --min and --max to disko_draw so allow manual setting the range of images
0.9.4b6 Fix bug in drawing PDF.
0.9.4b5 Import Resolution in disko to get array beam width.
        Fix sphere power.
0.9.4b4 Expose parent parsers.
        Refer to min_res() rather than nside for spheres
        Fix bugs in display of mesh spheres
        add disko.fov namespace
        serialize to hdf5 files
        New disko_draw CLI tool
        Conjugate visibilities from files.
0.9.4b3 Move sphere args parser to the sphere object.
0.9.4b2 Add helper method to calculate beam size. 
        Add area(), get_power() method to sphere. Make it work.
        add rms(), copy() methods for rms() and deep copying of spheres
0.9.4b1 Use read_ms from tart2ms (moved there). 
0.9.3b6 Use speed of light from astropy.constants. 
        Add a --version option to print the current version and exit.
0.9.3b5 Fix bug in the Matrix Free Linear Operator which wasn't conjugated.
0.9.3b4 Raise nicer errors when arguments aren't provided
0.9.3b2 Fix indexing error in read_ms when the number of visibilities requested exceeded the number available.
        clean up the meshing 
        rework the command line interface. New resolution specification
        Output residuals to the terminal (still no way to include them back in...). The ms needs to be flagged externally.
        Use Natural weighting when reading from measurement sets
0.9.3b1 Add --h5 option to allow sequential inference from a visiblity file.
0.9.2b1 No longer require arcmin for construction of spheres
0.9.1b1 Remove constraint that nside is a power of two now that healpy has accepted the pull request.
        Add new parameter l1_ratio
        Dont scale the alpha parameter.
        Allow negative solutions for Tikhonov regression.
        Allow full skies using --nside option
        Add a colour bar to the SVG output.
0.9.0b4 Improve measurement set reading. 
        Use the mean RMS value for a single noise estimate on visibilities.
        Use the correct rank value in overdetermined skies.
        Truncate the SVD to keep the condition number of the telescope less than 50.
0.9.0b3 Full Bayesian Inference is workin. Fix bug in meshio (after upgrade beyond 4)
0.9.0b2 Add a multivariate gaussian object. Fix ms_helper. Handle the case where the rank of the telescope operator is not full.
0.9.0b1 Move to a real telescope operator.
0.8.0b5 Allow FISTA to calculate its own largest eigenvalue if negative values are passed in.
0.8.0b4 Clean up code and avoid recalculating harmonics. 
        Added a DirectImagingOperator that performs the discrete Fourier Transform.
0.8.0b3 Add --fista command line option to use the FISTA solver.
0.8.0b2 Add an lsqr option to force the slightly slower lsqr algorithm in place of lsmr.
0.8.0b1 Add a matrix-free operator that actually works. Process UVW in meters.
0.7.0b10 Clean up tests. Rename the DiSkOOperator and get it going.  Fix up timestamp loading, use the correct frequency (based on channel parameter)
0.7.0b9 Fix up timestamp loading
0.7.0b8 Optimize mesh at each stage of refinement.
0.7.0b7 Better refinement.
0.7.0b6 Limit gradient calculation to cells above nyquist limit
0.7.0b5 Improve channel selection
0.7.0b4 Allow selection of the channel number
0.7.0b2 New adaptive meshing on gradient
0.7.0b1 Add adaptive meshing and --adaptive option
0.6.0b9 Report Nyquist resolution
0.6.0b7 MS were being read incorrectly - the UVW are measured in meters, not wavelengths!
0.6.0b6 Correct field pointing from measurement sets.
0.6.0b5 Reduce memory requirements by around 25%.
0.6.0b4 Report the r^2 value.
0.6.0b2  Use dask for very large jobs (use the --dask switch)
0.6.0b1  Get data from Measurement Sets!
0.5.0b5 Allow sources not to be shown.
0.5.0b4 Override plot in HPSubSphere to allow for non-normal pixels.
0.5.0b3 Added elliptical source circle projections in SVG.
0.5.0 Getting imaging logic better. Added L2 regularization, and cross-validation