error recognition - australia telescope national facility · general form of errors › additive...
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CSIRO; Swinburne
Error RecognitionEmil Lenc (and Arin)
University of Sydney / CAASTRO
www.caastro.org
CASS Radio Astronomy School 2017
Based on lectures given previously by Ron Ekers and Steven Tingay
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Error Recognition
Some are hard to fix Some are easy to fix
Some errors are easy to recognise
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Where do errors occur?
› Most errors and defects occur in the (u,v) plane - Measurement errors (imperfect calibration – see Calibration talk). - Approximations made in the (u,v) plane. - Approximations made in the transform to the image plane.
› Some are due to manipulations in the image plane. - Deconvolution (see Deconvolution talk).
› What we usually care about are effects in the image plane (not always e.g. spectral line).
› The relative contribution of certain errors will vary depending on the nature of the observation.
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Image or uv plane?
› We need to work between the uv plane and the image plane. - Different types of errors may be more obvious in one plane than the
other. - A good understanding of the relationship between both planes.
› Errors obey Fourier transform relations. - Narrow features transform to wide features and vice versa. - Symmetries important – real/imaginary, odd/even, point/line/ring. - The transform of a serious error may not be serious! - Some effects are diluted by the number of other samples.
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General form of errors
› Additive errors (out-of-field sources, RFI, cross-talk, baseline-based errors, noise) - V + ε ➔ I + F[ε]
› Multiplicative errors (uv-coverage effects, gain errors, atmospheric effects) - V•ε ➔ I ★ F[ε]
› Convolutional errors (primary beam effect, convolutional gridding) - V ★ ε ➔ I•F[ε]
› Other errors - Bandwidth and time average smearing. - Non-coplanar effects (see Wide Field Imaging talk by Tim Cornwell) - Deconvolutional errors (see Deconvolution talk by Mark Wieringa) - Software!!! (see everyone!)
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Error Diagnosis
› If ε is pure real, then the form of the error in the (u,v) plane is a real and even function i.e. F[ε] will be symmetric. - Such errors are often due to amplitude calibration errors.
› If ε has an imaginary component, then the form of the error in the uv plane is complex and odd i.e. F[ε] will be asymmetric. - Such errors are often due to phase calibration errors.
› Short duration errors - Localized in (u,v) plane but distributed in image plane. - Narrow features in (u,v) are extended in orthogonal direction in image.
› Long timescale errors - Ridge in (u,v) plane causes corrugations in image plane - Ring in (u,v) plane causes concentric “Bessel” rings in image plane
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Gain Errors
Adapted from Myers 2002 and Ekers.
10 deg phase error
anti-symmetric ridges
20% amp error
symmetric ridges
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Additive Errors: RFI
Dirty Map PSF
Observation of 1 Jy source
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Finding RFI
Observation of 1 Jy sourceSee Mark’s talk for more on removing RFI.
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The Bigger Picture
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The Bigger Picture
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The Bigger Picture
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The Bigger Picture
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The Bigger Picture
>6 deg!
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The Bigger Picture
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The Bigger Picture
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Multiplicative Errors
Dirty Map PSF
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Primary Beam ErrorCommon in widefield imaging/instruments
Deconvolved Peeled
Peeling applicable to transient and variable sources too.
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Point Deconvolution Errors
Pixel centred Pixel not centred
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Point Deconvolution Errors
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Point Deconvolution Errors
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Deconvolution Errors (Large-scale Structure)
True sky Standard CLEAN
Standard CLEAN does not handle large-scale structure well – results in negative bowls. More modern algorithms such as Multi-scale CLEAN are necessary to minimise deconvolution errors.
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Deconvolution Errors (Large-scale Structure)
True sky Standard CLEAN
Standard CLEAN does not handle large-scale structure well – results in negative bowls. More modern algorithms such as Multi-scale CLEAN are necessary to minimise deconvolution errors.
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Wideband Deconvolution Errors
Dirty Image (2.1 GHz CABB Obs) PSF
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Wideband Deconvolution Errors
Deconvolved Image
Standard CLEAN
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Wideband Deconvolution Errors
Source SED
What standard CLEAN fits with
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Wideband Deconvolution Errors
Deconvolved Image
Multi-frequency CLEAN
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Missing short baselines
No short baselines
Paul Rayner 2001Can only be fixed with additional data. See Shari’s talk on observing strategies.
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Smearing Errors
Bandwidth average smearing
Average 512x1MHz band
Time-average smearing Averaging 1000s
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Reality check
Model Stokes I to Stokes V leakage for each beam-former setting
Subtract modelled component of leakage from Stokes V.
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Reality check
Model Stokes I to Stokes V leakage for each beam-former setting
Subtract modelled component of leakage from Stokes V.
PSR J1157-6224
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Finding the errors in your way
› Avoid sausage factory processing (at least initially) - Try to understand each processing step. - Look closely at the data after each step, check and image calibrators. - Does the data look plausible.
› Take a different perspective - Look at your data in different domains (time, (u,v), image, frequency). - Plot different combinations of variables in different spaces. - Look at residuals, FT your dirty image, FT your beam.
› Process your data in different ways - Try different software, algorithms. - Partition and process your data in different ways - Try split in time chunks, split up frequency band - Different weighting, different uv tapers.
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Error reduction …
Process
Have errors been beaten
to submission?
Determine greatest
contributing error
Do Science
Attempt to reduce effect of
error
No
Yes
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What’s happening?
5.5 GHz observation, 3 configurations, 2 GHz bandwidth
2000:1 dynamic range
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What’s happening?
5.5 GHz observation, 3 configurations, 2 GHz bandwidth
2000:1 dynamic range
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What’s happening?
5.5 GHz observation, 3 configurations, 2 GHz bandwidth
2000:1 dynamic range
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What’s happening
› Amplitude calibration errors. › Hot spot near edge of 4.5 GHz beam (outside 6.5 GHz beam)
- Causes steepening of source spectra. - Causes position dependent effects. - Will need to consider peeling techniques.
› Spectral variation throughout the image (flat and steep) - Must use multi-frequency deconvolution.
› Structures on many different scales. - Must use appropriate deconvolution algorithms.
› North-west hot spot is bright and slightly extended. - Difficult to deconvolve accurately. - Small cell size or uv-subtract component.
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What’s happening?
38,000:1 dynamic range
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1. Can you deal with something new? What’s happening?
A. Heat haze B. Antenna deformation C. Ionosphere D. Compression artifacts
Low frequency MWA obs.
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1. Can you deal with something new? What’s happening?
A. Heat haze B. Antenna deformation C. Ionosphere D. Compression artifacts
Low frequency MWA obs.
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1. Can you deal with something new? What’s happening?
A. Heat haze B. Antenna deformation C. Ionosphere D. Compression artifacts
Low frequency MWA obs.
✔
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1. Can you deal with something new? What’s happening?
A. Heat haze B. Antenna deformation C. Ionosphere D. Compression artifacts
Low frequency MWA obs.
✔
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2. Be daring in your searchWhat’s happening?
A. Primary Beam error B. RFI C. Venetian blinds left open D. Deconvolution error
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2. Be daring in your searchWhat’s happening?
A. Primary Beam error B. RFI C. Venetian blinds left open D. Deconvolution error
✔
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3. Can you work this out? What’s happening?
A. Amplitude errors B. Cosmic ray C. Bandwidth smearing D. RFI
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3. Can you work this out? What’s happening?
A. Amplitude errors B. Cosmic ray C. Bandwidth smearing D. RFI✔
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4. Dare to solve this!What’s happening?
A. Amplitude errors B. Phase of moon incorrect C. Position-dependent errors D. Source outside imaged field
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4. Dare to solve this!What’s happening?
A. Amplitude errors B. Phase of moon incorrect C. Position-dependent errors D. Source outside imaged field✔
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5. Don’t give up! What’s happening?
A. RFI B. Bandwidth smearing C. Daylight savings not set D. Position-dependent errors
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5. Don’t give up! What’s happening?
A. RFI B. Bandwidth smearing C. Daylight savings not set D. Position-dependent errors✔
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6. Are you able to solve this?What’s happening?
A. Amplitude errors B. Tartan from wrong clan C. Data stored in HEX D. Phase errors
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6. Are you able to solve this?What’s happening?
A. Amplitude errors B. Tartan from wrong clan C. Data stored in HEX D. Phase errors
✔
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7. Can this be real?What’s happening?
A. Ionospheric effects B. Faraday rotation C. Polarisation leakage D. Galactic circular polarisation
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7. Can this be real?What’s happening?
A. Ionospheric effects B. Faraday rotation C. Polarisation leakage D. Galactic circular polarisation✔
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8. A tricky problem What’s happening?
A. Missing short baselines B. Missing long baselines C. Missing astronomer D. Alien Resurrection
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8. A tricky problem What’s happening?
A. Missing short baselines B. Missing long baselines C. Missing astronomer D. Alien Resurrection
✔
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9. End of game question What’s happening?
A. Amplitude errors B. Phase errors C. Deconvolution errors D. Position-dep. errors E. Almost everything
![Page 59: Error Recognition - Australia Telescope National Facility · General form of errors › Additive errors (out-of-field sources, RFI, cross-talk, baseline-based errors, noise) - V +](https://reader033.vdocuments.net/reader033/viewer/2022060316/5f0bfb547e708231d4332cb1/html5/thumbnails/59.jpg)
9. End of game question What’s happening?
A. Amplitude errors B. Phase errors C. Deconvolution errors D. Position-dep. errors E. Almost everything✔
![Page 60: Error Recognition - Australia Telescope National Facility · General form of errors › Additive errors (out-of-field sources, RFI, cross-talk, baseline-based errors, noise) - V +](https://reader033.vdocuments.net/reader033/viewer/2022060316/5f0bfb547e708231d4332cb1/html5/thumbnails/60.jpg)
Acknowledgements
› This talk is based on talks by: - Steven Tingay - Ron Ekers - ASP Conference Series Vol. 180, p.321 – available online
› Special thanks to Arin Lenc for running the pop quiz.