1. 1 Synthesis Imaging Workshop Error recognition R. D. Ekers Narrabri, 20 Sep 2006

2. R. Ekers2 Summary n Follows Chapter 10 “Error Recognition” in NRAO Synthesis Workshop closely n Educational –Use of basic concepts and analogies –Fourier transform practice n Practical information for diagnosing errors n Diagnostic tools n Making discoveries

3. Larson cartoon

4. 12 May 20034 Cygnus A  Raw data VLA continuum  Deconvolution Uses non-linear algorithms to correcting for errors due to missing information in the Fourier domain  Self Calibration Uses the corrupted image of the object to remove antenna based gain errors

5. 5 Image or Aperture Plane? n Most errors occur in the measurements (aperture plane) but effect the science in the image plane n Errors obey Fourier transform relations: – narrow features transform to wide features (and visa versa) – symmetries important - real/imag, odd/even, point/line/ring n Some errors more obvious in particular domain – switch between image and uv planes n The transform of a serious error may not be serious! – effects are diluted by the number of other samples

6. Paul Rayner 20016 Bad Scan - Visibilities: Flagged:Unflagged: Only two scans on 1/15 baselines affected.

7. Paul Rayner 20017 Bad Scan - Images: Flagged:Unflagged:

8. Paul Rayner 20018 Bad Gain - Visibilities: Properly Calibrated: 2.5% Gain error one ant: Gain error affects all visibilities on 5/15 baselines

9. Paul Rayner 20019 Bad Gain - Visibilities: Properly Calibrated:2.5% Gain error:

10. S.T. Myers - 8th Synthesis Summer School 20 June 2002 10 The 2D Fourier Transform n x,y (radians) in tangent plane relative to phase center n spatial frequency u,v (wavelengths) n adopt the sign convention of Bracewell:

11. S.T. Myers - 8th Synthesis Summer School 20 June 2002 11 The Fourier Theorems n shift in one domain is a phase gradient in the other n multiplication in one domain is convolution in the other

12. R.Ekers12 Fourier Symmetries n symmetries determined by Fourier kernel n real sky brightness  Hermitian uv plane – complex conjugate of visibility used for inverse baseline exp( i φ ) = cos φ + i sin φ –Real & Even  Real & Even –Real & Odd  Imag & Odd Symmetric image errors are often due to amplitude errors image errors with odd symmetry or asymmetric often due to phase errors

13. Transform Pairs - 1 Figure used without permission from Bracewell (1986). For educational purposes only, do not distribute.

14. Transform Pairs - 2 Figure used without permission from Bracewell (1986). For educational purposes only, do not distribute.

15. Transform Pairs - 3 Figure used without permission from Bracewell (1986). For educational purposes only, do not distribute.

16. Transform Pairs - 4 Figure used without permission from Bracewell (1986). For educational purposes only, do not distribute.

17. S.T. Myers - 8th Synthesis Summer School 20 June 2002 17 Error Diagnosis n amplitude or phase errors: – phase errors usually asymmetric or odd symmetry – amplitude errors usually symmetric (even) n short duration errors: – localized in uv plane  distributed in image plane – narrow  extended orthogonal direction in image n long timescale errors: – ridge in uv plane  corrugations in image – ring in uv plane  concentric “Bessel” rings in image

18. Adapted from Myers 200218 Example Gain Error - 2 10 deg phase error anti-symmetric ridges 20% amp error symmetric ridges

19. S.T. Myers - 8th Synthesis Summer School 20 June 2002 19 Example Gain Error - 3 10 deg phase error 1 ant all times rms 2.0 mJy rings – odd symmetry 20% amp error 1 ant all times rms 2.3 mJy rings – even symmetry

20. Adapted from Myers 200220 Additive n some errors add to visibilities – additive in conjugate plane – examples: noise, confusion, interference, dc offsets, cross- talk, variable source, source outside field (eg sun)

21. Paul Rayner 200121 DC offset: Additive error - Image plane

22. Paul Rayner 200122 DC offset: Additive error - aperture plane

23. Paul Rayner 200123 DC offset: Additive error - Image plane

24. S.T. Myers - 8th Synthesis Summer School 20 June 2002 24 Multiplicative n others multiply or convolve visibilities – multiplication  convolution in conjugate planes » examples - multiplicitive: sampling, gain errors, atmosphere, missing spacings »Examples - convolution: primary beam, gridding

25. 25 Image errors caused by rotation of the primary beam (alt-az telescope) n Gain error in UV plane = Convolution in image plane n Stokes Q image showing effect of off-axis instrumental polarization n HII region in direction of the HESS gamma ray source Robert Reinfrank

26. Paul Rayner 200126 CLEAN Errors in the Image:

27. Paul Rayner 200127 CLEAN errors in UV-plane:

28. Paul Rayner 200128 n Use different array configuration. n Different frequency, if possible. n North-spur, when available. Recognizing Poor UV Coverage: Fourier Transform the Source Model and Beam!

29. Paul Rayner 200129 Effect of missing short baselines No short baselines

30. S.T. Myers - 8th Synthesis Summer School 20 June 2002 30 Radially Dependent Errors n not expressible as simple operations in image/uv plane –sometimes convertible to standard form via coordinate change n smearing effects –bandwidth: radial - like coadding images scaled by frequency –time-average: tangential – baselines rotated in uv plane n baseline, shadowing n pointing –dependent on source position in the field –polarization effects worse (e.g. beam squint)

31. 31 Non-linear processing errors n 2 ATCA snapshots –Only 30 visibilities measured –Deconvolution interpolates 10 4 additional points –Selfcal adds 10 more degrees of freedom!

32. 32 Surprising non-linear effects n Polarization –Stokes parameters U and Q »Have gausian noise »Obey the convolution algorithm »But not very easy to interpret –P=  U 2 +Q 2 »Noise biased and not gausian »Not a convolution of the sky with anything »Angular resolution can appear to be infinite! –P/I and θ are even worse

33. R Ekers33 Diagnostics n Good image display –Negativity –Complex numbers n Polarization n Low resolution image of large field n Source subtraction n Fourier transform n Statistics

34. 34 IRAM-GIPSY Single bad point (& conjugate) Bad point removed Stripes in image Stripes gone! image UV plane

35. 35 IRAM-GIPSY

36. 36 Discoveries Radio emission from Cyg X2 n WSRT 21cm n Braes and Miley (1972) Variable source only present in 1 day of a 4 day synthesis

37. 37 Discoveries Flare in RS CVn system II Peg –WSRT 83cm

38. 38 Galactic Centre n VLA 6cm n Big picture missed by first observers

39. 39 Galactic Centre n VLA 6cm n Big picture missed by first observers n Too much resolution too small FOV