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A user's guide to lucky imaging

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A user's guide to lucky imaging

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An invited talk given to an audience interested in using lucky imaging for microlensing studies. I tried to give an overview of where the challenges lie in getting good science data using lucky imaging techniques.

An invited talk given to an audience interested in using lucky imaging for microlensing studies. I tried to give an overview of where the challenges lie in getting good science data using lucky imaging techniques.

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A user's guide to lucky imaging

  1. 1. A User’s Guide to Lucky Imaging (With a focus on mosaic cameras) Tim Staley, Craig Mackay & the Cambridge LI Group RS Lucky Imaging Meeting Chicheley Hall, April 2013 WWW: timstaley.co.uk
  2. 2. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS OUTLINE WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS
  3. 3. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS OUTLINE WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS
  4. 4. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS WHY MOSAIC? Spatial sampling High spatial resolution surveys require many pixels on sky. At the frame rates lucky imaging requires, it becomes difficult to run a 1Kx1K CCD without getting swamped by readout noise. Unfortunately, current generation of EMCCDs cannot be abutted (though this is changing).
  5. 5. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS WHY MOSAIC? Dynamic range Independent gain control is a bonus. Dynamic range on a single EMCCD is limited to about 10 magnitudes by electron well depth in multiplication register. (Disclaimer: back of envelope calculation.) Can’t guide well on a saturated star. Ok if GS is Mag ≈ 16. Potentially problematic if GS is Mag ≈ 10.
  6. 6. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS WHY MOSAIC? But... Significant hardware challenge – alignment, synchronisation, multiple control systems... High data rates. More calibration. (See below...)
  7. 7. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS OUTLINE WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS
  8. 8. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS A HARDWARE IMPLEMENTATION
  9. 9. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS A HARDWARE IMPLEMENTATION
  10. 10. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS A HARDWARE IMPLEMENTATION
  11. 11. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS A HARDWARE IMPLEMENTATION
  12. 12. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS FIELD TEST
  13. 13. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS FIELD TEST Visitor Instrument @ 2.5m Nordic Optical Telescope Awarded 8 nights on sky, + technical time for setup and take-down. Off we went to La Palma.
  14. 14. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS FIELD TEST
  15. 15. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS FIELD TEST
  16. 16. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS RESULTS? Minor software glitches slowed progress for first couple of days. Overall, system performed very well. Some good weather. Now, we hads lots of data to reduce (∼ 8TB).
  17. 17. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS OUTLINE WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS
  18. 18. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS PHOTOMETRIC CALIBRATION Specific to EMCCDs: Accurate on-sky gain calibration via pixel histograms is desirable (CCDs independently adjustable). Histogram based bias map estimation is the most robust way to separate bias pedestal and internally generated signal (i.e. dark current + clock induced charge).
  19. 19. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS PHOTOMETRIC CALIBRATION
  20. 20. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS PIPELINE OVERVIEW Custom pipeline written in C++. A 2 stage process, comprising evaluation and reduction. Almost always I/O limited (∼60–80MB/sec). Make use of thread-friendly STL based containers and Intel Thread Building Blocks library to implement a ‘pipeline of filters’ pattern.
  21. 21. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS PIPELINE OVERVIEW First, frame evaluation: Load frame, debias, gain normalize, subtract internally generated signal (DC + CIC). Interpolate GS region, cross-correlate with Airy core to estimate tip-tilt and (relative) Strehl. Save position and quality estimates to file.
  22. 22. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS PIPELINE OVERVIEW Second, selection and recombination: Sort frames by estimated quality, selecting according to user-criteria. Load and calibrate, optionally applying thresholding to a copy. Drizzle to create final image, writing to file whenever a user-selection criteria is met.
  23. 23. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS PIPELINE OVERVIEW
  24. 24. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS PIPELINE OVERVIEW
  25. 25. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS ASTROMETRIC CALIBRATION How do you calibrate a high-resolution mosaic detector?
  26. 26. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS ASTROMETRIC CALIBRATION How do you calibrate a high-resolution mosaic detector? Using standard binaries would take at least (2n-1) pointings per focal lens. So we observe a crowded field.
  27. 27. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS ASTROMETRIC CALIBRATION How do you calibrate a high-resolution mosaic detector? Using standard binaries would take at least (2n-1) pointings per focal lens. So we observe a crowded field. But: standard catalogues (2MASS, USNO) do not have the resolution (2MASS pixels are 2 arcseconds width).
  28. 28. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS ASTROMETRIC CALIBRATION How do you calibrate a high-resolution mosaic detector? Using standard binaries would take at least (2n-1) pointings per focal lens. So we observe a crowded field. But: standard catalogues (2MASS, USNO) do not have the resolution (2MASS pixels are 2 arcseconds width). Solution: Create own catalogues from HST/WFPC fields, cross match using custom code.
  29. 29. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS ASTROMETRIC CALIBRATION
  30. 30. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS ASTROMETRIC CALIBRATION
  31. 31. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS OUTLINE WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS
  32. 32. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS What resolution improvement do we obtain?
  33. 33. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS PSF PROFILES 1.0 0.5 0.0 0.5 1.0 Radius in arcseconds 0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040 Strehlratioatpeak 5% selection 100% selection Seeing profile How does this vary as we move away from GS?
  34. 34. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS FWHM ACROSS THE FIELD
  35. 35. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS FWHM ACROSS THE FIELD 0 10 20 30 40 50 60 Distance from guide star [arcsec] 100 200 300 400 500 FWHM[mas] 10% selection 100% selection Seeing FWHM
  36. 36. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS How faint can we go?
  37. 37. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS FAINT LIMITS: GUIDE STARS 102 103 Source flux (photo-electrons per exposure) 0.00 0.05 0.10 0.15 0.20 0.25 Strehlratio Normalised Airy reference CC Standard Airy reference CC Brightest pixel Ideal case (upper horizontal) Seeing disc (lower horizontal) (Simulated)
  38. 38. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS FAINT LIMITS: GUIDE STARS A real world example: the Einstein Cross. (Left: HST. Right: LI. GS Mi ≈ 17)
  39. 39. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS FAINT LIMITS: SCIENCE TARGETS 3C 405 (Cygnus-A) Left: HST. Right: LI.
  40. 40. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS FAINT LIMITS: SCIENCE TARGETS 3C 405 (Cygnus-A) Left: HST. Right: LI. One hour observation, 50% selection, ∼0.5” seeing. Galaxy approx 30” from guide star. Weakly detect stars of estimated Mi ≥ 23. Can do better with latest hardware.
  41. 41. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS OUTLINE WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS
  42. 42. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS OPTIMAL PIXEL SIZE AND FRAMERATE TRADE-OFFS Spatial sampling vs detector noise. (Old issue with a new twist: Drizzle many frames with subpixel offsets.) Longer exposures for faint targets vs atmospheric blurring above coherence time. Data rates.
  43. 43. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS OBSERVATION PLANNING Will there be a close / bright enough guide star? Will de-saturating my guide star ruin my faint limit? (Can I position the mosaic to avoid this?) What resolution / magnitude depth do I need to achieve, and what weather conditions / selection criteria does this require? (For very bright sources:) Would I be better off turning off the EM amplifier altogether?
  44. 44. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS ASTROMETRIC CALIBRATION This is non-trivial! Worth investigating: http://www.astromatic.net/ software/scamp (Cannot read custom catalogues when I last checked, but this is probably an easy feature to add.)
  45. 45. WHY MOSAIC? LUCKYCAM 2009 & THE NOT REDUCTION PERFORMANCE TRADEOFFS, RECOMMENDATIONS OPEN QUESTIONS / RESEARCH PROBLEMS Use of multiple guide stars. Crowded field photometry with a slowly varying PSF.

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