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Complete Characterization of sub-pixel Response of Near-Infrared Detectors ( Spot s -O-Matic )

Complete Characterization of sub-pixel Response of Near-Infrared Detectors ( Spot s -O-Matic ). Tomasz Biesiadzinski, Greg Tarl é , Michael Howe, Curtis Weaverdyck, Michael Schubnell, Wolfgang Lorenzon. Outline. Why do we care about sub-pixel response? The Spot-O-Matic

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Complete Characterization of sub-pixel Response of Near-Infrared Detectors ( Spot s -O-Matic )

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  1. Complete Characterization of sub-pixel Response of Near-Infrared Detectors (Spots-O-Matic) Tomasz Biesiadzinski, Greg Tarlé, Michael Howe, Curtis Weaverdyck, Michael Schubnell, Wolfgang Lorenzon Detectors for Astronomy, Garching

  2. Outline • Why do we care about sub-pixel response? • The Spot-O-Matic • Our first look at intra-pixel response • Barron et al., “Subpixel Response Measurement of Near-Infrared Detectors”, PASP (2007) • Spots-O-Matic • Concept • Effects of sub-pixel structure • Simulation of errors on point source photometry and simulated Spots-O-Matic correction • Weak lensing considerations • Possible errors • Shape projection capabilities • Spots-O-Matic Progress • Lens characterization Detectors for Astronomy, Garching

  3. Under-sampling for Survey Telescopes Under-sampled (Assuming Perfect Pixel) Well Sampled • Modern survey telescopes employ under-sampling to improve survey speed. • Precision photometry or galaxy shape measurements (for Weak Lensing) in under-sampled telescopes requires dithering and/or well-characterized intra-pixel response. • For under-sampled NIR survey telescopes, sub-pixel detector properties become important • Charge Diffusion (~1.87mm) • Capacitive Coupling (~2%) • Sub-pixel structure (pixel geometry, defects…) Detectors for Astronomy, Garching

  4. Looking Inside a Pixel - the Pixel Response Function (PRF) as measured by the Spot-O-Matic • Single ~1 mm spot projector (Spot-o-Matic) • 2 dimensional scan over several pixels mapping the internal response • Objective: Determine the largest plate scale a telescope can have while still delivering 1% photometric precision for point sources (e.g. SNe). H2RG-102 one dimensional scan (and fit to data points) showing effects of charge diffusion and capacitive coupling Detectors for Astronomy, Garching

  5. Spot-O-Matic Results 2 dimensional scan of an individual pixel response Summation over multiple pixels showing uniform photometry • For detectors with high quantum efficiency, better than 1% photometry is achieved with PSF’s > ½ the size of a pixel • Note small random defects (~5%) that could affect galaxy shape reconstruction for weak lensing. Detectors for Astronomy, Garching

  6. Spots-O-MaticConcept • Simultaneously scan an array of 160000 spots (400 x 400) to rapidly characterize the sub-pixel response of an entire detector • Standard 17.5 cm x 17.5cm photolithography mask • Illuminated by NIR laser • Each spot scans a 5 by 5 pixel array • 6 axis computer controlled stage • X and Y axis to perform a 2D scan of the entire detector • Z axis to sample focal “plane” over depth of focus • tip, tilt, rotation stages to make sure the image and detector are co-planar and scan is aligned with rows/columns • Use a commercial 50mm lens to demagnify and focus the image • Zeiss Planar T* 1.4/50ZF IR, optimized for NIR light (optics, AR coatings) • 28 cm object distance, 6.2 cm image distance, -1/4.5 magnification • Cold laser line filter inside dewar blocks out-of-band light Detectors for Astronomy, Garching

  7. Simulation of the use the Spots-o-Matic to Improve Photometry • Simulated Spots-o-Matic signal obtained by convolving Spot-o-Matic Scan with 6mm PSF Detectors for Astronomy, Garching

  8. Point Source Photometry (SNe) withoutSpots-o-Matic correction • Small plate scale • 0.23 arcsec/pixel • s = 0.39 Pixels • Large plate Scale • 0.81 arcsec/pixel • s = 0.11 Pixels • 3.6% of SNe have fluxes with more than 1% error • 0.4% of SN have fluxes with more than 2% error • All fitted fluxes are within ±0.3% of the true values Detectors for Astronomy, Garching

  9. Point Source Photometry (SNe) withSpots-o-Matic correction • Small plate scale • 0.23 arcsec/pixel • s = 0.39 Pixels • Large plate Scale • 0.81 arcsec/pixel • s = 0.11 Pixels • All fitted fluxes are within ±0.2% of the true values • Errors are now at the sub 0.1% level Detectors for Astronomy, Garching

  10. Spots-O-Matic & Weak Lensing • Dithering • Reconstructs diffraction limited seeing in under-sampled telescopes at the expense of survey speed • Compensates for intrapixel structure • A complete Spots-o-Matic scan can reduce the number of dithers required to achieve a given level of shape discrimination • Simulations have yet to be performed to quantify this • Can large plate scale HgCdTe detectors be used for shape reconstruction? Spots-o-Matic data will provide the answer • Elliptical “galaxies” and point sources (PSF calibration “stars”) will be projected onto real detectors and shapes will be extracted • Sub-pixel features mapped by the Spots-O-Matic will be used to correct the shapes for known intrapixel response and determine the errors after correction Detectors for Astronomy, Garching

  11. Questions Can a lens produce small enough spots? What is the f-stop that results in minimum spot size? Lens quality vs. diffraction limit How does the spot size change with location in the field of view? Knife edge technique Scaned the spot repeatedly across a knife edge1 (razor blade) while focusing in z. The spatial derivative of the signal at best focus gives a one dimensional profile of the lens PSF Spots-O-Matic Lens Characterization 1Firester, A. H., Heller, M. E., & Sheng, P. 1977, Appl. Opt., 16, 1971 Detectors for Astronomy, Garching

  12. Lens Characterization Runs Focusing in Z Axis Raw Signal • Obtained Scans at f / # 5.6, 2.8 and 1.4 on lens axis • At f / # 5.6, 7.5 cm off axis • Planned Spots-O-Matic mask range Detectors for Astronomy, Garching

  13. Measured PSF’s of the Lens at Different F-stops Detectors for Astronomy, Garching

  14. Lens Characterization Results • Obtained spot sizes (Demagnified by factor of 4.5): • f / # 1.4: on axis • FWHM = 5.4 mm • Fitted s = 6.6 mm • f / # 2.8: on axis • FWHM = 4.7 mm • Fitted s = 2.9 mm • f / # 5.6: on axis • FWHM = 5.8 mm • Fitted s = 2.8 mm • f / # 5.6: off axis • Fitted s = 3.4 mm • Manufacturer indicated diffraction limited psf at f/# 5.6 • Complicated structure at lower f-stops likely due to lens defects • Can’t measure FWHM in the off-axis scan (due to low signal to noise) but the fit indicates ~17% deterioration of  from on axis • Plan to scan at f/# 2.8 which appears to be a better choice for the Spots-O-Matic • Spots-o-Matic spot size will be s = 2.0 mm (as determined by fwhm/2.35) Detectors for Astronomy, Garching

  15. Conclusion • The Spots-O-Matic will provide  ~ 2mm resolution scans of an entire HgCdTe detector in ~ 1 day. • With a standard lens it can be used on visible CCDs with likely better resolution • The spot size is not significantly degraded at the periphery of the field of view • Now that we have characterized the optics we are proceeding with final design and construction of the Spots-o-Matic. • We expect first scans by Spring 2010 in time to influence JDEM instrument design. Detectors for Astronomy, Garching

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