Fast Solar Polarimeter A. Feller , F . Iglesias, K. Nagaraju, S. K. - PowerPoint PPT Presentation

Fast Solar Polarimeter A. Feller , F . Iglesias, K. Nagaraju, S. K. Solanki Max Planck Institute for Solar System Research and colleagues from the Max Planck semiconductor lab A. Feller FSP IAUS 305 1 / 15

Overview Fast Solar Polarimeter (FSP) in a nutshell Novel ground-based solar imaging polarimeter developed by MPS in collaboration with the MPG semiconductor lab (HLL) and PNSensor Based on fast low-noise pnCCD sensor and ferro-electric liquid crystals for polarization modulation Polarimetry of small and dynamic solar structures at increased polarimetric sensitivity (< 10 − 3 ) or at high temporal cadence in particular also in the chromosphere Development in 2 phases: 2012-2014 Proof of concept with small pnCCD prototype (264x264 pixels 2 ), single-beam 2014-2016 Development of full-scale, science-ready version with 1kx1k pnCCD, dual-beam Funded by MPG and European Commission (SOLARNET) A. Feller FSP IAUS 305 2 / 15

Why FSP? Photon budget and solar evolution Tradeoff between solar evolution and noise: Δ t s Maximum integration time ∆ t allowed by solar evolution: Δ t e ∆ t e = 2 ∆ x / v Δ t Minimum integration time to reach a given required rms optimum ( Δ x, Δ t) ~ F -1/3 σ -2/3 noise level σ : ∆ t s = ( F σ 2 ∆ x 2 ) − 1 Δ x ∆ x : spatial sampling v : evolution speed F : Flux [phot / (s · arcsec 2 )] A. Feller FSP IAUS 305 3 / 15

Why FSP? Photon budget and solar evolution 1m telescope: 0.320",33.1s 0.160",16.6s ( Δ x, Δ t) = (0.12", 12.5s) 0.547", 6.6s 0.274", 3.3s Fe I 525.0 nm 10 −4 CaII 393.3 nm RMS noise Sr I 460.7 nm 0.080", 8.3s 0.040", 4.1s 0.137", 1.7s 0.068", 0.8s 10 −3 0.020", 2.1s 0.034", 0.4s 0.010", 1.0s 0.017", 0.2s 10 −2 10 7 10 8 10 9 10 10 Flux [phot / (s · arcsec 2 )] A. Feller FSP IAUS 305 4 / 15

Why FSP? Why fast modulation? Slow dual-beam modulation is not sufficient for . . . high accuracy in the presence of strong polarization signals high spatial resolution The demodulated images still suffer from crosstalk between Q, U, V ... ... which is not reduced by AO (see poster by Nagaraju) Only corrective: Keep modulation cycle as close as possible to seeing time scale ( ∼ 10 ms) → 100 Hz modulation! A. Feller FSP IAUS 305 5 / 15

Why FSP? FSP is beneficial for 2 dedicated observing regimes High-precision polarimetry ( σ < 10 − 3 ) Fast modulation suppresses systematic errors Image reconstruction and statistical techniques like Feature-based spatial averaging (image segmentation) Feature tracking in time conserve small-scale spatial information Low-precision, high-cadence polarimetry High duty cycle (95%) → S/N in shortest possible ∆ t 1 reconstructed Stokes image set per s possible, due to high frame rate (400 fps) short mod. cycle (4 states) A. Feller FSP IAUS 305 6 / 15

How does FSP work? Main specifications FSP I FSP II Sensor size 264 px x 264 px 1024 px x 1024 px Max. frame rate 800 fps 400 fps Pixel pitch 48 µ m 36 µ m QE > 90% 500 nm - 870 nm 350 nm - 500 nm Duty cycle 97% 95% RMS readout noise 3 - 4 e − Sensitive subst. depth 450 µ m Readout ASICS x number CAMEX x 4 VERITAS-1 x 16 Max. data rate 0.78 Gb/s 6.7 Gb/s A. Feller FSP IAUS 305 7 / 15

How does FSP work? pnCCD camera Key concepts Sensor layout scheme Fast split frame transfer Column-parallel readout No shutter → numerical frame transfer correction (Iglesias et al. 2015) Multi-correlated double-sampling to reduce noise Custom coating to optimize QE From Ordavo et al. 2011 Thick substrate → no internal fringing A. Feller FSP IAUS 305 8 / 15

Does FSP work as expected? VTT test campaigns Campaigns Setups (for data shown later) Jun 2013 Spectrograph VTT aperture 0.7 m Nov 2013 TESOS Spectrograph, 422.7 nm Jun 2014 TESOS Sampling 0.8" x 17 mÅ FOV 72" x 3.7 Å 6 · 10 − 4 Efficiency TESOS, 630.2 nm Sampling 0.08" x 0.08" FOV 20" x 20" Spec. bandwidth 25 mÅ 1 · 10 − 2 Efficiency A. Feller FSP IAUS 305 9 / 15

Does FSP work as expected? Ca I 4227 Å, Scattering polarization I I 140 60 e - /(frame*pixel) 120 100 arcsec 40 80 60 20 40 20 0 0 422.50 422.60 422.70 422.80 422.50 422.60 422.70 422.80 Q/I Q/I 2.5 60 2.0 arcsec 1.5 40 % 1.0 20 0.5 0 0.0 422.50 422.60 422.70 422.80 422.50 422.60 422.70 422.80 nm nm Figure: Black: FSP obs. at µ ∼ 0 . 15; Blue line: atlas of the Second Solar Spectrum (Gandorfer 2000) A. Feller FSP IAUS 305 10 / 15

Does FSP work as expected? Figure: Time series of 19 MFBD reconstructed line scans (1.6s / spectral position) A. Feller FSP IAUS 305 11 / 15

Does FSP work as expected? Fe I 6302 Å, Quiet Sun Figure: Top: Simple averaging; Bottom: MFBD reconstructed A. Feller FSP IAUS 305 12 / 15

Does FSP work as expected? Fe I 6302 Å, noise behaviour (modulator off) Figure: RMS noise vs. number of averaged frames A. Feller FSP IAUS 305 13 / 15

What’s next? DEPFET/Infinipix - on-sensor charge caching In a nutshell . . . Decoupling of frame rate and modulation frequency Periodic on-sensor charge caching, in phase with pol. modulation No covered sensor areas, no charge transfer, 100% fill factor Switching time ∼ 100 ns Essential FSP sensor properties (e.g. QE, frame rate, noise char., . . . ) are conserved Heritage from particle physics and X-ray astronomy (BELLE-II, MIXS, ATHENA, . . . ) EC "Horizon 2020" proposal submitted: polarimetry tests with 32x32 4-DEPFET prototype sensor (2016-2018) A. Feller FSP IAUS 305 14 / 15

Summary Summary For high-precision polarimetry the light gathering capability of a large-aperture telescope is more important than pushing diffraction-limited resolution! FSP combines high duty cycle and fast modulation, which is essential for polarimetry at increased spatial resolution The FSP I prototype has successfully demonstrated the potential of this novel polarimetry concept With future large-aperture solar telescopes at the horizon we will try to improve solar polarimetry, based on pnCCD (and potentially DEPFET) sensor technology A. Feller FSP IAUS 305 15 / 15

Appendix Why fast modulation? Why fast modulation? seeing, jitter, ... pol. beamsplitter I u (t) u S sensor modulator d I d (t) Dual-beam modulation 4 I u ( t 1 ) = 1 ( I + δ I 1 ) + 1 � g S i + δ S i , 1 2 2 i = 2 4 I d ( t 1 ) = 1 2 ( g + δ g )( I + δ I 1 ) − 1 � S i + δ S i , 1 2 i = 2 A. Feller FSP IAUS 305 1 / 9

Appendix Why fast modulation? Why fast modulation? seeing, jitter, ... pol. beamsplitter I u (t) u S sensor modulator d I d (t) Dual-beam modulation with 2nd beam-exchange measurement 4 I u ( t 2 ) = 1 ( I + δ I 2 ) − 1 � g S i + δ S i , 2 2 2 i = 2 4 I d ( t 2 ) = 1 2 ( g + δ g )( I + δ I 2 ) + 1 � S i + δ S i , 2 2 i = 2 A. Feller FSP IAUS 305 1 / 9

Appendix Why fast modulation? Why fast modulation? seeing, jitter, ... pol. beamsplitter I u (t) u S sensor modulator d I d (t) Modulated intensities after dual beam + beam exchange (neglecting higher-order errors) 4 � � � S i + δ S i , 1 + δ S i , 2 � g + δ g � I 1 = I u ( t 1 ) − I u ( t 2 ) − I d ( t 1 ) + I d ( t 2 ) ≈ m 1 , i 2 2 i = 2 Same for I 2 and I 3 . . . ( S 1 , 2 , 3 : Stokes Q, U, V; g : gain table; m : mod. matrix) A. Feller FSP IAUS 305 1 / 9

Appendix Why fast modulation? Why fast modulation? Slow dual-beam modulation is not sufficient for . . . high accuracy in the presence of strong polarization signals high spatial resolution The demodulated images still suffer from crosstalk between Q, U, V ... ... which is not reduced by AO (see poster by Nagaraju) Only corrective: Keep modulation cycle as close as possible to seeing time scale ( ∼ 10 ms) → 100 Hz modulation! A. Feller FSP IAUS 305 2 / 9

Appendix How does FSP work? FSP setup at VTT/TESOS A. Feller FSP IAUS 305 3 / 9

Appendix How does FSP work? FSP setup at VTT/TESOS A. Feller FSP IAUS 305 3 / 9

Appendix How does FSP work? Modulator SOLIS/ZIMPOL design: 2 static retarders + 2 FLCs Temp. controlled ( ± 0 . 1 K) Broadband efficiency optimization following Gisler 2006 A. Feller FSP IAUS 305 4 / 9

Appendix How does FSP work? Modulator Polarimetric efficiencies wavelength [nm] modulation frequency [Hz] A. Feller FSP IAUS 305 4 / 9

Appendix Does FSP work as expected? Fe I 6302 Å, Active region Figure: 33s averages of MFBD reconstructed frames A. Feller FSP IAUS 305 5 / 9

Appendix Does FSP work as expected? H α 6563 Å, Active region Figure: Line scan, 55s average / spectral position A. Feller FSP IAUS 305 6 / 9

Appendix Does FSP work as expected? Expected performance at a 2m telescope Fe I 6302 Å, active region VTT test meas. 2m telescope Aperture 0.7 m 2 m Efficiency 1% 2% (dual beam) Duty cycle 50% 90% Spatial sampling 0.08" 0.03" (diff. lim.) 1 spec. scan cycle (5 pos.) 15s 3.3s (solar evol.) No. of cycles 1 1 Obs. time 15s 3.3s 4 . 7 · 10 2 4 . 5 · 10 2 S/N A. Feller FSP IAUS 305 7 / 9