Fast Solar Polarimeter

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: Maximum integration time t allowed by solar evolution: t e = 2 x/v Minimum integration time to reach a given required rms noise level σ: t s = (Fσ 2 x 2 ) 1 Δt Δt s Δx Δt e optimum (Δx, Δt) ~ F -1/3 σ -2/3 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 10 4 0.320",33.1s 0.547", 6.6s 0.160",16.6s 0.274", 3.3s 1m telescope: (Δx, Δt) = (0.12", 12.5s) Fe I 525.0 nm RMS noise 10 3 10 2 0.080", 8.3s 0.137", 1.7s CaII 393.3 nm 0.040", 4.1s 0.068", 0.8s 0.020", 2.1s 0.034", 0.4s 0.010", 1.0s 0.017", 0.2s Sr I 460.7 nm 10 7 10 8 10 9 10 10 Flux [phot / (s arcsec 2 )] A. Feller FSP IAUS 305 4 / 15

Why fast modulation? Why FSP? 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 µm 36 µm QE > 90% 500 nm - 870 nm 350 nm - 500 nm Duty cycle 97% 95% RMS readout noise 3-4 e Sensitive subst. depth 450 µm 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 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 Thick substrate no internal fringing Sensor layout scheme From Ordavo et al. 2011 A. Feller FSP IAUS 305 8 / 15

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

Does FSP work as expected? Ca I 4227 Å, Scattering polarization arcsec 60 40 20 0 I 422.50 422.60 422.70 422.80 e - /(frame*pixel) 140 120 100 80 60 40 20 0 I 422.50 422.60 422.70 422.80 arcsec 60 40 20 0 Q/I 422.50 422.60 422.70 422.80 nm % 2.5 2.0 1.5 1.0 0.5 0.0 Q/I 422.50 422.60 422.70 422.80 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

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

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

Why fast modulation? Appendix Why fast modulation? S seeing, jitter,... modulator pol. beamsplitter I u (t) I d (t) u d sensor Modulated intensities after dual beam + beam exchange (neglecting higher-order errors) I 1 = I u (t 1 ) I u (t 2 ) I d (t 1 ) + I d (t 2 ) ( g + δg ) 4 ( m 1,i S i + δs ) i,1 + δs i,2 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

Modulator Appendix How does FSP work? 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

Modulator Appendix How does FSP work? 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 Hα 6563 Å, Active region Does FSP work as expected? 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 S/N 4.7 10 2 4.5 10 2 A. Feller FSP IAUS 305 7 / 9

Appendix Does FSP work as expected? Expected performance at a 2m telescope Example: Fe I 6302 Å, quiet Sun VTT test meas. 2m telescope Aperture 0.7 m 2 m Efficiency 1% 2% (dual beam) Duty cycle 50% 90% Spatial sampling 0.08" 0.06" 1 spec. scan cycle (2 pos.) 6.6s 6.4s (solar evol.) No. of cycles 35 2 Obs. time 230s 12.8s S/N 3 10 3 3 10 3 A. Feller FSP IAUS 305 7 / 9

Appendix What s next? DEPFET/Infinipix - on-sensor charge caching A few words on the working principle... Based on the combined detector-amplifier structure DEPFET (Treis et al. 2004) On-pixel, non-destructive charge sampling via FET conductivity measurement Superpixel with 4 DEPFET cells for charge storing and readout Shield electrodes induce periodic photo-charge drifting into each of the 4 DEPFETs A. Feller FSP IAUS 305 8 / 9

Appendix What s next? DEPFET/Infinipix - on-sensor charge caching Status and next steps... Prel. design of 4-DEPFET Infinipix sensor EC "Horizon 2020" proposal submitted: expected funding period 2016-2019 First conceptual study in terms of numerical simulations Test of a small prototype sensor (32 x 32 superpixels) to assess potential for polarimetry A. Feller FSP IAUS 305 9 / 9