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 pixels2), 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: ∆te = 2 ∆x/v Minimum integration time to reach a given required rms noise level σ: ∆ts = (Fσ2∆x2)−1 ∆x: spatial sampling v: evolution speed F: Flux [phot / (s · arcsec2)]

Δx Δt Δts Δte

• ptimum (Δx, Δt) ~ F-1/3 σ-2/3
• A. Feller

FSP IAUS 305 3 / 15

Why FSP?

Photon budget and solar evolution

107 108 109 1010 Flux [phot / (s · arcsec2)] 10−2 10−3 10−4 RMS noise

0.010", 1.0s 0.017", 0.2s 0.020", 2.1s 0.034", 0.4s 0.040", 4.1s 0.068", 0.8s 0.080", 8.3s 0.137", 1.7s 0.160",16.6s 0.274", 3.3s 0.320",33.1s 0.547", 6.6s Fe I 525.0 nm CaII 393.3 nm Sr I 460.7 nm

1m telescope: (Δx, Δt) = (0.12", 12.5s)

• 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 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 Spectrograph Nov 2013 TESOS Jun 2014 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

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FSP IAUS 305 9 / 15

Does FSP work as expected?

Ca I 4227 Å, Scattering polarization

I

422.50 422.60 422.70 422.80 20 40 60 arcsec

Q/I

422.50 422.60 422.70 422.80 nm 20 40 60 arcsec

I

422.50 422.60 422.70 422.80 20 40 60 80 100 120 140 e-/(frame*pixel)

Q/I

422.50 422.60 422.70 422.80 nm 0.0 0.5 1.0 1.5 2.0 2.5 %

Figure: Black: FSP obs. at µ ∼ 0.15; Blue line: atlas of the Second Solar Spectrum (Gandorfer 2000)

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FSP IAUS 305 10 / 15

Does FSP work as expected?

Figure: Time series of 19 MFBD reconstructed line scans (1.6s / spectral position)

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FSP IAUS 305 11 / 15

Does FSP work as expected?

Fe I 6302 Å, Quiet Sun

Figure: Top: Simple averaging; Bottom: MFBD reconstructed

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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

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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)

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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

• f 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?

modulator

sensor

S

• pol. beamsplitter

Iu(t) Id(t) seeing, jitter, ... u d

Dual-beam modulation Iu(t1) = 1 2 g (I + δI1) + 1 2

4

• i=2

Si + δSi,1 Id(t1) = 1 2(g + δg)(I + δI1) − 1 2

4

• i=2

Si + δSi,1

• A. Feller

FSP IAUS 305 1 / 9

Appendix Why fast modulation?

Why fast modulation?

modulator

sensor

S

• pol. beamsplitter

Iu(t) Id(t) seeing, jitter, ... u d

Dual-beam modulation with 2nd beam-exchange measurement Iu(t2) = 1 2 g (I + δI2) − 1 2

4

• i=2

Si + δSi,2 Id(t2) = 1 2(g + δg)(I + δI2) + 1 2

4

• i=2

Si + δSi,2

• A. Feller

FSP IAUS 305 1 / 9

Appendix Why fast modulation?

Why fast modulation?

modulator

sensor

S

• pol. beamsplitter

Iu(t) Id(t) seeing, jitter, ... u d

Modulated intensities after dual beam + beam exchange (neglecting higher-order errors) I1 = Iu(t1) − Iu(t2) − Id(t1) + Id(t2) ≈

• g + δg

2

• 4
• i=2

m1,i

• Si + δSi,1 + δSi,2

2

• Same for I2 and I3 . . . (S1,2,3: 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

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Appendix Does FSP work as expected?

Hα 6563 Å, Active region

Figure: Line scan, 55s average / spectral position

• A. Feller

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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 · 102 4.5 · 102

• 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 · 103 3 · 103

• 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

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