Fast Solar Polarimeter
Transcript of Fast Solar Polarimeter
Fast Solar Polarimeter
A. Feller, F. Iglesias, K. Nagaraju, S. K. SolankiMax Planck Institute for Solar System Research
and colleagues from the Max Planck semiconductor lab
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Overview
Fast Solar Polarimeter (FSP) in a nutshell
Novel ground-based solar imaging polarimeter developed by MPS incollaboration with the MPG semiconductor lab (HLL) and PNSensorBased on
fast low-noise pnCCD sensor andferro-electric liquid crystals for polarization modulation
Polarimetry of small and dynamic solar structuresat increased polarimetric sensitivity (< 10−3) orat high temporal cadence
in particular also in the chromosphereDevelopment 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 with1kx1k pnCCD, dual-beam
Funded by MPG and European Commission (SOLARNET)
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Why FSP?
Photon budget and solar evolution
Tradeoff between solar evolutionand noise:
Maximum integration time ∆tallowed by solar evolution:
∆te = 2 ∆x/v
Minimum integration time toreach a given required rmsnoise level σ:
∆ts = (Fσ2∆x2)−1
∆x : spatial samplingv : evolution speedF : Flux [phot / (s · arcsec2)]
Δx
Δt
Δts
Δte
optimum (Δx, Δt) ~ F-1/3 σ-2/3
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Why FSP?
Photon budget and solar evolution
107 108 109 1010
Flux [phot / (s · arcsec2)]
10−2
10−3
10−4
RM
S no
ise
0.010", 1.0s
0.017", 0.2s
0.020", 2.1s
0.034", 0.4s
0.040", 4.1s
0.068", 0.8s0.080", 8
.3s
0.137", 1.7s
0.160",16.6s
0.274", 3.3s0.320",3
3.1s
0.547", 6.6s
Fe I 525.0 nm
CaII 393.3 nmSr I 460.7 nm
1m telescope:(Δx, Δt) = (0.12", 12.5s)
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Why FSP?
Why fast modulation?
Slow dual-beam modulation is not sufficient for . . .high accuracy in the presence of strong polarization signalshigh spatial resolution
The demodulated imagesstill 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!
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Why FSP?
FSP is beneficial for 2 dedicated observing regimes
High-precision polarimetry (σ < 10−3)Fast modulation suppresses systematic errorsImage 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 ∆t1 reconstructed Stokes image set per s possible, due to
high frame rate (400 fps)short mod. cycle (4 states)
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How does FSP work?
Main specifications
FSP I FSP IISensor size 264 px x 264 px 1024 px x 1024 pxMax. frame rate 800 fps 400 fpsPixel pitch 48 µm 36 µmQE > 90% 500 nm - 870 nm 350 nm - 500 nmDuty cycle 97% 95%RMS readout noise 3 - 4 e−
Sensitive subst. depth 450 µmReadout ASICS x number CAMEX x 4 VERITAS-1 x 16Max. data rate 0.78 Gb/s 6.7 Gb/s
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How does FSP work?
pnCCD camera
Key conceptsFast split frame transferColumn-parallel readoutNo shutter→ numerical frametransfer correction(Iglesias et al. 2015)Multi-correlateddouble-sampling to reducenoiseCustom coating to optimize QE
Thick substrate→ no internalfringing
Sensor layout scheme
From Ordavo et al. 2011
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Does FSP work as expected?
VTT test campaigns
Campaigns
Jun 2013 SpectrographNov 2013 TESOSJun 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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Does FSP work as expected?
Ca I 4227 Å, Scattering polarization
I
422.50 422.60 422.70 422.80
0
20
40
60
arcsec
Q/I
422.50 422.60 422.70 422.80
nm
0
20
40
60
arcsec
I
422.50 422.60 422.70 422.80
020
40
60
80
100
120
140
e-/(fram
e*pixel)
Q/I
422.50 422.60 422.70 422.80nm
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 SolarSpectrum (Gandorfer 2000)
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Does FSP work as expected?
Figure: Time series of 19 MFBD reconstructed line scans (1.6s / spectralposition)
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Does FSP work as expected?
Fe I 6302 Å, Quiet Sun
Figure: Top: Simple averaging; Bottom: MFBD reconstructed
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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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What’s next?
DEPFET/Infinipix - on-sensor charge caching
In a nutshell . . .Decoupling of frame rate and modulation frequencyPeriodic on-sensor charge caching, in phase with pol. modulationNo covered sensor areas, no charge transfer, 100% fill factorSwitching time ∼ 100 nsEssential FSP sensor properties(e.g. QE, frame rate, noise char., . . .) are conservedHeritage from particle physics and X-ray astronomy (BELLE-II,MIXS, ATHENA, . . .)EC "Horizon 2020" proposal submitted: polarimetry tests with32x32 4-DEPFET prototype sensor (2016-2018)
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Summary
Summary
For high-precision polarimetry the light gathering capability of alarge-aperture telescope is more important than pushingdiffraction-limited resolution!FSP combines high duty cycle and fast modulation, which isessential for polarimetry at increased spatial resolutionThe FSP I prototype has successfully demonstrated the potentialof this novel polarimetry conceptWith future large-aperture solar telescopes at the horizon we willtry to improve solar polarimetry, based on pnCCD (and potentiallyDEPFET) sensor technology
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Appendix Why fast modulation?
Why fast modulation?
modulator
senso
r
S
pol. beamsplitterIu(t)
Id(t)
seeing, jitter, ...
u
d
Dual-beam modulation
with 2nd beam-exchange measurement
Iu(t1) =12
g (I + δI1) +12
4∑i=2
Si + δSi,1
Id (t1) =12
(g + δg)(I + δI1)− 12
4∑i=2
Si + δSi,1
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Appendix Why fast modulation?
Why fast modulation?
modulator
senso
r
S
pol. beamsplitterIu(t)
Id(t)
seeing, jitter, ...
u
d
Dual-beam modulation with 2nd beam-exchange measurement
Iu(t2) =12
g (I + δI2)− 12
4∑i=2
Si + δSi,2
Id (t2) =12
(g + δg)(I + δI2) +12
4∑i=2
Si + δSi,2
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Appendix Why fast modulation?
Why fast modulation?
modulator
senso
r
S
pol. beamsplitterIu(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 +δg2
) 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 signalshigh spatial resolution
The demodulated imagesstill 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
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Appendix How does FSP work?
FSP setup at VTT/TESOS
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Appendix How does FSP work?
Modulator
SOLIS/ZIMPOL design: 2 static retarders + 2 FLCsTemp. controlled (±0.1 K)Broadband efficiency optimization following Gisler 2006
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Appendix How does FSP work?
Modulator
Polarimetric efficiencies
wavelength [nm]
modulation frequency [Hz]
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Appendix Does FSP work as expected?
Fe I 6302 Å, Active region
Figure: 33s averages of MFBD reconstructed framesA. Feller FSP IAUS 305 5 / 9
Appendix Does FSP work as expected?
Hα 6563 Å, Active region
Figure: Line scan, 55s average / spectral position
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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 telescopeAperture 0.7 m 2 mEfficiency 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 1Obs. time 15s 3.3sS/N 4.7 · 102 4.5 · 102
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Appendix Does FSP work as expected?
Expected performance at a 2m telescope
Example: Fe I 6302 Å, quiet Sun
VTT test meas. 2m telescopeAperture 0.7 m 2 mEfficiency 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 2Obs. time 230s 12.8sS/N 3 · 103 3 · 103
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Appendix What’s next?
DEPFET/Infinipix - on-sensor charge caching
A few words on the workingprinciple . . .
Based on the combineddetector-amplifier structureDEPFET (Treis et al. 2004)On-pixel, non-destructive chargesampling via FET conductivitymeasurementSuperpixel with 4 DEPFET cellsfor charge storing and readout
Shield electrodes induce periodicphoto-charge drifting into each ofthe 4 DEPFETs
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Appendix What’s next?
DEPFET/Infinipix - on-sensor charge caching
Status and next steps . . .Prel. design of 4-DEPFET Infinipix sensorEC "Horizon 2020" proposal submitted: expected funding period2016-2019First conceptual study in terms of numerical simulationsTest of a small prototype sensor (32 x 32 superpixels) to assesspotential for polarimetry
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