The SPHERE/ZIMPOL polarimeter for
extra-solar planetary systems
Hans Martin SCHMID, ETH Zurich
and many collaborators in the SPHERE consortium
IPAG Grenoble, F
J.L. Beuzit, D. Mouillet, P. Puget, J. Charton, G. Chauvin,
J.C. Augerau, F. Menard, P. Martinez, A. Eggenberger, et al.
ETH Zurich, CH
D. Gisler, A. Bazzon, P. Steiner, F. Joos, et al.,
ASTRON, NL
R. Rolfsema, J. Pragt, F. Rigal, J. Kragt, et al.
Univ. of Amsterdam NL C. Domink, Ch. Thalmann, R. Waters (SRON),
Leiden University NL C. Keller, F. Snik
MPIA Heidelberg, D
M. Feldt, A. Pavlov, Th. Henning, R. Lenzen, et al.
LAM Marseille F
K. Dohlen, M. Langlois (now Lyon), et al.
ESO, Garching,
M. Kasper, M. Downing, S. Deires, N. Hubin, et al.
LESIA, Meudon, F
A. Boccaletti, et al.
ONERA, F
T. Fusco et al.
INAF-Padova, I
A. Baruffolo, R. Gratton, S. Desidera, et al.
Obs. de Geneve, CH
F. Wildi, S. Udry, et al.
1. Why polarimetry?
2. Polarimetric concept for SPHERE/ZIMPOL
3. Outlook to EPOL / E-ELT Planet Finder
Why polarimetry? Reflected light from planets is polarized
at the poles:
- haze scattering
at equator:
- cloud reflection
- thin layer of Rayleigh
scattering
Jupiter in blue light
p > 40 % at poles
p ~ 5-10 % at equator
p ~ 19 % integrated
Jupiter in red light
p > 40% at poles
p < 5% at equator
p ~ 11% integrated
Why polarimetry? Reflected light from disks is polarized
Why polarimetry? Differential technique for detecting planets
12
basic problem:
planet much fainter than
residual PSF halo!
PSF
10
If not, simulate!
log(counts)
8
simulated PSF
6
photon noise level
4
planet signal
2
0.0”
0.1”
0.2”
0.3”
0.4”
0.5”
differential technique: (speckle rejection)
reflection from planets and disks produce a polarization signal
on top of the unpolarized PSF from the central star
Polarimetry with VLT / SPHERE
ZIMPOL (Zurich Imaging Polarimeter)
• FoV (detector): 3.5 x 3.5 arcsec; resolution of 15 mas at 600 nm
• wavelength range 550-890 nm
• filters: broad-band R,I, …; narrow band CH4, KI…; line filters, Hα, OI….
• Polarimetric sensitivity 10 -5
SPHERE
• Extreme AO system (9mag star), Strehl up to 50% for 600-900 nm
• coronagraphy (Lyot coronagraphs, 4QPM)
• IRDIS: polarimetry in the 1 – 2.2 µm range
Goals:
• polarization contrast limit 10-8 for bright stars
• detect planets around nearby stars d < 5pc
• characterize scattered light from circumstellar disks
your high resolution and high contrast polarimetric imager at the VLT
 What about your science?
SPHERE-Design
Jan 2012  Dec 2012
ZIMPOL: basic polarimetric principle
(fast modulation)
synchronization (kHz)
polarizer
modulator
demodulating
CCD detector
S
polarization
S(t)
modulated
I(t)
modulated
signal
polarization
signal
intensity
signal
Advantages:
• images of two opposite polarization modes are created almost simultaneously
 modulation faster than seeing variations
• both images are recorded with same pixel
• both images are subject to almost exactly the same aberrations
• integration over many modulation cycles without readout (low RON)
Polarimeter implementation SPHERE
mutual constraints:
• polarimeter should not affect the AO
• AO should not destroy polarization
telescope
Nasmyth focus
pol.-switch
derotator
AO
1. telescope polarization compensated
with rotating λ/2-plate and M4 mirror
2. instrument polarization calibrated with
pol. switch
3. Instrument polarization compensated
by inclined plate
adaptive optics
compensator plate
near-IR
instruments
λ>0.95μ λ<0.9μ
BS
BS
WFS
wave front sensor
coronagraph
imaging
polarimeter
Polarimetric Details
derotator
HWP1
HWP2
M4
Pol.Cal.
pol.comp.
filters
HWPZ
Pol.Cal
FLC
Mod.
BS
SPHERE/ZIMPOL concept
• Telescope polarization corrected with HWP1 and mirror M4
pQ = Q/I = (I0–I90)/(I0+I90)
– as polarization switch to
separate instrument
polarization and
sky+telescope polarization
– to orientate the selected
polarization into the correct
direction for the derotator
Polarization pQ [%]
• HWP2 is used
0.4
0.3
+pQ(tel.+sky)
0.2
pQ(inst)
0.1
0.0
– 0.1
–pQ(tel.+sky)
• The derotator polarization is corrected with a (co-rotating)
polarization compensator
• HWPz rotates the polarization into the ZIMPOL system
• ZIMPOL performs the high precision measurement
time
ZIMPOL/SPHERE calibration plan
for (``user-friendly’’) data reduction pipeline
• Science Calibrations
–
–
–
–
Astrometric calibrations
Photometric calibrations
Telescope polarization calibrations (unpolarized standard stars)
Telescope zero point polarization angle (polarized standard stars)
• Technical Calibrations
–
–
–
–
–
Bias
Dark
Intensity flat (bad pixels)
Sky flat
Modulation/demodulation efficiency
• Instrument monitoring
–
–
–
–
–
AO+C polarization efficiency
AO+C instrument polarization
AO+C polarization crosstalk
ZIMPOL modulation crosstalk
Telescope crosstalk
Let‘s think big: ZIMPOL-SPHERE/VLT is just a test for
EPOL-EPICS/E-ELT
ZIMPOL  EPOL „optimum“ concept
HWP near intermediate focus
- rotates polarization from sky into the
direction (p or s) of M4, M5
- polarization switch (+/--) and allows
a polarimetric (self)-calibration of system
HWP near Nasmyh focus
- rotates sky and telescope polarization
into direction of instrument plane
No M6
- else variable cross talks are introduced
- else switch calibration is compromised
no M6
Publications survey 2000 to 2006 (Schmid 2007, ESO calibration workshop)
on polarimetric observations with ESO telescopes: 58 refereed papers
Distribution of polarimetric papers with respect to:
scientific topic
instrument used
other
sol. system 7%
7%
CS scatt.
9%
stellar
magn. fields
38%
AGN scatt.
17%
GRB / SN
22%
other
5%
SOFI
3%
NACO
5%
EFOSC
14%
FORS1
72%
Message:
Only well designed polarimetric systems produce a lot of science
Thank you