1. Standard stars for the ZIMPOL polarimeter
in the SPHERE/VLT planet finder instrument

2. SPHERE / ZIMPOL Science goal:
detection of reflected=polarized light from extra-solar planets
108
log I
Example:
Sun – Jupiter system at 5 pc
107
log I
1´´
103
log I
coro
X-AO
tiny polarized planetary signal in bumpy and variable PSF halo

3. SPHERE-Design
Grenoble, Dec. 2011

6. ZIMPOL: basic polarimetric principle
(fast modulation)
synchronization (kHz)
modulator
polarizer
demodulating
CCD detector
S(t)
I(t) S
polarization
signal
modulated
intensity
Advantages:
images of two opposite polarization modes are created almost simultaneously
 modulation faster than seeing variations
both images are recorded with same pixel (buffers are different)
both images are subject to almost exactly the same aberrations
integration over many modulation cycles without readout (low RON)

7. Implementation SPHERE ZIMPOL CPI ZIMPOL IFS IRDIS
ITTM
PTTM DM
DTTP
DTTS
WFS
De-rotator
VIS ADC
NIR ADC
Focus 1
Focus 2
Focus 3
Focus 4
NIR corono
VIS corono
HWP2
HWP1
Polar Cal
Implementation
SPHERE
Derotator
40x40 X-AO system
vis. Wavefront sensor
IR and vis. Coronagraph
2 IR and 1 vis. Focal plane inst.
ZIMPOL
fast mod. Imaging pol.
2 arms

8. Polarimetry with VLT / SPHERE
ZIMPOL (Zurich Imaging Polarimeter)
FoV (detector): 3.5 x 3.5 arcsec; res. 15 mas at 600 nm
wavelength range nm
filters: broad R,I, …; narrow CH4, KI…; line, Hα, OI….
Polarimetric sensitivity 10-5
SPHERE
Extreme AO, (9mag star), Strehl ~50% for 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

9. On-sky polarimetric calibration and testing needed:
Nasmyth focus plus polarization compensation
 polarization depends on pointing direction and filter
needs polarimetric calibration with 0-pol. and high-pol. standard stars (each night)
Complicated instrument: derotator, AO, beamsplitter, coronagraph
Instrument polarization depends on system configuration
Internal lamps and polarization components available
Needs on sky tests with 0-pol. and high pol standards and twilight sky polarimetry, for checks of polarimetric calibration, polarimetric structure of PSF, field dependent effects (initial characterization, yearly updates, instrument polarization checks for each run)
Fast modulation imaging polarimeter
pol. efficiency depends on wavelength, mod. frequency, frame transfer and detector X-Y position
needs on sky checks with high pol. stars and twilight sky polarimetry (initial characterization, check pol. efficiency for each run)

10. Science requirements Extra-solar planets (S/N ~ 5-10):
Measure tiny signal p< 0.01% on top of coronagraphic PSF of bright star (= 0-pol.reference)
rough rel. pol. calibration ΔQ/Q < ± 0.05 (same for U)
rough pos. angle calibration: Δθ < 5 degr
Bright targets:
binaries, circumstellar disks, resolved objects (red giants, asteroids), etc. as accurately as possible:
accurate calibration at the ΔQ/I ~ ± 0.1% (same for U)
accurate pos. angle calibration Δθ ~ 1 degr
and much higher on a relative scale within FoV

11. ZIMPOL polarization filters (and Bessell V, R, I)

12. 0-pol. calibration stars
calibrate/check telescope + instrument pol. for setup (each run)
polarization offset due to instrument / field dependence?
polarized diffraction/ghost features?
polarized structure in coronagraphic PSF?
Requirements:
brightness between mR = 3 – 8 mag (like targets)
low pol. p<0.03%, if possible p<0.01%
similar colors because of broad-band filters
no narrow-band polarization in H-alpha

13. Selection of 0-pol. calibrators
Serkowski (1974): mv<5mag (includes binaries like 36 Oph AB
Serkowski (1974): nearby 7mag G-stars (UKIRT) only few in the south
Tinbergen (1979): mv<5mag, mixed list incl. e.g. Sirius,
Hsu and Breger (1982), few good, well established objects
Turnshek et al. (1990), few good, well established objects
Other list provide not much more
Bailey et al. (2010)  mv<5 mag, 10-5 accuracy, limited sky coverage
Selection for ZIMPOL / SPHERE:
30 stars: nearby <20 pc, 4<mv<8, late F to early K, single, ``normal’’ MS
(most have measurements, but not homogeneous)
evenly distributed calibrators with similar color
7 stars from Bailey (2010) – high precision calibrators

14. high-pol. calibration stars
calibrate/check telescope + instrument pol. position angle and pol. efficiency (each run / regularly)
position angle offset for this system configuration?
polarization efficiency for this instrument mode?
Requirements:
brightness mR < 8 mag (AO compatible)
position angle accurate to Δθ < ± 1 degr
polarization known to ΔQ/I < ± 0.1%, ΔQ/Q ~ ± 0.01
polarization constant in H-alpha

15. Selection of high-pol. calibrators
Serkowski (1974): ~ 15 high polarization stars – good start
Serkowski (1975): ~300 stars (large reservoir)  mostly UBV only
Hsu and Breger (1982), few good, well established objects  UBVR[750]
Bailey & Hough (1982), few well established objects  UBVRI + IR
Whittet et al. (1992)  mostly mv>8mag (too faint for SPHERE AO)
FORS list  the classical objects, and mV>8mag objects (too faint)
Selection for ZIMPOL / SPHERE:
17 stars, mostly selected from the above sources
no Cepheids
Still many problems
Many are variables (0.1 mag level)
Supergiants, e.g. A2Iab have line emission
only few well established: o Sco, ζ Oph, HD154445, HD
miserable sky distribution

16. Distribution of ZIMPOL/SPHERE polarization standard stars
HD 23512
z~45 degr

17. For high precision polarimetry:
Problem:
For each standard star we need
polarization value for each
ZIMPOL filter
Not available:
accurate spectropolarimetry
investigation of pol. variability
For high precision polarimetry:
quality of standard stars must
still be established p wl

18. Special issues for polarimetry with AO systems
Is the instrumental polarization of off-axis point sources different from
central bright source  are all faint sources potential polarized planets

19. There is hardly a field on the sky with several bright stars for the testing of FoV effects
Figure 1. Trapezium cluster with rough outline of the ZIMPOL field of view for θ1 Ori A,B,C,D, and E (from Petr-Gotzens et al. (2008), Jour. Phys. Conf. Ser. 131

20. Wish list and things to do for SPHERE/ZIMPOL 0-pol standards:
0-pol. standard stars: study individual object to reject
critical objects (var., binarity, dust)
 observe more than one 0-pol standards per run
High polarization standards:
produce from existing photomety and polarimety
 synthetic absolute spectropolarimetry
high-pol. standard stars: investigate the existence of
spectropolarimetric data in the archives.
- study polarimetric variability limits
Test targets:
investigate extended/multiple high pol. sources on-sky
 unpolarized bright star in front of rich field with high interstellar polarization