Standard stars for the ZIMPOL polarimeter
in the SPHERE/VLT planet finder instrument
SPHERE / ZIMPOL Science goal:
detection of reflected=polarized light from extra-solar planets
log I
Example:
Sun – Jupiter system at 5 pc
108
log I
1´´
log I
107
coro
103
X-AO
tiny polarized planetary signal in
bumpy and variable PSF halo
SPHERE-Design
Grenoble, Dec. 2011
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 (buffers are different)
• both images are subject to almost exactly the same aberrations
• integration over many modulation cycles without readout (low RON)
CPI
Focus 1
HWP2
De-rotator
HWP1
ITTM
PTTM
Polar Cal
Implementation
Focus 2 DM
DTT
S
NIR ADC
Focus 4
VIS ADC
VIS
corono
Focus 3
WF
NIR
S
corono
DTT
P
IRDIS
ZIMPOL
IFS
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
Polarimetry with VLT / SPHERE
ZIMPOL (Zurich Imaging Polarimeter)
• FoV (detector): 3.5 x 3.5 arcsec; res. 15 mas at 600 nm
• wavelength range 550-890 nm
• filters: broad R,I, …; narrow CH4, KI…; line, Hα, OI….
• Polarimetric sensitivity 10-5
SPHERE
• Extreme AO, (9mag star), Strehl ~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
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
 Internal lamps and polarization components available
 needs on sky checks with high pol. stars and twilight sky polarimetry (initial
characterization, check pol. efficiency for each run)
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
ZIMPOL polarization filters (and Bessell V, R, I)
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
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
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
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 183143
• miserable sky distribution
Distribution of ZIMPOL/SPHERE polarization standard stars
HD 23512
z~45 degr
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
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
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
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