standard stars for the zimpol polarimeter in the sphere/vlt planet finder instrument
DESCRIPTION
Standard stars for the ZIMPOL polarimeter in the SPHERE/VLT planet finder instrument. log I. log I. log I. 10 8. 10 7. 10 3. SPHERE / ZIMPOL Science goal: detection of reflected=polarized light from extra-solar planets. Example: Sun – Jupiter system at 5 pc. 1´´. coro. X-AO. - PowerPoint PPT PresentationTRANSCRIPT
Standard stars for the ZIMPOL polarimeter in the SPHERE/VLT planet finder instrument
1´´
108
log I
103
log I
107
log I
Example: Sun – Jupiter system at 5 pc
X-AO
tiny polarized planetary signal in bumpy and variable PSF halo
SPHERE / ZIMPOL Science goal: detection of reflected=polarized light from extra-solar planets
coro
SPHERE-Design
Grenoble, Dec. 2011
synchronization (kHz)
modulator
polarizer
demodulating CCD detector
S(t) I(t)Spolarization signal
modulatedpolarization signal
modulatedintensitysignal
ZIMPOL: basic polarimetric principle (fast modulation)
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)
SPHERESPHERE•DerotatorDerotator•40x40 X-AO system 40x40 X-AO system •vis. Wavefront sensorvis. Wavefront sensor•IR and vis. CoronagraphIR and vis. Coronagraph•2 IR and 1 vis. Focal plane inst.2 IR and 1 vis. Focal plane inst.
CPICPI
IRDISIRDIS IFSIFS
ZIMPOLZIMPOL
ITTMITTMPTTMPTTM
DMDM
DTTDTTPP
DTTDTTSS
WFWFSS
De-rotatorDe-rotator
VIS ADCVIS ADCNIR ADCNIR ADC
Focus 1Focus 1
Focus 2Focus 2
Focus 3Focus 3
Focus 4Focus 4
NIR NIR coronocorono
VIS VIS coronocorono
HWP2HWP2
HWP1HWP1
Polar CalPolar Cal
ZIMPOLZIMPOL•fast mod. Imaging fast mod. Imaging pol.pol.•2 arms2 arms
ImplementatiImplementationon
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 requirementsExtra-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
z~45 degr
Distribution of ZIMPOL/SPHERE polarization standard stars
HD 23512
p
wl
Problem:For each standard star we needpolarization value for each ZIMPOL filter
Not available:- accurate spectropolarimetry- investigation of pol. variability
For high precision polarimetry: quality of standard stars muststill be established
Special issues for polarimetry with AO systems
Is the instrumental polarization of off-axis point sources different fromcentral bright source are all faint sources potential polarized planets
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
There is hardly a field on the sky with several bright stars for the testing of FoV effects
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