ZIMPOLCHEOPS a Polarimetric Imager for the Direct Detection

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					Astronomical Polarimetry: Current Status and Future Directions
ASP Conference Series, Vol. 343, 2005
Adamson, Aspin, Davis, and Fujiyoshi

       ZIMPOL/CHEOPS: a Polarimetric Imager for the Direct
       Detection of Extra-solar Planets

       H. M. Schmid,1 D. Gisler,1 F. Joos,1 H. P. Povel,1 J. O. Stenflo,1 M.
       Feldt,2 R. Lenzen,2 W. Brandner,2 J. Tinbergen,3 A. Quirrenbach,4 R.
       Stuik,4 R. Gratton,5 M. Turatto,5 and R. Neuh¨user6
       1 Instituteof Astronomy, ETH Zentrum, CH-8092 Zurich, Switzerland
       2 MPI   u                 o
              f¨r Astronomie, K¨nigstuhl, D-69117 Heidelberg, Germany
       3 ASTRON, P.O. Box 2, NL-7990 AA Dwingeloo, Netherlands
       4 Leiden Observatory, P.O. Box 9513, NL-2300 RA Leiden, Netherlands
       5 INAF-Osservatorio Astronomico di Padova, I-35122 Padova, Italy
       6 Astrophys. Institut Jena, Schillerg¨sschen 2,D-07745 Jena, Germany

       Abstract.      In a phase A study supported by ESO, we are currently plan-
       ning a dedicated VLT instrument for the direct detection of extra-solar planets:
       CHEOPS (CHaracterizing Exoplanets by Opto-infrared Polarization and Spec-
       troscopy). The envisaged instrument consists of an extreme adaptive optics
       system with a coronagraph, and two science channels for differential imaging,
       which are (1) a spectroscopic integral field unit in the J- and H-band opti-
       mized for the detection of methane bands, and (2) a polarimetric I-band imager
       based on the ZIMPOL technique for the search of reflected (polarized) light. We
       highlight here some aspects of the polarimetric part of this project.

1.   The CHEOPS Science Case and Polarimetric Requirements

Light reflected from Jupiter, Saturn and other planets is polarized mainly due
to Rayleigh scattering. The measured linear polarization is in the range p =
5 − 50% for phase (scattering) angles near 90◦ (e.g., Smith & Tomasko 1984;
Gratton et al. 2004; Stam et al., these proceedings). This basic property of
planets provides the possibility of detecting directly the light from extra-solar
planets using polarimetry. Moreover, the polarization has significant diagnostic
potential as it can strongly depend on latitude. For example, in Jupiter and
Saturn the poles exhibit an enhanced polarization because they are covered
with an optically thick Rayleigh scattering layer above the cloud tops. At the
equator, the radiation is predominantly reflected by clouds and the resulting
polarization is small. Spectropolarimetry shows further that the polarization is
enhanced in the methane bands (see Joos et al., these proceedings).
   The CHEOPS instrument is optimized for the search of a very weak plane-
tary signal in the adaptive optics (AO) corrected speckle halo of a bright star.
The bright star serves as wave front probe for an extreme AO system with a
deformable mirror with 1000 – 2000 actuators. Thus the AO system provides,
in combination with the VLT, coronagraphic imaging with a very high contrast
(Strehl > 0.5 in the I-band) and angular resolution (≈ 25 mas). The differential
90                 Schmid et al.

imagers are used to enhance the contrast further for the detection of a very weak
signal in the residual speckle halo from the AO system.
   Even with the VLT and such an extreme AO system the polarimetric detec-
tion of extra-solar planets is very demanding due to the very large brightness
contrast of ≈ 109 . The peak intensity of a Jupiter-like planet in a system at
5 pc (separation of 5 AU or 1 arcsecond) will be of the order of 10 −4 weaker
than the (residual) halo of the bright star at the position of the planet. Thus
the ZIMPOL imaging polarimeter must achieve a precision of order 10 −5 for a
successful detection of a polarization signal from an extra-solar planet. Because
CHEOPS is a Nasmyth focus instrument this performance has to be achieved
despite the 4% instrumental polarization from mirror M3 (see Stuik et al., these

     Figure 1.    The principles of the ZIMPOL technique. From left to right: half
     wave plate for choosing between Stokes Q and U, polarization modulator,
     polarizer, and ZIMPOL CCD. The modulator is synchronized with the CCD
     demodulator. In CHEOPS/ZIMPOL the second beam from the polarizer
     (beam splitter) goes to a second camera system.

2.   The ZIMPOL Technique

ZIMPOL (Zurich Imaging Polarimeter) is an instrument principally developed
at the Institute of Astronomy of ETH Zurich (e.g., Povel 1995). It is based on a
fast polarization modulator, e.g., a photo-elastic or ferro-electric liquid crystal
retarder, working in the kHz range in combination with a special CCD camera
performing the on-chip demodulation of the modulated signal (see Figure 1).
   The modulator and polarizer convert the polarization signal into a fractional
modulation of the intensity signal. This intensity modulation is converted back
into a polarization signal by a special ZIMPOL-CCD camera which measures
for each active pixel the intensity difference between the modulation states. In
the ZIMPOL system every second row of the CCD is masked so that charge
packages created in the unmasked row during one half of the modulation cycle
are shifted for the second half of the cycle to the next masked row, which is used
as temporary buffer storage. After many thousands of modulation periods the
CCD is read out in less than 1 second. The sum of the two images is proportional
to the intensity while the normalized difference is the polarization degree of one
ZIMPOL/CHEOPS Polarimetric Imager for Extra-solar Planets                      91

Stokes component. The CCD will be equipped with cylindrical micro-lenses
which focus the light onto the open CCD rows.
  Because the measurement is fully differential, systematic errors are reduced
to a very low level. Key advantages of this technique are:

     • images for the two opposite polarization modes are created practically
       simultaneously (the modulation is faster than the seeing variations), and,
     • both images are recorded with the same pixel, so that the two images with
       opposite polarizations would be subject to exactly the same aberrations
       introduced by the atmosphere and the telescope/instrument.

   ZIMPOL has proved to be an extremely accurate technique for polarimetric
imaging. It has achieved a precision of better than about 10−5 in spectropo-
larimetric mode for solar applications (e.g., Stenflo & Keller 1996). The same
level of precision was recently also achieved for imaging polarimetry (Gisler et
al. 2004).

3.    ZIMPOL within CHEOPS

CHEOPS consists of an extreme adaptive optics system with a coronograph and
two science channels: the I-band (650–950 nm) ZIMPOL polarimeter, and the
near-IR (950–1700 nm) integral field spectrometer (Feldt et al. 2003; Claudi et
al. 2004; Gisler et al. 2004). Both channels are designed to work simultaneously
using a dichroic beam splitter after the AO-system.
   In the planned ZIMPOL/CHEOPS setup the beam from the AO-system passes
first through the polarization modulator package, including a modulator and
polarization beam splitter producing two beams (see Figure 1). In both beams
the same polarization signal is encoded as intensity modulation. This double
beam mode provides two full ZIMPOL observations which can be reduced and
analyzed independently of each other. If different filters are introduced in the
two beams, then ZIMPOL can be used at the same time as differential imager
for the mapping of circumstellar material, for example in the Hα emission line.


Claudi, R. U., et al. 2004, SPIE, 5492, in press
Feldt, M., et al. 2003, SPIE 4860, 149
Gisler, D., et al. 2004, SPIE, 5492, in press
Gratton, R., et al. 2004, SPIE, 5492, in press
Povel, H. P. 1995, Opt. Eng., 34, 1870
Smith, P. H., & Tomasko, M. G. 1984, Icarus, 58, 35
Stenflo, J. O., & Keller, C. U. 1996, Nat, 382, 588

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