Compensation of instrumentally induced Polarization at the VLT

1
Compensation of instrumentally induced
Polarization at the VLT

• Franco Joos
• 03.02.04

2
Contents

• CHEOPS
• Polarization of Planets
• The VLT
• Telescope polarization
• Simulation of a compensation
• Summary
• Outlook

3
1. CHEOPS
Faculdade de Ciencias de Universidade de Lisboa
Max-Planck Institut für Astronomie, Heidelberg
Astronomisches Institut und Universitäts-Sternwart
e Jena
Osservatorio di Padova
ETH Zürich, Institut für Astronomie
Thüringer Landessternwarte Tautenburg
Universiteit van Amsterdam Sterrenkundig Instituut
Max-Planck Institut für extraterrestrische
Physik, Garching
Astrophysikalisches Institut Potsdam
Sterrewacht Leiden
Dipartimento di Astronomia dell' Università di
Padova
Osservatorio Astronomico di Capodimonte
4
1. CHEOPS

• Characterizing Exoplanets by Opto-Infrared
  Polarimetry and Spectroscopy
• Search for Jupiter-like planets at a distance of
  several pc's at separations of more than 3 AU
  from the central star.
• NGST (launched around 2012) will provide much
  better images than possible from ground for
  wave-lengths more than 2500 nm.

5
1. CHEOPS

• This favours for a ground-based planet- finder
  the short wavelengths lt 2500 nm
• Two possibilities for planet candidates - old
  cold planets (only reflected light) -
  young hot planets (thermal emission)
  distant objects (gt50 pc) at large separation
  

6
1. CHEOPS
Stars
Brown Dwarfs
Young
Planets
Old
Jupiter
Burrows et al. 1993, 1997
7
1. CHEOPS

• Expected photon flux from an old planet has a
  contrast of about 10-8 compared to the central
  star (solar type)
• Idea of polarimetry to enhance the contrast
• Unpolarized central star expected (solar type)

8
1. CHEOPS

• Highly polarized planet, due to Rayleigh
  scattering at nearly 90 degrees phase angle.

9
1. CHEOPS
Basic concept
10
1. CHEOPS
11
2. Polarization of Planets
Slide by Hans Martin Schmid
12
2. Polarization of Planets
Polarization signals from Jupiter and Saturn in
the range of 8 and 2, respec- tively (maximum
at the poles). High polarization of gas
planets, where the gas layers are obove the
clouds.
I
Q/I
U/I
Jupiter Saturn with ZIMPOL at Kitt Peak, Daniel
Gisler, 2002
13
3. The VLT

• 8.2 m in diameter
• Alt-azimuth mounting
• Defraction limited resolution of 0.03'' at
  1micron
• Four foci (Cassegrain, Coudé and 2 Nasmyth)
• Low telescope induced polarization for the
  Cassegrain focus, but not feasible for large and
  heavy instruments
• -gt Nasmyth focus, but high polarization by M3

14
3. The VLT
Optical layout
15
4. Telescope polarization
Zenith angle 0 deg
Polarization induzed by the Telescope mirror M
3 3 to 5 , depending on the wavelength
16
5. Simulation of a compensation

• Mirror M3 produces linear polarization due to a
  45 degree reflection.
• Inserting an additional mirror 'M4' after M3, at
  a relative angle of 90 degrees compared to M3
  and an incident angle of also 45 degrees
  avoids the induced polarization by M3.
• But this only works at a zenith angle of 0
  degrees because 'M4' is fix.

17
5. Simulation of a compensation

• To 'simulate' a permanent zenith angle of 0
  degrees for each altitude we
  intend to insert a halfwaveplate
  between M3 and 'M4' with the fast axis
  at an angle a to the horizon, when the
  zenith angle is 2 a.

18
5. Simulation of a compensation
Halfwave plate
M4
Q
-Q
M3
19
5. Simulation of a compensation
Zenith angle 0 deg
White only M3 red M3 with additional 'M4'
20
5. Simulation of a compensation
Zenith angle 45 deg
white only M3 red M3 with additional 'M4'
21
5. Simulation of a compensation
Zenith angle 90 deg
white only M3 red M3 with additional 'M4'
22
5. Simulation of a compensation
Zenith angle 0 deg
white only M3 red M3 with additional
'M4' yellow M3, 'M4' and halfwaveplate in between
23
5. Simulation of a compensation
Zenith angle 45 deg
white only M3 red M3 with additional
'M4' yellow M3, 'M4' and halfwaveplate in between
24
5. Simulation of a compensation
Zenith angle 90 deg
white only M3 red M3 with additional
'M4' yellow M3, 'M4' and halfwaveplate in between
25
5. Simulation of a compensation
Zenith angle 0 deg
M3, 'M4', halfwaveplate in between AO
26
5. Simulation of a compensation
Zenith angle 45 deg
M3, 'M4', halfwaveplate in between AO
27
5. Simulation of a compensation
Zenith angle 90 deg
M3, 'M4', halfwaveplate in between AO
28
6. Summary

• Complete elimination of telescope induzed
  polarization
• Holds only in theory, where the two mirrors have
  exactly the same optical properties
• Loss of total transmittance about 10 at 800 nm
• Gold layers instead of Aluminium would be better,
  but ...

29
6. Summary

• Easy to manage
• Not too expensive if the halfwaveplate is near to
  the focus (superachromat of 25 mm diameter about
  9000 euros, Halle )
• Always the same orientation of polarization
  reference system (no altitude dependence)
• Feasible for every Nasmyth focus

30
7. Outlook

• Further studies on feasibility
• what about space on the Nasmyth platform?
• Decision by ESO on project in spring 2005
• Final design 2005 2006
• Construction 2006 2008

31
(No Transcript)
32
THE END
33
8. Appendix
Zenith angle 0 deg
Entering Stokes (I,Q,0,0)
34
8. Appendix
Zenith angle 45 deg
Entering Stokes (I,Q,0,0)
35
8. Appendix
Zenith angle 90 deg
Entering Stokes (I,Q,0,0)