Astronomy 920
Commissioning of the Robert Stobie
Prime Focus Imaging Spectrograph
Meeting 11
Polarimetry: Waveplate Modulation;
Calibration
Ken Nordsieck
Dec 9, 2005
Ast 920 Meeting 12
1
Polarimetric Optics
NIR Beam (future)
850 - 1700 nm
Fold/ Dichroic
Fold
Polarizing
Beamsplitter
Collimator
320 - 1700 nm
Visible Camera
320 - 900 nm
1/4 wave plate
Focal plane
Dec 9, 2005
Ast 920 Meeting 12
1/2 wave plate
2
Polarizing Beamsplitter
Calcite
E (y)
O (x)
• In Collimated beam, just before camera
• Mosaic of 9 Calcite “Wollaston” Prisms
• Splitting 4.8 – 5.8 deg (color dependent): 240 – 280 arcsec
at CCD
Dec 9, 2005
Ast 920 Meeting 12
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Grating Spectropolarimetry
No Beamsplitter
Beamsplitter
Unpolarized
E
O
900 nm
HN32 Polaroid
600 nm
• Setup
– Tungsten Continuum
– Test slitmask masked to center 4’
– Polaroid screwed to slitmask
•
•
Polarization signal = (E-O)/(E+O)
Dual Beam cancels out transparency
changes
Dec 9, 2005
Ast 920 Meeting 12
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Modulating the Signal with
Waveplates
• 1/2 and 1/4 wave retardation plates
– switch which beam has which sense of polarization to
remove response variations with time by rotating plate
– remove polarization sensitivity of optical elements after
waveplates
– measure different Stokes parameters:
• (Q,U): linear; V: circular
– Q/I = (I(y) – I(x)) / (I(y) + I(x)) (measured directly by
beamsplitter)
– U/I = (I(+45) – I(-45)) / (I(+45) + I(-45))
– V/I = (I(rcp) – I(lcp)) / (I(rcp + I(lcp)
Dec 9, 2005
Ast 920 Meeting 12
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Waveplate Optics
• Pancharatnam “superachromatic”
waveplates
– at beginning of collimator
– sandwich of 6 very thin pieces of
crystal quartz and magnesium
fluoride (birefringent materials)
– 320 nm – 1.7 microns to
accommodate visible and NIR
beams
• 1/2 wave: 100 mm diameter (full
field
• 1/4 wave: 60 mm diameter (3
arcmin field)
Dec 9, 2005
Ast 920 Meeting 12
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Dual-Beam Polarimeter Basics
Waveplate
Beamsplitter

I''(O), 1, 0, 0
I, Q, U, V
I', Q', U', V'
I''(E), -1, 0, 0

• Signal:
f = [ I’’(E) – I’’(O) ] / [ I’’(E) + I’’(O) ]
= Q’ / I’
= 1/2 (1-cos τ) (Q/I cos 4ψ + U/I sin 4ψ) + sin τ V/I sin 2ψ
• Linear pol (Q,U) modulated 100% at 4ψ by τ = 1/2 λ waveplate
• Circular pol (V) modulated 100% at 2ψ by τ = 1/4 λ waveplate
Dec 9, 2005
Ast 920 Meeting 12
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PFIS waveplate patterns
• 1/2 waveplate followed optionally by 1/4 waveplate
Linear
1/2 λ
0
45
22.5
67.5
11.25
56.25
33.75
78.75
1/4 λ
0
0
0
0
0
0
0
0
Dec 9, 2005
Circular
1/2 λ
0
0
22.5
22.5
45
45
67.5
67.5
1/4 λ
+45
-45
-45
+45
+45
-45
-45
+45
All-Stokes
1/2 λ
1/4 λ
0
0
22.5
33.75
45
67.5
67.5
101.25
90
135
112.5
168.75
135
202.5
147.5
236.25
Ast 920 Meeting 12
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Halfwave Efficiency
1
50
40
Angle
Efficiency
0.99
0.98
30
0.97
Efficiency
-- collimated
Axis angle
-- collimated
0.96
300
500
700
900
1100
1300
1500
20
1700
Wavelength (nm)
•
•
•
Dec 9, 2005
Efficiency = (1 – cos τ ) / 2
Measure efficiency with polaroid at slitmask
Absolute axis angle measured with stars of known polarization
Ast 920 Meeting 12
9
Halfwave Modulation test
(T-B)/T+B)
20050721 Halfwave Spectropolarimetry Test
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
556nm Data
556nm Fit
450nm Data
450nm Fit
0
22.5
45
67.5
90
Waveplate angle (deg)
Dec 9, 2005
Ast 920 Meeting 12
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Quarterwave Efficiency
1
40
0.98
Angle
Efficiency
35
0.96
30
Efficiency
-- collimated
Axis angle
-- collimated
0.94
quarterwave_09
0.92
300
500
700
900
1100
1300
1500
25
1700
Wavelength (nm)
• Predicted efficiency = sin τ
• Measure with polaroid + 1/4 waveplate at slitmask
Dec 9, 2005
Ast 920 Meeting 12
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Instrumental Polarization
• Linear instrumental: from nonnormal reflections off mirrors +
net asymmetry due to
– off-axis in FOV
– asymmetric illumination of mirror
at ends of track
• Predict from ray-trace < ~ 0.1%
(typical)
• Measure with known
unpolarized stars
Dec 9, 2005
Ast 920 Meeting 12
12
Commissioning Status
• Ready, limited by telescope: Imaging, Long Slit
• Not ready, with my guesses
– Fabry-Perot: March
– MOS: April
– Polarimetry: May
Dec 9, 2005
Ast 920 Meeting 12
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