1. Polarimetry: Waveplate Modulation; Calibration
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

2. Polarimetric Optics
Dec 9, 2005
Ast 920 Meeting 12

3. 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

4. Grating Spectropolarimetry
No Beamsplitter
Beamsplitter E
Unpolarized O
600 nm
900 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
HN32 Polaroid
Dec 9, 2005
Ast 920 Meeting 12

5. 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

6. 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

7. Dual-Beam Polarimeter Basics
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

8. PFIS waveplate patterns
1/2 waveplate followed optionally by 1/4 waveplate
Linear Circular All-Stokes
1/2 λ 1/4 λ 1/2 λ 1/4 λ 1/2 λ 1/4 λ
Dec 9, 2005
Ast 920 Meeting 12

9. Halfwave Efficiency Efficiency = (1 – cos τ ) / 2
Measure efficiency with polaroid at slitmask
Absolute axis angle measured with stars of known polarization
Dec 9, 2005
Ast 920 Meeting 12

10. Halfwave Modulation test
Dec 9, 2005
Ast 920 Meeting 12

11. Quarterwave Efficiency
Predicted efficiency = sin τ
Measure with polaroid + 1/4 waveplate at slitmask
Dec 9, 2005
Ast 920 Meeting 12

12. Instrumental Polarization
Linear instrumental: from non-normal 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

13. 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