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

Polarimetric Optics Dec 9, 2005 Ast 920 Meeting 12

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

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

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

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

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

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 0 0 0 +45 0 0 45 0 0 -45 22.5 33.75 22.5 0 22.5 -45 45 67.5 67.5 0 22.5 +45 67.5 101.25 11.25 0 45 +45 90 135 56.25 0 45 -45 112.5 168.75 33.75 0 67.5 -45 135 202.5 78.75 0 67.5 +45 147.5 236.25 Dec 9, 2005 Ast 920 Meeting 12

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

Halfwave Modulation test Dec 9, 2005 Ast 920 Meeting 12

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

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