Astronomy 920 Commissioning of the Robert Stobie Prime Focus Imaging Spectrograph

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Astronomy 920Commissioning of the Robert Stobie
Prime Focus Imaging Spectrograph

• Meeting 11
• Polarimetry Waveplate Modulation Calibration
• Ken Nordsieck

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Polarimetric Optics
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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

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Grating Spectropolarimetry
No Beamsplitter
Beamsplitter
E
Unpolarized
O
900 nm
600 nm

• Setup
• Tungsten Continuum
• Test slitmask masked to center 4
• Polaroid screwed to slitmask
• Polarization signal (E-O)/(EO)
• Dual Beam cancels out transparency changes

HN32 Polaroid
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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)

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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)

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Dual-Beam Polarimeter Basics

• Signal
• f I(E) I(O) / I(E) I(O)
• Q / I
• 1/2 (1-cos t) (Q/I cos 4? U/I sin 4?) sin
  t V/I sin 2?
• Linear pol (Q,U) modulated 100 at 4? by t 1/2
  ? waveplate
• Circular pol (V) modulated 100 at 2? by t 1/4
  ? waveplate

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PFIS waveplate patterns

• 1/2 waveplate followed optionally by 1/4
  waveplate
• Linear Circular All-Stokes
• 1/2 ? 1/4 ? 1/2 ? 1/4 ? 1/2 ? 1/4 ?
• 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

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Halfwave Efficiency

• Efficiency (1 cos t ) / 2
• Measure efficiency with polaroid at slitmask
• Absolute axis angle measured with stars of known
  polarization

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Halfwave Modulation test
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Quarterwave Efficiency

• Predicted efficiency sin t
• Measure with polaroid 1/4 waveplate at slitmask

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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 lt 0.1 (typical)
• Measure with known unpolarized stars

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Commissioning Status

• Ready, limited by telescope Imaging, Long Slit
• Not ready, with my guesses
• Fabry-Perot March
• MOS April
• Polarimetry May