Samuel C. Barden Frank Hill

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PSI: Polarimetric Spectroscopic Imager - A Simple, High Efficiency, High Resolution Spectro-­Polarimeter. Samuel C. Barden Frank Hill. Volume Phase Holographic Gratings. Four configurations of VPHGs: A – Littrow Transmission B – Non- Littrow Transmission
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PSI: Polarimetric Spectroscopic Imager - A Simple, High Efficiency, High Resolution Spectro-­PolarimeterSamuel C. BardenFrank HillVolume Phase Holographic GratingsFour configurations of VPHGs:A – Littrow Transmission B – Non-Littrow TransmissionC – Non-dispersive Reflection D – Dispersive ReflectionVPHGs diffract light via modulations of refractive index in thin gelatin layer.Very high efficiency set by d (grating thickness) and Δn (index modulation).VPHGs are now in use in numerous night time astronomical spectrographs and are planned for many future spectrographs.PSI Concept DescriptionThe Polarimetric Spectroscopic Imager uses a key aspect of VPHG technology to simultaneously observe two orthogonal polarization modes with spectrally dispersed images plus a non-dispersed white light image.A VPHG with a line frequency diffracting light at a total angle of 90° inside the grating is a perfect polarizing beam splitter at that wavelength.Such devices are used for spatial filtering of unwanted laser lines (Kaiser Optical Systems, Inc. Holographic Laser BandpassFilters or HLBFs).PSI Concept SchematicTwo VPHG’s with the second rotated 90° to the first.The three beams are imaged onto 3 separate detectors.A ½ wave plate can be used between VPHG’s to rotate the second channel. (Required if using slits)Predicted RCWA efficiency of a grating operating at 650 nm Rigorous Coupled Wave Analysis~100% diffraction efficiency at design wavelength!Note that the desired efficiency target might be more like 90-95% in order to allow sufficient light from the bandpass to illuminate the 3rd channel.Model shows minimum P-pol diffraction efficiency of ~4x10-8 at design wavelength.Efficiency of Diffracted LightEfficiency of Transmitted LightSample VPHG Elements
  • Demonstration of PSI concept with two sample HLBFs from Kaiser Optical Systems, Inc.
  • ~15 mm clear aperture
  • Design wavelength unknown, but near-IR
  • HLBF-1S-polHLBF-2P-polHLBF-2P-polHLBF-1S-polPolarizer RemovedBoth polarizations visiblePolarizerHalf Wave PlatePolarizer Position 1S polarization visiblePolarizer Position 2P polarization visible30 second video showing effect of rotating input polarizer.PSI Optical ModelParaxial 40 cm f/16.4 telescopeReal f/16.4 Collimator and Cameras4kx4k 15 micron Detectors80 mmBeam DiameterCollimator and Camera lenses have same prescriptionDoublet Lens CameraS-Pol ChannelDoublet Lens CameraImage ChannelTel Focal PlaneDoublet ObjectiveCollimatorDoublet Lens CameraP-Pol ChannelBandpass Filter andPolarization ModulatorsHalf Wave Plate to rotate P-Pol Channel by 90°6301.500 ÅPSI Optical ModelSpot Diagrams for 6301.5 and 6302.5 Å at center, mid radius, full RBoxes = 2x2 pixels, Circle = Airy Disk2 x 2 pixels = 30 μm0.024 Å/pixel Dispersion0.47"/pixel spatial scale2 pix λ/Δλ resolution = 131,280Zoom in of 6301.5 to 6301.788 in 0.048 Å steps.6301.788 Å.PSI Image FormatPSI has minimal slit curvature.Distance along slit0°0.1°0.2°0.25°0.2°One quadrant of spectrally dispersed detector shown.Field Positions (degrees):0.0 0.0, 0.1, 0.2, 0.250.1 0.0, 0.1, 0.2, 0.250.2 0.0, 0.1, 0.2 ,0.25for constant wavelength+ is center of detectorDetector edge indicated by black borderSpectral Dispersion0.1°0°Possible PSI Configurations
  • Dichroic beam splitters allowing simultaneous multiple wavelength channels.
  • Multi-slit with spatial scanning.
  • No slit with image deconvolution / tomographic reconstruction.
  • Alternating wavelength regions by use of VPHG containing two gratings in single assembly (see next slide). Filter bandpass would be interchanged to “activate” alternative grating. For example a channel alternating between Hαand CaII IRT.
  • PSI could also be used for night-time surveys of star clusters for flares, etc. with either slit aperture plate or no slits at telescope focal plane.Hα GratingOn-sky test of grating (18th mag galaxy)
  • Sample multiplex grating containing two gratings within one unit.
  • 1200 l/mm grating diffracts Hα and a
  • 1620 l/mm grating diffracts Hβlight to the same angle of diffraction.
  • Grating fringes rotated to each other to separate spectra. PSI would only see one grating at a time depending on which bandpass filter is installed, so no need to rotate grating structures to each other.
  • Hβ GratingPSI Doubled DispersionBy daisy-chaining two VPHGs in series, the dispersion can be doubled without significant loss of efficiency due to the inherently high VPHG efficiency.The proposed PSI concept could have a dispersion of 0.012 Å/pixel at 6301.5 Åor a 2 pixel λ/Δλ resolution = 262,560.Dichromated GelatinTransmittance of dichromated gelatin as a function of wavelength for a 15 mm thick layer which has been uniformly exposed and processed.Typical VPHGs can be fabricated to work at design wavelengths across the optical and near-infrared (0.3-2.7 μm).PSI Estimated Efficiency63% of incident photons detected in combined channels(70% detected if filter is removed)Assumes telescope is two mirrors with corrector.For solar observationsFor nighttime stellar observationsPotential nighttime usage:PSI could measure S/N(2px) = 500 in ~300 sec for R~3 magnitude star at full resolution in each polarimetric channel.PSI VPHG Technology Cost100 mm VPHG gratings ~ $10k each or $20k per wavelength channelPSI offers an option of highly efficient, high resolution spectro-polarimetry with relative simplicity and low cost for a network of solar synoptic telescopes.ConclusionThanks to Frank for giving this presentation!
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