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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 C – Non-dispersive Reflection D – Dispersive Reflection VPHGs 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 Description The 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 Bandpass Filters or HLBFs). PSI Concept Schematic Two 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! Efficiency of Diffracted Light Efficiency of Transmitted Light Model shows minimum P-pol diffraction efficiency of ~4x10-8 at design wavelength. Note that the desired efficiency target might be more like 9095% in order to allow sufficient light from the bandpass to illuminate the 3rd channel. Sample 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-1 S-pol Polarizer HLBF-1 S-pol HLBF-2 P-pol Half Wave Plate Polarizer Removed Both polarizations visible Polarizer Position 1 S polarization visible Polarizer Position 2 P polarization visible 30 second video showing effect of rotating input polarizer. HLBF-2 P-pol PSI Optical Model Paraxial 40 cm f/16.4 telescope Collimator and Camera lenses have same prescription Real f/16.4 Collimator and Cameras 4kx4k 15 micron Detectors Doublet Lens Camera Doublet Lens Camera S-Pol Channel 80 mm Beam Diameter Image Channel Tel Focal Plane Doublet Objective Collimator Bandpass Filter and Polarization Modulators Half Wave Plate to rotate P-Pol Channel by 90° Doublet Lens Camera P-Pol Channel 6301.500 Å 2 x 2 pixels = 30 μm 0.024 Å/pixel Dispersion 0.47"/pixel spatial scale 2 pix λ/Δλ resolution = 131,280 Zoom in of 6301.5 to 6301.788 in 0.048 Å steps. 6301.788 Å PSI Optical Model Spot Diagrams for 6301.5 and 6302.5 Å at center, mid radius, full R . Boxes = 2x2 pixels, Circle = Airy Disk PSI Image Format Distance along slit 0° 0.1° 0.2° 0.25° PSI has minimal slit curvature. 0.2° One quadrant of spectrally dispersed detector shown. Spectral Dispersion 0.1° 0° Field Positions (degrees): 0.0 0.0, 0.1, 0.2, 0.25 0.1 0.0, 0.1, 0.2, 0.25 0.2 0.0, 0.1, 0.2 ,0.25 for constant wavelength + is center of detector Detector edge indicated by black border 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 Hα and 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. 12000 Night Sky Emission 1.0 SIGNAL H H/H Multiplex Grating Measured Efficiency for 1200 l/mm Component 10000 Measured Super-Blaze 8000 Hα Grating 0.9 6000 RCWA Predicted Super-Blaze 0.8 On-sky test of grating (18th mag galaxy) EFFICIENCY () 4000 0.7 2000 [SII] 0.6 0 0.5 650 660 = 23o 680 670 690 = 33o WAVELENGTH (nm) 0.4 0.3 700 [OIII] H Channel Spectrum = 17o 20000 0.2 H [OIII] SIGNAL 0.1 Sample multiplex grating containing two gratings 1.0 Measured Super-Blaze within one unit. 0.9 0.9 Grating0.3 fringes rotated to each other to separate = 17o spectra. PSI would only see one grating at a 0.2 time depending on which bandpass filter is 0.1 installed, so no need to rotate grating structures 0.0 300 to each other.400 500 600 700 800 900 1000 1100 WAVELENGTH (nm) 400 500 600 700 800 900 1000 1100 WAVELENGTH [OIII] (nm) H H/H Multiplex Grating Measured Efficiency for 1620 l/mm Component 0 480 490 500 510 520 WAVELENGTH (nm) RCWA Predicted Super-Blaze 0.8 EFFICIENCY () = 33o 0.4 300 10000 1.0 RCWA Predicted and aSuper-Blaze 1200 l/mm grating diffracts Hα 0.7 1620 l/mm grating diffracts Hβ light to the 0.6 = 23o same angle of diffraction. 0.5 EFFICIENCY () • • 0.0 5000 H/H Multiplex Grating Measured Efficiency for 1200 l/mm Component 0.8 15000 0.7 0.6 0.5 = 17o = 23o Measured Super-Blaze 0.4 = 33o 0.3 0.2 Hβ Grating 0.1 0.0 300 400 500 600 700 800 WAVELENGTH (nm) 900 1000 1100 PSI Doubled Dispersion By 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 Å or a 2 pixel λ/Δλ resolution = 262,560. Dichromated Gelatin Transmittance 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 nearinfrared (0.3-2.7 μm). PSI Estimated Efficiency Component Primary Mirror Secondary Mirror Corrector Lens Collimator Waveplate Analyzers Filter VPHG-1 Half Wave Plate VPHG-2 Optional Fold Mirror Camera Lens Detector S-Pol 0.98 0.98 0.97 0.97 0.97 0.90 0.92 Imaging 0.98 0.98 0.97 0.97 0.97 0.90 0.05 0.97 0.97 0.98 0.97 0.90 P-Pol 0.98 0.98 0.97 0.97 0.97 0.90 0.95 0.97 0.92 0.98 0.97 0.90 Total 0.62 0.58 0.032 Efficiency per Channel Total Fraction Photons Detected with filter 0.31 0.29 0.032 0.635 For solar observations Total Fraction Photons without filter 0.34 0.32 0.036 0.706 For nighttime stellar observations Protected Silver Protected Silver Decent AR coatings Decent AR coatings Assumes telescope is two mirrors with corrector. Optimistic? Assume want 5% light for imaging channel 63% of incident photons detected in combined channels (70% detected if filter is removed) 5% already accounted for in both polarization states 0.97 0.90 Potential 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 Cost 100 mm VPHG gratings ~ $10k each or $20k per wavelength channel Conclusion PSI offers an option of highly efficient, high resolution spectro-polarimetry with relative simplicity and low cost for a network of solar synoptic telescopes. Thanks to Frank for giving this presentation!