PSI: Polarimetric Spectroscopic Imager - A Simple, High Efficiency, High Resolution Spectro-­Polarimeter Samuel C. Barden Frank Hill

Volume Phase Holographic Gratings 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. Four configurations of VPHGs: A – Littrow Transmission B – Non-Littrow Transmission C – Non-dispersive ReflectionD – Dispersive Reflection

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 Efficiency of Diffracted Light Efficiency of Diffracted Light Efficiency of Transmitted Light Efficiency of Transmitted Light Rigorous Coupled Wave Analysis ~100% diffraction efficiency at design wavelength! Note that the desired efficiency target might be more like % in order to allow sufficient light from the bandpass to illuminate the 3 rd channel. Model shows minimum P-pol diffraction efficiency of ~4x10 -8 at design wavelength.

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 PolarizerHalf Wave Plate HLBF-1 S-pol HLBF-2 P-pol Polarizer Removed Both polarizations visible Polarizer Position 1 S polarization visible Polarizer Position 2 P polarization visible HLBF-1 S-pol HLBF-2 P-pol 30 second video showing effect of rotating input polarizer.

PSI Optical Model Paraxial 40 cm f/16.4 telescope Real f/16.4 Collimator and Cameras 4kx4k 15 micron Detectors 80 mm Beam Diameter Tel Focal Plane Doublet Objective Collimator Doublet Lens Camera S-Pol Channel Doublet Lens Camera P-Pol Channel Doublet Lens Camera Image Channel Half Wave Plate to rotate P-Pol Channel by 90° Bandpass Filter and Polarization Modulators Collimator and Camera lenses have same prescription

PSI Optical Model Spot Diagrams for and Å at center, mid radius, full R Boxes = 2x2 pixels, Circle = Airy Disk Å Å Å/pixel Dispersion 0.47"/pixel spatial scale 2 pix λ/Δλ resolution = 131,280 2 x 2 pixels = 30 μm. Zoom in of to in Å steps.

PSI Image Format PSI has minimal slit curvature. One quadrant of spectrally dispersed detector shown. Field Positions (degrees): , 0.1, 0.2, , 0.1, 0.2, , 0.1, 0.2,0.25 for constant wavelength + is center of detector Detector edge indicated by black border Spectral Dispersion 0°0.1°0.2°0.25° Distance along slit 0° 0.1° 0.2°

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.

Sample multiplex grating containing two gratings within one unit l/mm grating diffracts Hα and a 1620 l/mm grating diffracts Hβ light 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. Hα Grating Hβ Grating On-sky test of grating (18 th mag galaxy)

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 Å/pixel at Å or a 2 pixel λ/Δλ resolution = 262,560.

Dichromated Gelatin Typical VPHGs can be fabricated to work at design wavelengths across the optical and near- infrared ( μm). Transmittance of dichromated gelatin as a function of wavelength for a 15  m thick layer which has been uniformly exposed and processed.

PSI Estimated Efficiency ComponentS-PolP-PolImaging Primary Mirror0.98 Protected Silver Secondary Mirror0.98 Protected Silver Corrector Lens0.97 Decent AR coatings Collimator0.97 Decent AR coatings Waveplate Analyzers0.97 Filter0.90 Optimistic? VPHG Assume want 5% light for imaging channel Half Wave Plate0.97 VPHG % already accounted for in both polarization states Optional Fold Mirror0.98 Camera Lens0.97 Detector0.90 Total Efficiency per Channel Total Fraction Photons Detected with filter Total Fraction Photons without filter % of incident photons detected in combined channels (70% detected if filter is removed) 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. Assumes telescope is two mirrors with corrector. For solar observations For nighttime stellar observations

PSI VPHG Technology Cost 100 mm VPHG gratings ~ $10k each or $20k per wavelength channel PSI offers an option of highly efficient, high resolution spectro-polarimetry with relative simplicity and low cost for a network of solar synoptic telescopes. Conclusion Thanks to Frank for giving this presentation!