Samuel C. Barden Frank Hill

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

2. 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.

3. 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 BandpassFilters or HLBFs).

4. 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)

5. 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 Light Efficiency of Transmitted Light

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

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

8. 6301.500 Å PSI Optical ModelSpot Diagrams for 6301.5 and 6302.5 Å at center, mid radius, full RBoxes = 2x2 pixels, Circle = Airy Disk 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 .

9. PSI Image Format PSI has minimal slit curvature. Distance along slit 0° 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, 0.25 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 Spectral Dispersion 0.1° 0°

10. 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.

11. Hα Grating On-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 Grating

12. 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.

13. 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 near-infrared (0.3-2.7 μm).

14. PSI Estimated Efficiency 63% of incident photons detected in combined channels (70% detected if filter is removed) Assumes telescope is two mirrors with corrector. For solar observations For nighttime stellar observations 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.

15. 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!