PSI: Polarimetric Spectroscopic Imager - A Simple, High Efficiency, High Resolution Spectro-Polarimeter

PSI: Polarimetric Spectroscopic Imager - A Simple, High Efficiency, High Resolution Spectro-PolarimeterSamuel C. BardenFrank HillVolume Phase Holographic Gratings

VPHGs diffract light via modulations of refractive index in thin gelatin layer.

Very high efficiency set by d (grating thickness) and n (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 TransmissionC Non-dispersive ReflectionD Dispersive ReflectionVPH Background Information. ESO, AAO, HET, SALT amongst international observatories utilizing VPHG instrumentation. 2PSI 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 Bandpass Filters or HLBFs).

PSI Concept SchematicTwo VPHGs with the second rotated 90 to the first.

The three beams are imaged onto 3 separate detectors.

A wave plate can be used between VPHGs to rotate the second channel. (Required if using slits)

Predicted RCWA efficiency of a grating operating at 650 nm

Efficiency of Diffracted LightEfficiency of Transmitted LightRigorous 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.Note that P-polarization is at 0% (model shows only 4e-8 efficiency) efficiency at the peak wavelength. Fabrication processes may not achieve full 100%, but typically well over 90% is achieved. If peak S-pol efficiency isnt achieved, the P-pol efficiency is still at 0%. 5Sample VPHG ElementsDemonstration of PSI concept with two sample HLBFs from Kaiser Optical Systems, Inc. ~15 mm clear apertureDesign wavelength unknown, but near-IR

PolarizerHalf Wave PlateHLBF-1S-polHLBF-2P-pol

Polarizer RemovedBoth polarizations visiblePolarizer Position 1S polarization visiblePolarizer Position 2P polarization visibleHLBF-1S-polHLBF-2P-pol30 second video showing effect of rotating input polarizer.Movie will run once. Can click on it to run again.6

PSI Optical Model

Paraxial 40 cm f/16.4 telescopeReal f/16.4 Collimator and Cameras4kx4k 15 micron Detectors80 mm Beam DiameterTel Focal PlaneDoublet ObjectiveCollimatorDoublet Lens CameraS-Pol ChannelDoublet Lens CameraP-Pol ChannelDoublet Lens CameraImage ChannelHalf Wave Plate to rotate P-Pol Channel by 90Bandpass Filter andPolarization ModulatorsCollimator and Camera lenses have same prescriptionFold mirrors are optional and shown to provide a compact arrangement where detectors could be housed next to each other.7

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

6301.500 6301.788 0.024 /pixel Dispersion0.47"/pixel spatial scale2 pix / resolution = 131,2802 x 2 pixels = 30 m.Zoom in of 6301.5 to 6301.788 in 0.048 steps.These are spots for the S-pol channel. P-pol shows identical performance. Imaging channel is also similar.8

PSI Image FormatPSI has minimal slit curvature.

One quadrant of spectrally dispersed detector shown.

Field Positions (degrees):0.00.0, 0.1, 0.2, 0.250.10.0, 0.1, 0.2, 0.250.20.0, 0.1, 0.2 ,0.25for constant wavelength

+ is center of detectorDetector edge indicated by black borderSpectral Dispersion00.10.20.25Distance along slit00.10.2I was going to state that slit curvature is better than classical spectrographs, but I havent really checked that. It may be about the same.9Possible PSI ConfigurationsDichroic 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 H 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.Image reconstruction may be difficult with on-disk images of sun. This is more of a night time application for nebula observations not really relevant. So I dont know how much you want to emphasis bullet 3.10

Sample multiplex grating containing two gratings within one unit. 1200 l/mm grating diffracts H and a1620 l/mm grating diffracts H 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.H GratingH GratingOn-sky test of grating (18th mag galaxy)This was for a grating operating at 23 degree incident angle, so is not the same configuration as for PSI, but still represents the ability to implement two gratings into a single assembly.11PSI 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 /pixel at 6301.5 or a 2 pixel / resolution = 262,560.

Raytrace shown here is from a different optical design, but is representative of the concept.12Dichromated Gelatin

Typical VPHGs can be fabricated to work at design wavelengths across the optical and near-infrared (0.3-2.7 m).Transmittance of dichromated gelatin as a function of wavelength for a 15 mm thick layer which has been uniformly exposed and processed.Gratings in the 300-400 nm regime may still have some fabrication issues due to the slight drop in transmittance of the gelatin. There are also other materials that may actually work well for the PSI style of gratings, but would need to be researched.13PSI Estimated EfficiencyComponentS-PolP-PolImagingPrimary Mirror0.980.980.98Protected SilverSecondary Mirror0.980.980.98Protected SilverCorrector Lens0.970.970.97Decent AR coatingsCollimator0.970.970.97Decent AR coatingsWaveplate Analyzers0.970.970.97Filter0.900.900.90Optimistic?VPHG-10.920.950.05Assume want 5% light for imaging channelHalf Wave Plate0.970.97VPHG-20.920.975% already accounted for in both polarization states Optional Fold Mirror0.980.98Camera Lens0.970.970.97Detector0.900.900.90Total0.620.580.032Efficiency per ChannelTotal Fraction Photons Detected with filter0.310.290.0320.635Total Fraction Photons without filter0.340.320.0360.706 63% 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 observationsFor nighttime stellar observationsI hope I got the S/N numbers right. For nighttime use, one could remove the filter since one is looking at a set of point sources rather than a continuous disk.14PSI VPHG Technology Cost100 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.ConclusionThanks to Frank for giving this presentation!