Download PSI: Polarimetric Spectroscopic Imager

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!