demonstration of a dualband ir imaging spectrometer

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1 PHYSICS Demonstration of a Dualband IR imaging Spectrometer Brian P. Beecken Physics Dept., Bethel University Paul D. LeVan Air Force Research Lab, Kirtland AFB Benjamin D. Todt Physics Dept., Bethel University 27 August 2007 San Diego, CA SPIE Conference 6660A Infrared Detectors and Focal Plane Arrays IX

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Demonstration of a Dualband IR imaging Spectrometer. 27 August 2007 San Diego, CA SPIE Conference 6660A Infrared Detectors and Focal Plane Arrays IX. Brian P. Beecken Physics Dept., Bethel University Paul D. LeVan Air Force Research Lab, Kirtland AFB Benjamin D. Todt - PowerPoint PPT Presentation

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Page 1: Demonstration of a Dualband IR imaging Spectrometer

1

PHYSICS

Demonstration of aDualband IR imaging Spectrometer

Brian P. BeeckenPhysics Dept., Bethel University

Paul D. LeVanAir Force Research Lab, Kirtland AFB

Benjamin D. TodtPhysics Dept., Bethel University

27 August 2007San Diego, CA

SPIE Conference 6660AInfrared Detectors and Focal Plane Arrays IX

Page 2: Demonstration of a Dualband IR imaging Spectrometer

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PHYSICS Classic “2 channel” Spectrometer

• Efficiencies change with λ

– Gratings

– FPA detectors

• Classic Solution: 2 channels

– Common aperture & FOV

– Beamsplitter

– 2 Dispersive elements and 2 FPAs

– Each channel optimized for roughly 1 octave of λ

• Issues

– Size

– Mass

– Power consumption

– λ Registration

– Complex

Dispersive Elements

FPA

Page 3: Demonstration of a Dualband IR imaging Spectrometer

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PHYSICS

Spectral Image, but only 1 spatial dimension

Sp

atia

l Dim

ensi

on

Dualband FPA Diffraction Concept

DispersiveElementSpectral

Dimension

DualbandFPA

Multispectral IR

Page 4: Demonstration of a Dualband IR imaging Spectrometer

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PHYSICS Using Dual-band FPA

• Gratings

– nλ = d sin θ

– Peak efficiencies atλB, λB/2, λB/3,…

• Designed Bands:

3.75 – 6.05 µm (MWIR)

7.5 – 12.1 µm (LWIR)

• λ Gap chosen to prevent spectral crosstalk

• Advantages:

– Reduced Complexity

– Smaller mass & size

– Less cooling required

– Perfect λ registration

2nd order is MWIR1st order is LWIR

320 cols x 240 rows

Page 5: Demonstration of a Dualband IR imaging Spectrometer

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PHYSICS Schematic of Dewar Optics

DualbandFPA

gratingImage formed on slit

Page 6: Demonstration of a Dualband IR imaging Spectrometer

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PHYSICS Solar Observations

• Goal: demonstrate functionality

• Why the Sun?

– Distant

– Extended body for imaging

– Significant IR signature

– It fits: solar θ ~ 0.5°, spectrometer θ ~ 1°

– Demonstrate imaging thru Earth’s atmosphere

– It is there everyday

• Issue: too much radiation → Solar filter Required

Page 7: Demonstration of a Dualband IR imaging Spectrometer

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PHYSICS

4 useable wavebands 3.75 – 4.1 µm 4.5 – 4.7 µm 8.2 – 8.5 µm 9.9 – 10.1 µm

4 4.5 5 5.50

0.5

1

Wavelength (microns)

Atm

osph

eric

tra

nsm

issi

on,

Ban

d 1

0 64 128 192 256 3200

0.5

1

Pixel number

Atm

osph

eric

tra

nsm

issi

on,

Ban

d 2

MWIR

LWIR

4 4.5 5 5.5

8 9 10 11

Atm

osph

eric

Tra

nsm

issi

on

0.5

0.5

1

1

0

0

Useable Wavebands

Diminishing Detector Response

Page 8: Demonstration of a Dualband IR imaging Spectrometer

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PHYSICS

Dual-band Spectral Image of Diameter of Solar Disk

Page 9: Demonstration of a Dualband IR imaging Spectrometer

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PHYSICS Experimental Setup

Sun

FPA

Solar image formed by telescope is allowed to pass over spectrometer slit

Solar Filter

Page 10: Demonstration of a Dualband IR imaging Spectrometer

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PHYSICS Concatenation of Single Column

Plot of Column 516 (λ = 4.6 µm)

Frame #

Row #

Page 11: Demonstration of a Dualband IR imaging Spectrometer

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PHYSICS Timing is Important

Plot of Column 600 (λ = 3.95 µm)

Frame #Row #

Page 12: Demonstration of a Dualband IR imaging Spectrometer

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PHYSICS Why is the image elliptical?

Page 13: Demonstration of a Dualband IR imaging Spectrometer

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PHYSICS Data fits to an Ellipse

Page 14: Demonstration of a Dualband IR imaging Spectrometer

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PHYSICS Circularization Process

• Concatenate one column from successive frames for composite image

• Find FWHM of each column

• Find Midpoint of cols

• Slide each col to align midpoints

• Fit top/bottom halves separately to eq for ellipse

• Find ratio of ellipse axes

• Use ratio to scale composite image horizontally

Page 15: Demonstration of a Dualband IR imaging Spectrometer

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PHYSICS Image can be Circularized

Page 16: Demonstration of a Dualband IR imaging Spectrometer

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PHYSICS Transit Angle and Time

• Astronomical calculations predict:

92 seconds at 90°

131 seconds at 45°

• Data analysis yields:

132 ± 1 seconds at 42.4° ± 0.5°

• Gratifying!

Page 17: Demonstration of a Dualband IR imaging Spectrometer

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PHYSICS Circularized Sun: MWIR

Page 18: Demonstration of a Dualband IR imaging Spectrometer

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PHYSICS Circularized Sun: LWIR

Page 19: Demonstration of a Dualband IR imaging Spectrometer

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PHYSICS Median Smoothed Sun: LWIR

Smoothing window of 5 pixels Smoothing window of 3 pixels

Page 20: Demonstration of a Dualband IR imaging Spectrometer

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PHYSICS Finding Full Width at Half Max

• Must work with bad pixels

• Find column mean value

• Avg top 10% above

• Avg bottom 10% below

• Determine halfway

• Two methods:

– Pixel values

– Contiguous pixels

– Essentially identical

Page 21: Demonstration of a Dualband IR imaging Spectrometer

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PHYSICS Solar Diameter vs. λ

122 rows108 rows

Design value of 15 arcsec for IFOV implies solar diameter of 125 pixels

Page 22: Demonstration of a Dualband IR imaging Spectrometer

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PHYSICS Sharpness vs. Size

Page 23: Demonstration of a Dualband IR imaging Spectrometer

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PHYSICS Focus Issues

• Apparatus is hard to focus on infinity

– Normally take smallest image

– Sun moves

– Therefore solar chord continually growing and shrinking!

• Two focus settings used

– First: larger image, but sharper edges

– Second: smaller image, softer edges

• Does magnification change with focus?

Page 24: Demonstration of a Dualband IR imaging Spectrometer

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PHYSICS Summary: Focus on the Future

• Blackbody at 100 m → done

• Blackbody at 1000 m

– In planning, strobe to facilitate acquisition

– Still not at infinity!

• Star

– Not possible w/o optimal focus

– Recent Dewar modification to facilitate

• Full Moon

– Limited opportunity, once per month

– Tried, but too many clouds

– Plan again for January

• Improved dualband FPA would lead to

dramatic increase in capability, in LWIR!