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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

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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

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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

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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

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PHYSICS Schematic of Dewar Optics

DualbandFPA

gratingImage formed on slit

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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

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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

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PHYSICS

Dual-band Spectral Image of Diameter of Solar Disk

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PHYSICS Experimental Setup

Sun

FPA

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

Solar Filter

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PHYSICS Concatenation of Single Column

Plot of Column 516 (λ = 4.6 µm)

Frame #

Row #

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PHYSICS Timing is Important

Plot of Column 600 (λ = 3.95 µm)

Frame #Row #

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PHYSICS Why is the image elliptical?

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PHYSICS Data fits to an Ellipse

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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

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PHYSICS Image can be Circularized

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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!

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PHYSICS Circularized Sun: MWIR

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PHYSICS Circularized Sun: LWIR

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PHYSICS Median Smoothed Sun: LWIR

Smoothing window of 5 pixels Smoothing window of 3 pixels

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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

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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

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PHYSICS Sharpness vs. Size

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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?

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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!