Demonstration of a dualband ir imaging spectrometer
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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

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Demonstration of a Dualband IR imaging Spectrometer

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Demonstration of aDualband 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

Physics Dept., Bethel University


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

    • λRegistration

    • Complex

FPA

Dispersive Elements


Spectral Image, but only 1 spatial dimension

Dualband FPA Diffraction Concept

Dualband

FPA

Multispectral IR

Spatial Dimension

Dispersive

Element

Spectral

Dimension


Using Dual-band FPA

2nd order is MWIR

  • Gratings

    • nλ = d sin θ

    • Peak efficiencies at

      λB, λB/2, λB/3,…

  • Designed Bands:

    3.75 – 6.05 µm (MWIR)

    7.5 – 12.1 µm (LWIR)

  • λGap chosen to prevent spectral crosstalk

  • Advantages:

    • Reduced Complexity

    • Smaller mass & size

    • Less cooling required

    • Perfect λ registration

1st order

is LWIR

320 cols x 240 rows


Schematic of Dewar Optics

Dualband

FPA

grating

Image formed on slit


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


Useable Wavebands

MWIR

1

4 useable wavebands

3.75 – 4.1 µm

4.5 – 4.7 µm

8.2 – 8.5 µm

9.9 – 10.1 µm

0.5

0

4

4.5

5

5.5

Atmospheric Transmission

LWIR

1

0.5

0

8

9

10

11

Diminishing Detector Response


Dual-band Spectral Image of Diameter of Solar Disk


Experimental Setup

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

Sun

FPA

Solar Filter


Concatenation of Single Column

Plot of Column 516 (λ = 4.6 µm)

Row #

Frame #


Timing is Important

Plot of Column 600 (λ = 3.95 µm)

Row #

Frame #


Why is the image elliptical?


Data fits to an Ellipse


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


Image can be Circularized


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!


Circularized Sun: MWIR


Circularized Sun: LWIR


Median Smoothed Sun: LWIR

Smoothing window of 5 pixels

Smoothing window of 3 pixels


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


Solar Diameter vs. λ

122 rows

108 rows

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


Sharpness vs. Size


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?


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!


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