Demonstration of a Dualband IR imaging Spectrometer

1. 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

2. 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

3. Spectral Image, but only 1 spatial dimension Dualband FPA Diffraction Concept Dualband FPA Multispectral IR Spatial Dimension Dispersive Element Spectral Dimension

4. 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

5. Schematic of Dewar Optics Dualband FPA grating Image formed on slit

6. 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

7. 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

8. Dual-band Spectral Image of Diameter of Solar Disk

9. Experimental Setup Solar image formed by telescope is allowed to pass over spectrometer slit Sun FPA Solar Filter

10. Concatenation of Single Column Plot of Column 516 (λ = 4.6 µm) Row # Frame #

11. Timing is Important Plot of Column 600 (λ = 3.95 µm) Row # Frame #

12. Why is the image elliptical?

13. Data fits to an Ellipse

14. 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

15. Image can be Circularized

16. 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!

17. Circularized Sun: MWIR

18. Circularized Sun: LWIR

19. Median Smoothed Sun: LWIR Smoothing window of 5 pixels Smoothing window of 3 pixels

20. 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

21. Solar Diameter vs. λ 122 rows 108 rows Design value of 15 arcsec for IFOV implies solar diameter of 125 pixels

22. Sharpness vs. Size

23. 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?

24. 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!