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

2 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

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

4 PHYSICS Using Dual-band FPA Gratings – nλ = d sin θ – Peak efficiencies at λ B, λ B /2, λ B /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 2 nd order is MWIR 1st order is LWIR 320 cols x 240 rows

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

6 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

7 PHYSICS 4 useable wavebands 3.75 – 4.1 µm 4.5 – 4.7 µm 8.2 – 8.5 µm 9.9 – 10.1 µm MWIR LWIR Atmospheric Transmission Useable Wavebands Diminishing Detector Response

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

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

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

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

12 PHYSICS Why is the image elliptical?

13 PHYSICS Data fits to an Ellipse

14 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

15 PHYSICS Image can be Circularized

16 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!

17 PHYSICS Circularized Sun: MWIR

18 PHYSICS Circularized Sun: LWIR

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

20 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

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

22 PHYSICS Sharpness vs. Size

23 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?

24 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!