Optical & Radiometric Conceptual Design of EMAS Thermal Port Upgrade Kickoff Meeting June 29, 2010 Roy W. Esplin
Jan 05, 2016
Optical & Radiometric Conceptual Design of EMAS Thermal Port Upgrade
Kickoff Meeting
June 29, 2010
Roy W. Esplin
Optics and Radiometry Overview of E-MAS Cooled Optical Bench (COB)
The COB will replace the current thermal ports 3 and 4
The COB will contain a12-channel LWIR spectrometer and a single channel MWIR radiometer with space available to convert the MWIR radiometer to a 12-channel MWIR spectrometer in the future
SDL will design the LWIR spectrometer, specify and purchase its components, fabricate and test it
SDL will design the MWIR radiometer, specify its components, fabricate mechanical hardware and test it
SDL anticipates that the government will furnish the MWIR/LWIR dichroic, the MWIR lens, and the MWIR detector and preamplifier
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COB Optical Layout with Panels Removed
3LWIR Beam
MWIR Beam
LWIR Spectrometer Optical Components
Fold Mirror
Grating
Lens
Detector Array with Cold shield
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Cold Shield and Cold Optics Aperture
Cold shield with cover removed
Cold optics aperturemounted to cold shield
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Parametric model used to compute area of warm bench seen by detectors
parameter d
0.430 deg = 7.5 mradAfocal magnification = 6(7.5 mrad)/6 =1.25 mradFOV = 2*1.25mrad = 2.5 mrad
Cross-hatched, crescent-shaped area is the warm area of bench that is seen by detectors
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Background Reduction Factor of COB
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Background Change to Signal for 10K Warm Bench Temperature Change for Channel 12 and
300K Signal
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Signal-To-Noise Ratio Improvement Factor
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Temperature Stability Requirements
The cold optical bench can change more than 10K with no significant change in the detector output
The cryo-cooler temperature is controlled to 0.1K
Thus, the detector temperature is controlled to ~0.1K
The detector responsivity changes ~0.14% for a 0.1K detector temperature change
The signal change due to a 0.1K detector temperature change will be dominated by warm bench temperature changes
A warm bench temperature change of only 0.0028K produces a 0.14% change in detector output
As a point of reference the temperature control of the SABER focal plane, which uses the same type detectors, is ~0.1K
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LWIR Detector Array Solid Model
Increasing wavelength
Section View
Linear Variable Filter (LVF)
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LWIR Detector Array Dimensions
• To meet budget we need to reduce number of detector widths from 3 to 2• Based on Linear dispersion of 1.667 mm/µm• Detector array length may be too long for F/1 lens
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LWIR Spectrometer with F/1 Lens
Grating64.7 line-pairs/mm
Fold Mirror
F/1 Lens
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Polarization of LWIR Same As Current Design, MWIR will be rotated 90 Degrees
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Conclusions
SDL is already working with Teledyne Judson Technologies to finalized detector array specifications
Effects of field stop width on spectral purity not yet modeled
Dispersion may need to be reduced or F/number increased to make LWIR lens practical
We need to decide if dispersion of MWIR orthogonal to LWIR is an issue
The Cooled Optical Bench (COB) will significantly reduce measurement errors due to background emissions
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