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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