Optical Subsystem Roy Esplin Dave McLain

Internal Optics Bench Subassembly 2 Gut Ray Dichroic Beamsplitter (MWIR reflected, LWIR transmitted) LWIR Lens Cold Entrance Aperture Fold Mirror MWIR Lens Fold Mirrors Thermal Isolator Mount LN2 Heat Exchanger LWIR FPA Fold Mirror (MWIR Grating Replacement) LWIR Grating

Cold Entrance Aperture Position of cold aperture precisely controlled by mounting cold aperture assembly on MWIR/LWIR dichroic mount Cold aperture covers larger hole in cold shield that is less accurately positioned Cold aperture size can be changed by replacing the aperture assembly Making this cold aperture the aperture stop rather than the telecope exit pupil would reduce the sensitivity to temperature changes of the optics in front of COB, but it would decrease the optical throughput. 3 Cold Aperture Assembly Cold Shield MWIR/LWIR Dichroic Mount Replacement P2 Splitter Mount

Optical Design Highlights New optical prescriptions for the Port 4 (LWIR) and Port 3 (MWIR) were developed starting from the ZMAX prescriptions MASModelPort4.zmx and MASModelPort3.zmx and provided to SDL by UCSC New optical prescriptions use the telescope ±0.065º by ±0.065º IFOV used in MASModelPort4.zmx and MASModelPort3.zmx Telescope field stop mm x mm, entrance pupil diameter 150mm, afocal magnification 6X, exit pupil diameter 25 mm Lenses optimized together with the telescope and other optics in front of COB using Code V LWIR prescription name: EMAS_Port4_V5f_Cold.len MWIR Single-Element prescription name: EMAS_Port4_SE_V2f_Cold.len MWIR Path-Forward prescription name: EMAS_Port3_V3e_Cold.len Focal length of both the LWIR and MWIR lenses combined with the telescope is 250 mm. The focal length for the lenses alone is 250/6=41.7 mm Lens optimized with operational temperature indices of refraction Fabrication specifications for lens radii and the length of lens spacers adjusted for CTE difference between operational and ambient temperatures 4

5 Lens elements made of Germanium and ZnSe as shown on drawing View looking down at lens shows direction of dispersion Rays for only bands L1 and L12 are shown 5 LWIR Lens ZnSe Ge ZnSe Ge

Location of L1 Image of Field Stop 6 LWIR Grating LWIR Fold Mirror LWIR Filter L1 rays (Zoom 1) L1 Image of Field Stop

LWIR Lens Elements 7 Lens 1 Lens 3 Lens 2 Lens 4 Lens 5 Coatings R ≤ 1.5% avg. 6.4µm-14.0µm wavelengths both ZnSe & Ge elements Supplier: RMI

MWIR Single-Element Lens 8 Lens Material: Silicon First surface, S2 is an ellipse with conic constant K= Coating R ≤ 1.0% avg. 3.0 µm µm wavelengths Supplier: ISP

Worst-Case Total Wavefront Errors (Design Residual + 2σ Tolerance Contribution) Worst-case means across IFOV Optics with a wavefront error of 0.07 RMS waves are essentially diffraction limited Tolerances include all optical components including nominal tolerances on telescope and other optics in front of COB 1 mrad primary tilt 1.5 mrad secondary tilt 9

Lens barrel showing spring-loading of lenses 10

Lens barrel with first element removed to show hard stops for radial locating 11

Selection of Grating Blaze Wavelength µm Blaze Selected

Incidence and Diffracted Rays for µm Wavelength 13 Incident Gut Ray Diffracted Gut Ray at µm Grating Bisector of Angle Formed by Incident and Diffracted Rays is Facet Normal LWIR Fold Mirror LWIR Lens

Plane of Grating facet and Plane Containing Incident and Diffracted Rays and Facet Normal for µm Wavelength 14 Plane of Grating Facet Plane of Incident & Diffracted Rays and Facet Normal at µm Diffracted Gut Ray at µm Incident Gut Ray Facet Normal

Grating Blazed for µm Wavelength with Facets Shown Oversize (Grating Blaze Angle 15.73º) 15 Yellow Line Outlines Plane of Grating Facet

Grating 16 Material: Stress-Relieved Aluminum Type: Original Burnished Rulings Coating: Gold Grating spacing: mm Ruling Spatial Frequency: 50 lines/mm Operational temperature: 80K

Optical Beam Footprint on Grating 17

LWIR Images of Field Stop on Detector Array 18 BandCENTER WAVELENGTH label(µm) L L L L L L L L L L L L

LWIR Detector Active Areas & Field Stop Images 19 Dimensions: millimeters (mm) Solid lines: Active Areas Phantom Lines: Field Stop Images

LWIR Channel Passbands 50% points estimated by ascertaining wavelength gut ray strikes the left and right edges of detector active area 0% points estimated by finding wavelength for with ray moves from the left edge to right edge and vice versa The values given for Band L4 need to be moved slightly longer because the spatial gap between L3 and L4 recently had to be increased from 13µm to 19µm to accommodate the capabilities of the detector fabrication process 20 Band0%50%100%50%0%Width at 50%Width at 0% label(µm) L L L L L L L L L L L L

LWIR Detector Array Layout 21

FPA Mother Board 22

LWIR Detector Package Solid Model Order Sorting Filter µm µm passband Order Sorting Filter µm µm passband L1 detector L12 detector

LWIR Detector Package Drawing L6 L7 Drawing dimensions are in inches Connector mounted on circuit board mm space between filter and detectors

LWIR Order-Sorting Filter Two bandpass filters on a monolithic Germanium substrate No uncoated areas except at edges Filter 1 and Filter 2 coatings overlap in mm wide transition region Filter 1 Transmittance: ≥ 80% average µm µm ≥ 72% absolute µm µm ≤ 0.1% absolute UV – 5.85 µm ≤ 0.1% absolute 11.6 µm – 18 µm Filter 2 Transmittance: ≥ 80% average µm – µm ≥ 72% absolute µm – µm ≤ 0.1% absolute UV – 9.45 µm ≤ 0.1% absolute µm – 18 µm 25 Drawing dimensions in millimeters (mm) mm wide region where Filter 1 and Filter 2 coatings overlap Diffraction orders greater than 1 are blocked: Wavelengths > 5.85 µm in diffraction orders > 1 are dispersed to the right of Filter 1. (2*5.85 = 11.7) Wavelengths > 9.45 µm in diffraction orders > 1 are dispersed to the right of Filter 2. (2* 9.45 = 18.9)

Zero-Order Diffraction is Blocked by LWIR Lens 26

Spacer Between the First Two Lens Elements of LWIR Lens Painted To Minimize Scattered Light From Zero-Order 27 SpacerLens 1Lens 2 Zero-Order Rays

Mirrors Substrates: 6061-T6 Aluminum Machining, stress-relieving, and thermal cycling to be done by SDL Diamond turning, post-polishing, lapping and coating to be provided by NU-TEK Optical Surface Coating: Protected Gold Reflectance ≥ µm to 14.1 µm and 22.5º AOI Flatness ≤ 633 nm Surface Roughness ≤ 250 Angstroms Mounting Pads Lapped or Diamond Turned 3 Identical Fold Mirrors Diameter inches Clear Aperture inches 1 Grating Replacement Mirror Diameter inches Clear Aperture inches 28 Fold Mirrors MWIR Grating Replacement Mirror Drawing Dimensions in inches

New Port 3 Dichroic Beamsplitter New Port 3 Beamsplitter required for new cryogenic mounting inside COB Operational temperature: 80K Substrate: ZnSe Diameter: mm Thickness: 6.00 mm Clear Aperture: 34.0 mm Surface Flatness: Before coating: 633 nm After coating: 2λ power, λ/2 nm Angle of incidence: 25º Reflectance: ≥ 93% average 3.66µm to µm ≥ 83% minimum absolute 3.66µm to µm Transmittance: ≥ 80% average 6.56µm to µm ≥ 70% minimum absolute 6.56µm to µm 29 DSI Design

Window Substrate: ZnSe Diameter: mm Thickness: 3.00 mm Clear Aperture: 37.0 mm Coated Area: ≥ 41.0 mm Surface Flatness: ≤ 633 nm Angle of incidence: 30º BAR Coating Surface Reflectance: ≤ 2% avg. 3.6 µm to 14.1 µm ≤ 4% max. 3.6 µm to 14.1 µm 30 BAR Coating Reflectance Curve From ISP Optics

Optical Component Status & Drawing Numbers 31

32 “Path Ahead” There is room in COB for this MWIR lens and grating in future Si Ge Filter MWIR Grating Si Ge Prescription: EMAS_Port3_V3e_cold.len