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Ultraviolet Holographic Telescope for TWiLiTE

This presentation outlines the design and requirements of the Ultraviolet Holographic Telescope (TWiLiTE), including the auto-alignment system, HOE rotary drive, optical system, and opto-mechanical integrity. It also discusses the properties of the first UV HOEs, solar background light, alignment and test results, and backup information.

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Ultraviolet Holographic Telescope for TWiLiTE

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  1. Ultraviolet Holographic Telescope for TWiLiTE J. Hancock*, J. Swasey*, A. Shelley*, G. Schwemmer, C. Marx §, S. Schicker*, G. Bowen*, T. Wilkerson* Acknowledging contributions by others to the TWiLiTE Scanning Holographic Telescope Team: *Space Dynamics Laboratory Logan, UT 84341 § NASA-Goddard space Flight Center Greenbelt, MD 20771 Presentation for the Working Group on Space Based Lidar Winds Monterey, CA February 5 – 8, 2008 Marc Hammond (SDL & Diffraction Ltd.) – Consultant & Designer Matthew McGill (GSFC) – Scientist & Adviser Richard Nelson (SDL) – Designer Quinn Young (SDL) – Thermal Engineering Brent Bos (GSFC) – Optical Engineering, ex-COTR Richard Rallison (Wasatch Photonics) – HOE Consultant Elroy Pearson (Wasatch Photonics) – HOE Consultant

  2. Outline of Presentation • Background of TWiLiTE Telescope • Telescope Requirements • Auto-alignment System • HOE Rotary Drive • Optical System • Opto-mechanical Integrity • Properties of First UV HOEs • Solar Background Light • Alignment and Test Results • Backup Information Slides

  3. Receiver: UV HOE (355 nm) 45-deg off-axis FOV Folded optical path 3-rod metering structure Rotating HOE (Step/Stare) Coaxial laser transmission via periscope through HOE Designer: Marc Hammond Heritage: HOE Telescope Development PHASERS refl. HOE, 532 nm 1995/1999 HARLIE trans. HOE 1064 nm, 1998 Geary Schwemmer et al. SDL’s UV Cornerstone HOE 355 nm, (design 2003/2004)

  4. TWiLiTE Telescope SDL Optics Laboratory, December 2007 Design Concept TWiLiTE telescope delivered to NASA-Goddard December 14, 2007

  5. Telescope Functional Requirements • Rcvr FOV and laser beam; conical, 45° off-nadir, N-step-stare • Integrated rotating HOE and beam steering mirrors • Step Interval 1 – 2 seconds, Alignment settling time < 1 sec • Provide pointing knowledge to ± 1 mrad • Scan motor encoder + backlash ≤ 1 mrad • Throughput to Doppler RCVR • Aperture * efficiency > 296 cm2 • Automatic bore sight (± 40 urad) • Detector & beam steering mirror (AAS system) • AAS boresight specifications:

  6. Auto-alignment FOV 800 urads Telescope FOV 200 urads Focal spot size ~150 urads L1 BS L2 Auto-alignment Optics (AAS) HOE Laser Feedback to fast steeringmirror Automatic Boresight Alignment Design goals

  7. Requirements Step size and time – Turn HOE 90 deg in 1 second Velocity error budget for azimuth angle < 0.2m/s  1 mrad Drive System Motor - Animatics SM 3430 Encoder – 4000 counts/rev Low Pressure Grease Bearing – KaydonSG180XP0A 440C Stainless Steel Azimuth Angle Pointing Knowledge • Motor Resolution – 0.20 mrad • Pulley Backlash – 0.62 mrad • Sprocket Backlash – 0.16 mrad • RSS Total – 0.67 mrad • Sum Total – 0.98 mrad Drive Design for HOE Rotation • Sprockets and Belt – Gates GT2 • 176 Tooth Custom Sprocket • 22 Tooth Pulley • 1600 Tooth Belt • Gear Ratio 8:1

  8. Telescope Optical Design Secondary (flat) Tertiary HOE Receiver Fiber Beam Splitter • Advance in HOE technology • UV operation at 355 nm

  9. Displacement Analysis for Optical Elements • Displacements Due to • Thermal 20 +/-5C • Vibration • Critical Optics • Tertiary Mirror • Secondary Mirror Summary: Mechanical displacements are within optical tolerances

  10. Focal Length Focal Length, Diffraction Angle and Efficiency Focal Length: 998.2 mm Diffraction Angle: 44.86 degrees HOE # 1: Throughput ~ 60 % Fraction of energy (200 m spot) = 59 % Estimated size ~ 340 m

  11. Throughput Link Budget

  12. Predicted Solar Background Signals for TWiLiTE Telescope (Nadir FOV) • No solar background contribution below 300 nm: Borofloat glass absorption • Visible light (400 – 700 nm) produces background (per shot per bin) at most • 0.63 photon counts (small fiber) • 2.52 counts (AAS system) 1 range bin = 250 meters (range gate =1.67 sec) • Ultraviolet light (300 – 400 nm) background (per shot per bin): • 1.0 photon counts (small fiber, narrow filter) • 4.0 counts (AAS, narrow filter) • 2.2 counts (small fiber, wide filter) • 8.9 counts (AAS, wide filter) Estimated minimum total SNR 14 - 15 for the perfectly aligned AAS signal, integrated over all altitude range bins. Adjusted simulations needed to refine the predictions of SNR as a function of AAS degree of alignment

  13. Succesful Alignment & Test at SDL for Goddard Delivery, December 2007 Mutual alignment of all telescope optics with HOE normal and rotation axis:  10 radians Best spot size (~ 340 m) for HOE # 1 & 2 at 45.0º, but Diffraction angle for initial, bearing-centered HOE = 45.9º Small shims and tilt for HOE de-centration: adjusts to 45.0º Mutual alignment  20 rad between AAS and TWiLiTE sensors (requirement:  40rad) Alignment settling time = 0.6 seconds (requirement 1 sec) Pointing accuracy < 650 rad, SD = 250 rad (res.160 rad) (requirement: 1000 rad) FOV (TWiLiTE) 320-380 rad (required 200 rad) Inference: excess due to excess spot size FOV (AAS) ± 800rad per channel (required 800 rad) Improved performance expected with recent HOE fabrication

  14. Backup Slides on TWiLiTE Telescope

  15. A/D SIGNAL FIBER WATER POWER TWiLiTE System Block Diagram Power Dist/Sw DOPPLER RECEIVER INS/GPS Data PRESSURE VESSEL ETALON ETALON SPACING/PARALLELISM Etalon Control ANALOG/PHOTON COUNTS, SYS DATA Data Acq. SIGNAL FIBER SYNC Timing/Control AFT OPTICS Computer PRESSURE VESSEL Scanning Telescope Laser Cooling Laser Laser Power RECEIVER TEMP CONTROL INS/GPS Scanner Ctrl HOE PRESSURE VESSEL Window Det. Box Temp PRESSURE VESSEL

  16. TWiLiTE Telescope Requirements to meet System Measurement Goals Mechanical Optical

  17. TWiLITE Shot Noise Limited Velocity Error

  18. Solar Irradiance at the Top of Earth’s Atmosphere Source: Kitt Peak National Solar Observatory ftp://nsokp.nso.edu/pub/atlas/ HOE—Diffracted UV light Undiffracted visible light 300 nm 400 nm 355 nm 160 W/cm2-nm Wavelength of laser and interference filter λFilter = 0.15 or 0.25 nm blocking = 10-6 otherwise

  19. Principal Wavelength Bands of Upward Scattered Sunlight Fiber Fiber Diam. = 200 m FOV = 440 mrad Afiber = 0.00031 cm2 Diam. = 200 m FOV = 200 rad A tel = 786 cm2 AAS pickoff mirror (1.5 %) AAS HOE Undiffracted light (400 – 700 nm) Diffracted light (300 – 400 nm)

  20. Optical Design

  21. Alignment: Secondary, Tertiary, Periscope Diffraction angle alignment O-ring mount Diffraction plane alignment Translation adjustment Tilt adjustment

  22. Mechanical Interface Top Plate Center of Laser Metering rods (3) 3.00” 2.49” Mounting Points (3) 0.30” HOE Mount Structure HOE Face Envelope Dimensions: 25” Height 30” Diameter (includes mounts and motor, 25” without) Telescope Mass: 46kg (101lb)

  23. HOE and Bearing Mount Telescope Base Ring Bearing Sprocket Interface Sprocket Bearing HOE Tab HOE Ring

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