Acknowledging contributions by others to the TWiLiTE Scanning Holographic Telescope Team:

Marc Hammond (SDL & Diffraction Ltd.) – Consultant & Designer

Matthew McGill (GSFC) – Scientist & AdviserRichard Nelson (SDL) – Designer

Quinn Young (SDL) – Thermal EngineeringBrent Bos (GSFC) – Optical Engineering, ex-COTR

Richard Rallison (Wasatch Photonics) – HOE ConsultantElroy Pearson (Wasatch Photonics) – HOE Consultant

Ultraviolet Holographic Telescope for TWiLiTE J. Hancock*, J. Swasey*, A. Shelley*, G. Schwemmer, C. Marx §,

S. Schicker*, G. Bowen*, T. Wilkerson*

*Space Dynamics LaboratoryLogan, UT 84341

§ NASA-Goddard space Flight CenterGreenbelt, MD 20771

Presentation for the Working Group on Space Based Lidar WindsMonterey, CA February 5 – 8, 2008

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

Heritage: HOE Telescope Development

PHASERS refl. HOE, 532 nm 1995/1999

HARLIE trans. HOE 1064 nm, 1998

• 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

SDL’s UV Cornerstone HOE355 nm, (design 2003/2004)

Geary Schwemmer et al.

TWiLiTE Telescope

Design ConceptSDL Optics Laboratory, December 2007

TWiLiTE telescope delivered to NASA-Goddard December 14, 2007

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:

Parameter Value

Field of View 800 µrad

Nominal alignment time 1 sec

Maximum alignment time 2 sec

Feedback control time constant

100 ms

HOE

L1 BS L2

Auto-alignmentOptics (AAS)

Laser

Feedback to fast steering mirror

Auto-alignment FOV 800 urads

Telescope FOV 200 urads

Focal spot size ~150 urads

Automatic Boresight Alignment

Design goals

Drive Design for HOE Rotation

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 – Kaydon SG180XP0A– 440C Stainless Steel

• Sprockets and Belt – Gates GT2– 176 Tooth Custom Sprocket– 22 Tooth Pulley– 1600 Tooth Belt– Gear Ratio 8:1

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

Telescope Optical Design

HOE

Receiver Fiber

Beam Splitter

Advance in HOE technology• UV operation at 355 nm

TertiarySecondary (flat)

Displacement Analysis for Optical Elements• Displacements Due to

– Thermal 20 +/-5C– Vibration

• Critical Optics– Tertiary Mirror– Secondary Mirror

Tertiary Mirror Secondary Mirror

 Axial(μm)

Radial(μm)

Tilt(μrad)

Axial(μm)

Radial(μm)

Tilt(μrad)

Thermal ±25 ±35 ±5 ±40 ±10 ±20

Vibration ±40 ±25 ±30 ±70 ±94 ±121

Displacement Total ±65 ±60 ±35 ±110 ±104 ±141

Required Tolerances ±250 ±100 ±349 ±250 ±150 ±175

Summary: Mechanical displacements are within optical tolerances

Focal Length, Diffraction Angle and Efficiency

Focal Length: 998.2 mmDiffraction Angle: 44.86 degrees

Focal Length

HOE # 1: Throughput ~ 60 %Fraction of energy (200 m spot) = 59 %Estimated size ~ 340 m

Throughput Link Budget

Parameter CDR Value Basis / Explanation

Clear aperture diam. (cm) 38.8 TWiLiTE HOE #1 measurement

Effective area (cm2) 786 (area-obscuration) x cos 45°

HOE efficiency 0.60 Pre-TWILITE Laboratory measurement

Boresight and alignment losses 0.59 TWiLiTE HOE #2 measurement

Boresight pickoff .985 Calculation for S/N=10 per pixel

Fiber throughput 0.93

Other optics 0.96

Total Optical Efficiency 0.32 Product of optical efficiencies

Total throughput (cm2) 254 Area * Total Optical Efficiency(296 desired)

FOV (µrad) 200 200 µm field stop / 1m focal length

Slew time 1 s Selected motor and gearing

Bore sight time 1 s Auto-alignment SNR calculation

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)

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

1 range bin = 250 meters(range gate =1.67 sec)

Succesful Alignment & Test at SDLfor 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, butDiffraction 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

Backup Slides on TWiLiTE Telescope

TWiLiTE System Block Diagram

Scanner Ctrl

Laser Power

Timing/Control

Data Acq.

Laser

Com

puter

ScanningTelescope

Laser Cooling

SIGNAL FIBER

ANALOG/PHOTON COUNTS, SYS DATA

Etalon Control

SYNC

AFT OPTICS

Det. Box Temp

PRESSURE VESSEL

ETALON

RE

CE

IVE

R T

EM

P C

ON

TR

OL

ETALON SPACING/PARALLELISM

A/D SIGNALFIBERWATERPOWER

INS/GPS Data

Power Dist/Sw

INS

/GP

S

DOPPLER RECEIVER

PRESSURE VESSEL

PRESSURE VESSEL

PRESSURE VESSEL Window

HOE

TWiLiTE Telescope Requirements to meet System Measurement Goals

Parameter Value Telescope Impact

Velocity accuracy (LOS projected) (m/s) 1.5 Throughput

Nadir angle (deg) 45 Throughput

Step-stare scan pattern (1-16 steps) 8 nominal Scanner torque

Scan cycle time seconds (km) 112 s (22.4 km) Scanner torque

Horizontal integration per LOS (seconds) & ground track (km)

10 s (2 km) Throughput & torque

Slew period (s) 2 - 4 Scanner torque

Field Of View 200 µrad Focal spot size, Throughput

Pointing knowledge accuracy & precision 900 µrad Scan motor system

MechanicalOptical

TWiLITE Shot Noise Limited Velocity Error

Solar Irradiance at the Top of Earth’s Atmosphere

355 nm160 W/cm2-nm

Source: Kitt Peak National Solar Observatory

ftp://nsokp.nso.edu/pub/atlas/

400 nm300 nm HOE—Diffracted UV light Undiffracted visible light

Wavelength of laserand interference filterλFilter = 0.15 or 0.25 nmblocking = 10-6 otherwise

Diffracted light (300 – 400 nm)

Undiffracted light(400 – 700 nm)

FiberDiam. = 200 mFOV = 200 radA tel = 786 cm2

Diam. = 200 mFOV = 440 mradAfiber = 0.00031 cm2

Fiber

AAS

AAS

Principal Wavelength Bands of Upward Scattered Sunlight

HOE

pickoffmirror(1.5 %)

Optical Design

Radius of

Curvature

Diam. Substrate Description

Secondary flat 8.5”, 8” CA PyrexMade by Nu-Tek, 0.5” thick, flat to 2 waves PV, Coated R> 99%

Tertiary 515 mm 3” S Fused Silica CVI

Collimator 77.3 mm 1” Fused Silica CVI

Beam Splitter flat wedge 1.5” Fused SilicaCVI, 1 degree wedge, R = 1.5% front side, R<0.75 back side

Focus Lens 20.6 mm 1” Fused Silica CVI

Alignment: Secondary, Tertiary, Periscope

Diffraction planealignment

Diffraction anglealignment

Tilt adjustment

Translation adjustment

O-ring mount

Mechanical Interface

Envelope Dimensions:25” Height30” Diameter (includes mounts and motor, 25” without)

Mounting Points (3)

Metering rods (3)

Top Plate

2.49”

3.00”

0.30”

HOE Face

Center of Laser

HOE Mount Structure

Telescope Mass: 46kg (101lb)

HOE and Bearing Mount

Bearing Sprocket Interface

Telescope Base Ring

Sprocket

HOE Ring

HOE

Tab

Bearing