acknowledging contributions by others to the twilite scanning holographic telescope team:
DESCRIPTION
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 - PowerPoint PPT PresentationTRANSCRIPT
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