R&O Buoy Spectrograph System

NOAA Research and Operations Marine Optical Buoy Design Review July 18-19, 2006

R&O Buoy

Spectrograph System

Steve Brown

NIST


Research and Operations Objectives

– Transition MOBY vicarious calibration capabilities for NPP/NPOESS VIIRS & GOES-R HES– Like to maintain high spectral resolution for good matching to

satellite bands

– Adapt MOBY technology for complex coastal validation activities for GOES-R (e.g. HES)– Two spectrographs: one blue and one red

– Reduce MOBY operational costs– Reduce the size of the buoy– Operational for longer periods of time between servicing (extend

period from 3 mos. to 6 mos.)• Anti-bio-fouling more critical• Instrument stability and monitoring more critical


Reduce the measurement uncertainty

– Laboratory– Calibration source

– Stray light

– Environmental– Bio-fouling

– Self shading or shadowing

– Noise from wave-focusing, etc.

• MOBY: single channel spectrograph with optical fiber multiplexer

– each arm, each sensor data taken sequentially

– It can take 20 minutes for a scan.

• New Buoy: Simultaneous Data Acquisition


MOBY Uncertainty Elements Addressed with the new Optical System

– Reduce the stray or scattered radiation in the system

– Eliminate dichroic filter– Two separate spectrograph systems

• One for blue water

• Second to be added for coastal regions

– Systems designed for simultaneous acquisition (all ports)– Ideally like to have 8 ports to minimize self-shading effects

– Minimum # of ports required is 4


R&O Optical System Breadboard (Spring 2005)

– ISA (Jobin Yvon) f/2 spectrograph with reflective concave holographic grating; 25 m slit

– Andor 1024x256 cooled CCD array, 25 m pixels

– Four separate 1 mm diameter optical fiber inputs along entrance slit

Lens, aperture, & shutter

CCD & spectrograph

Fiber bundle

Input fibers

Shutter drive circuit

Lens, aperture, & shutter

CCD & spectrograph

Fiber bundle

Input fibers

Shutter drive circuit

Spectralon sphere

Input fibers

Spectralon sphere

Input fibers


Multi-track fibers

– Breadboard system had 1 mm fibers separated by ~500 m

Image expanded to 1 % full scale


Breadboard System Performed Well in the Laboratory'along-track' scattering – in the dispersion direction

– Stability, system response, and signal to noise ratio adequate for ocean color measurements

– Spectral stray light from optical system is better than MOBY

– Spatial stray light correction algorithm (to account for cross-track coupling) developed and successfully implemented


Cross-track CouplingWhite LED Illuminating Track 2 ONLY

0 200 400 600 800 1000 120010

0

101

102

103

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105

Pixel

Adj

uste

d

T1T2T3T4


At-Sea TestsThe breadboard system was implemented with four inputs and tested in Case 1 waters off Oahu in August 2005. The inputs were Es, Eu, Lu (0.75m) and Lu (3.25 m).


At-Sea Testing: Deployment


At-Sea Testing: Klaus Wyrtki


Preliminary Conclusions (end of August 2005)– Breadboard System

– All-fiber input simplified optical design– Superior stray light (compared to MOBY)

• A simple 2D stray light model was implemented– Satisfactory dynamic range and sensitivity demonstrated– Successfully balanced individual throughputs resulting in the same integration time,

independent of Es or Lu

– Simultaneous acquisition successful. • A full measurement, comparable to a MOBY data set, takes about 20 sec, not

20 min.• Meaningful reduction in measurement uncertainty achieved (Ken Voss)

– Outstanding issues:– Desirable to have eight fiber inputs to reduce shadowing effects– Increased spectral resolution

• Resolution degraded for top and bottom channels– Desirable to change CCD from Andor to Apogee Alta system

• Heritage: U of Miami Group is experienced with Apogee systems; some control software has been written

• Size: Andor power supply a disadvantage for buoy operation


Spectral Resolutioncompared with MOBY

Image Qualitybetween Tracks

Breadboard System


Spectrograph vendor search– Multi-channel input:

• minimum of 4 channels; 8 channels preferable– High throughput, f/# 2.4 or lower– High resolution

• Approx. 1 nm ideal• 2 nm might be acceptable

– Software control over the acquisition• Ethernet-based

– Compact – size is an issue

– Time and money concerns, looking for Commercial Off-the-Shelf (COTS) systems if possible

– Ruggedized or ruggedizable for field deployment– Heritage matters


Image Plane– Defined by choice of CCD

– 1024 by 256 element, 25 m pitch CCD– 25 mm (dispersion) x 6 mm (for multi-channel acq)

– Systems 1:1 imaging– 25 m pixel > 25 m entrance slit

System f/#– For simplicity and to maximize throughput, we wanted the f/# of

the system to match or be slightly smaller than the f/# of the input fiber– Fused silica fiber, NA of 0.22 or f/#=2.3

– Spectrograph f/# of 2.4 or smaller


System resolution defined by spectral band pass matching requirements

– High Resolution Spectra Convolved to Sensor’s Spectral Band Pass.

– Single site can service multiple sensors

–MODIS, MERIS, SeaWiFS > VIIRS, GOES-R HES, etc.

MODIS&SeaWiFS: 10 nm bands VIIRS: 20 nm bands HES: 20 nm threshold

10 nm goal MOBY: 1 nm bands > R&O: 2 nm bands

NASA new Ocean Color Satellite SeaWiFS-like: 10 nm bandsNASA: Vicarious calibration buoy: ~ 1 nm resolution

R&O - 2 nm resolution maybe okNASA - 1 nm resolution


Spectral Bandpass Matching Illustration

MODIS Terra In-Band Wavelength Uncertainty

400 450 500 5500.92

0.94

0.96

0.98

1

1.02

1.04

1.06

1.08

1.1

B8B9

B10

B11

B12

Wavelength (nm)

Lwn

ratio

(S

hifte

d/un

shift

ed)

MODIS-Terra In-Band Wavelength Uncertainty

-3.0 nm-2.0 nm-1.0 nm0.0 nm1.0 nm2.0 nm3.0 nm


Multi-track fiber consideration

– 4 input channels – keep 1 mm core diameter fiber

– or go to 500 m fiber

– 8 input channels– 500 m fibers; 250 m spacing

Future consideration

– New 1024 by 512, 25 m pitch chip coming out for the Apogee system (est. release this fall)– 12 mm slit height&image plane possible


Vendor Search

– Newport, ISA (JY), Zeiss, TECUSA, Headwall, Satlantic, Kaiser, Optronic, Instrument Systems …

– Headwall, Jobin Yvon, & Kaiser– Headwall provided aircraft instruments

• Same optical configuration as MOBY

– Kaiser

• Axial transmissive system

• Developed an in-situ ocean Raman system for MBARI

– Jobin-Yvon

• We had evaluated a JY system and it performed well.


Headwall Photonics

– Image plane not matched (6 mm horizontal, dispersion direction)

– Spectral coverage not well-matched to MOBY requirements

– Resolution: 2 nm at best

– Has required resolution– Needs custom grating– No characterization data

– Better horizontal image plane


Jobin Yvon

– CP140– COTS not a perfect match:

spectral coverage&resolution

– Custom gratings possible, but expensive (>10 K) with several months delivery

– f/2.4; simple, compact, good optical quality

– Aberrations affect imaging away from center 2 mm

– We need to develop input and CCD mounts/holders, etc.

– CP200– possible better imaging

(larger system)


Volume Transmissive Gratings

Axial Transmissive Design– Offer both standard and custom gratings – No imaging degradation over the full slit: > 10 mm

– High Throughput: f/#=1.8– High Spectral Resolution: ~ 1 nm– Minimal # of optical elements: No Moving

Parts

Improved Thermal Stability over Czerny-Turner design

Stray light < 1e-4

50 channel system demonstrated

Rugged Compact Design– Been deployed in an underwater Raman system (by MBARI)

HolographicTransmission

Grating

EntranceSlit

Multi-elementLenses

Output Plane

COTS: Kaiser Optical Systems


Kaiser Optical Systems

– Each grating a ‘master’– Relatively inexpensive and quick to modify grating

specifications

• Respond to changing vicarious calibration requirements

– Different gratings can be placed vertically within a single larger grating plane– Visible and NIR system within the same instrument


R&O Prototype: Status on Spectrograph Systems

– The JY/Andor prototype is in-house– Successful characterization measurements & field

trials with JY system

– One Kaiser system, new JY system ordered– Two Apogee camera systems ordered– Multi-track fiber inputs ordered (Romack)

– One 8-channel input system

– One 6-channel input system

All due in ~ September

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R&O Buoy Spectrograph System

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