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 This is a undesirable sampling feature. There can be variability due to changing solar zenith angle and atmospheric conditions, requiring normalization procedures that introduce measurement uncertainty

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 mm slit Andor 1024x256 cooled CCD array, 25 mm pixels Four separate 1 mm diameter optical fiber inputs along entrance slit

Image expanded to 1 % full scale Multi-track fibers Breadboard system had 1 mm fibers separated by ~500 mm 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 Coupling White LED Illuminating Track 2 ONLY

At-Sea Tests The 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 Resolution compared with MOBY Breadboard System Spectral Resolution compared with MOBY Image Quality between Tracks

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 System f/# Defined by choice of CCD 1024 by 256 element, 25 mm pitch CCD 25 mm (dispersion) x 6 mm (for multi-channel acq) Systems 1:1 imaging 25 mm pixel > 25 mm 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 bands NASA: Vicarious calibration buoy: ~ 1 nm resolution R&O - 2 nm resolution maybe ok NASA - 1 nm resolution

Spectral Bandpass Matching Illustration MODIS Terra In-Band Wavelength Uncertainty

Multi-track fiber consideration 4 input channels keep 1 mm core diameter fiber or go to 500 mm fiber 8 input channels 500 mm fibers; 250 mm spacing Future consideration New 1024 by 512, 25 mm 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) COTS: Kaiser Optical Systems Holographic Transmission Grating Entrance Slit Multi-element Lenses Output Plane

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