1. R&O Buoy Spectrograph System
Steve Brown
NIST

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. Cross-track Coupling White LED Illuminating Track 2 ONLY

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

10. At-Sea Testing: Deployment

11. At-Sea Testing: Klaus Wyrtki

12. 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

13. Spectral Resolution compared with MOBY
Breadboard System
Spectral Resolution compared with MOBY
Image Quality between Tracks

14. 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

15. 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

16. 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

17. Spectral Bandpass Matching Illustration MODIS Terra In-Band Wavelength Uncertainty

18. 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

19. 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.

20. 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

21. 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)

22. 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

23. 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

24. 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