VSWIR-Dyson Imaging Spectrometer: Design,

Alignment, Laboratory Calibration, and Testing

Robert O. Green, Byron van Gorp, Jose Rodriguez, Ian McKinley and the

Imaging Spectroscopy Team

Jet Propulsion Laboratory, California Institute of Technology

Copyright 2016 California Institute of Technology. U.S. Government sponsorship acknowledged.

• Imaging Spectroscopy

• VSWIR-Dyson Description

• Laboratory Tests and Calibration

• Sample Reflectance Measurements

• Vibration Testing

• Summary and Conclusions

Overview

Imaging Spectroscopy: Measurement of a

Complete Spectrum for Each Sample in the Image

Spectrometer

Telescope

Detector Array

Slit

1000s of Parallel SpectrometersCalibrated

Image Cube

Material Map

VSWIR-Dyson Imaging Spectrometer

• The VSWIR-Dyson demonstrates an F/1.8 optical speed, compact, wide-

swath imaging spectrometer covering a 2.5 octave wavelength range from

380 to 2510 nm.

• The VSWIR-Dyson implementation is much lower volume and mass than the

equivalent Offner imaging spectrometer design.

• The higher SNR and wider swath of the VSWIR-Dyson enables a new class

of spectroscopically based measurements for Earth System science.

• The reduced mass and volume support deployment of the VSWIR-Dyson on

a range of platforms and missions including hosted payloads

• Optical bench assembly achieves compact size and mass with total mass of

~6.5 kg.

Instrument Description

The VSWIR-Dyson is a compact imaging spectrometer system for the

solar reflected spectrum (380-2510 nm) with wide swath (1600 elements),

fast optical speed (F/1.8), and high uniformity (≥95%). The basic system

specifications are given in below.

SPECTROMETER SPECIFICATIONS

*: at an equivalent 10 nm sampling and 12 fps, reference radiance R = 0.05, 45o

zenith angle.

**: straightness of cross-track monochromatic slit image (smile <3% of pixel width).

***: mis registration of spectrum to array row (keystone)

Spectral Range 380-2510 nm

Sampling 7.4 nm

Spatial with test telescope Field of view 52 deg

Instantaneous FOV 0.56 mrad

Spatial swath 1600 pixels (1280 test array)

Radiometric Range 0 – 100% R

SNR >2000 *

Uniformity Spectral cross-track >95% **

Spectral IFOV >95% ***

• Spectrometer

optical bench and

thermal hardware

demonstrate low

mass.

• Vacuum

enclosure

assembly built for

cryogenic

laboratory testing.

• FPA mount

provides thermal

isolation from the

spectrometer

housing.

Instrument Description

Instrument layout showing the VSWIR-Dyson

spectrometer, two-mirror laboratory test

telescope, and cryo-cooler.

Instrument Description

• Spectrometer Assembly

– Diffraction Grating Assembly (Substrate, Mount)

– Spectrometer Bench (Bench, Baffles, Thermal Spacer)

– Dyson Block Assembly (AL & Moly Mounts, CaF2/Fused Silica Lens)

– Slit Assembly (Ti Mount, Blackened Si Slit)

– Focal Plane Array Assembly (FPA, Mount, OSF)

• Spectrometer Bipods

• Spectrometer Thermal Shields (Floating, FPA 210k shield)

• Two Mirror Laboratory Test Telescope(Not shown)

1600pixsplitblocksmalleratF1-8CAF2-silica7p4nmsamplingPLUSONEORDERoptimizedforcoldMTFv3correctedmeritfunction_optim6_tp3_aper_set_therm1_TP_asfab_L1sn2_L2sn1_reoptim3.zmxConfiguration 1 of 6

3D Layout

0.38-2.5micron, 30 micron pixels8/12/2014Scale: 0.6667

30.00 Millimeters

X

Y Z

1600pixsplitblocksmalleratF1-8CAF2-silica7p4nmsamplingPLUSONEORDERoptimizedforcoldMTFv3correctedmeritfunction_optim6_tp3_aper_set_therm1_TP_asfab_L1sn2_L2sn1_reoptim3.zmxConfiguration 1 of 6

3D Layout

0.38-2.5micron, 30 micron pixels8/12/2014Scale: 0.6667

30.00 Millimeters

X

Y

Z

48mm

Slit

VSWIR-Dyson spectrometer ray trace; top

spatial cross section and bottom spectral

cross section

Optical Design

Spectrometer Optical Design:• Challenging design because of

large spectral range (380-2510

nm) and wide swath

(48mm/1600 pix).

• CaF2-Fused Silica doublet was

required in order to meet

spectrometer uniformity

requirement.

• Dyson block has one cemented

flat near spectrometer input.

• Spectrometer is 325mm end-to-

end with a 125mm diameter

grating.

• Diffraction grating substrate is

post-polished diamond turned

Aluminum.

Cemented flat

OSF

Slit

systemtemp_no_fold_CAF2_54mmFL_conics_only.zmx

Configuration: All 1

3D Layout

8/12/2014

Scale: 0.5000

40.00 Millimeters

XY

Z

• E-Beam Fabricated Grating

• Dyson Block Doublet– Fused Silica meniscus

lens

– CaF2 Plano Convex Lens with Cemented Flat

• E-Beam Lithographic Slit

• Test Telescope Mirrors– Glass Substrates w/ AL

Coating

Optical Design

Optical Design Key Characteristics:

• Minimum number of optical

components for high throughput

• Compact Wide-Field design

• Specially designed grating groove

profile to tune SNR, reduce

polarization dependence and minimize

energy in negative orders

• Low angles of incidence on optical

components

SRF’s (Spectral Response Functions) for the middle of the field of

view centered at 535, 1535, and 2465nm.

Laboratory Tests and Calibration: SRF

• SRF’s are normalized to unity and compiled with measurements throughout the

field; excellent through-field FWHM uniformity is demonstrated.

• Representative SRF below; other field positions and wavelength SRF’s are

similar.

500 520 540 5600

0.2

0.4

0.6

0.8

1

(nm)

No

rm. R

esp

on

se

Fu

nctio

n

1500 1520 1540 15600

0.2

0.4

0.6

0.8

1

(nm)

No

rm. R

esp

on

se

Fu

nctio

n

2440 2460 2480 25000

0.2

0.4

0.6

0.8

1

(nm)

No

rm. R

esp

on

se

Fu

nctio

n

Plot above shows the spectral alignment and of the curvature

(smile) of a monochromatic slit image.

The inherent cross-track spectral uniformity is ≥95% or less than

5% smile.

Laboratory Tests and Calibration:

Cross-Track Spectral Uniformity

200 400 600 800 1000 1200-0.05

-0.04

-0.03

-0.02

-0.01

0

0.01

0.02

0.03

0.04

0.05

CE

NT

RO

ID -

Yin

t/A

vg S

lope

Spatial Pixel #

Centroid-Yint/Slope vs Spatial Pixel #

=416nm: Pixel #168

=541nm: Pixel #185

=642nm: Pixel #198

=840nm: Pixel #225

=1073nm: Pixel #257

=1560nm: Pixel #322

=2075nm: Pixel #391

200 400 600 800 1000 1200-0.1

-0.08

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

0.08

0.1

CE

NT

RO

ID -

Yin

t

Spatial Pixel #

Laser Sphere CENTROID for several 's [TV6]:Centroid-Yint vs Spatial Pixel #

=416nm: y = 8.4708e-05*x + 168.0598

=541nm: y = 0.0001008*x + 184.9018

=642nm: y = 9.6371e-05*x + 198.4971

=840nm: y = 0.00010702*x + 225.0884

=1073nm: y = 8.8488e-05*x + 256.521

=1560nm: y = 9.3053e-05*x + 322.0982

=2075nm: y = 0.00010599*x + 391.3033

Cross-track spatial response functions for 0

degree field, nominal telescope focus.

Laboratory Tests and Calibration: CRF

• Spatial characteristics established

through cross and along track

spatial response functions (CRF

and ARF).

• Scan sub-pixel slit, at focus of

collimator illuminating the

instrument aperture, oriented either

parallel (ARF) or perpendicular

(CRF) to the spectrometer slit.

• Representative CRF’s are shown

for adjacent pixels with

wavelengths spanning the spectral

range.

• The IFOV uniformity and alignment

meets requirements 80 100 120 140 160 180 200 220 2400

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Frame #

No

rm. R

esp

on

se

Fu

nctio

n

=501nm

FWHM=1.23

=1118nm

FWHM=1.22

=1801nm

FWHM=1.14

=2329nm

FWHM=1.09

Cross-track response function centroid’s

versus spectral channel for five fields

Laboratory Tests and Cal.: Keystone

• Keystone measurement made

with spectrally broad point

source forming a line in the

dispersion direction.

• Point source is scanned in

cross-track direction (CRF)

and centroid location of the

resulting distribution is

measured.

• Corresponding centroids

plotted against spectral

channel, the points cluster

within the ±2% band.

• The inherent spectral IFOV

uniformity is ≥95% (or below

5% keystone).

200 250 300 350 400 450-0.05

-0.04

-0.03

-0.02

-0.01

0

0.01

0.02

0.03

0.04

0.05

CR

F G

auss C

ente

r -

Yin

t/A

vg S

lope [P

ixels

]

Pixel # [Spectral Direction]

-16 Field [Pix#141]

-8 Field [Pix#390]

0 Field [Pix#641]

8 Field [Pix#892]

16 Field [Pix#1141]

Laboratory Tests and Calibration: ARF

• The telescope response (along scan direction) was measured at several different

spatial fields.

• Along-track response functions for several field positions and one wavelength (left),

and for a single field/several wavelengths (right).

• Representative ARFs show small variation through wavelength and all field

positions.

-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.50

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Pixels

No

rm. R

esp

on

se

Fu

nctio

n

Pix# 142, =1682.3nm,

-15.9deg Field, 6mm Focus

Pix# 392, =1682.3nm,

-8deg Field, 6mm Focus

Pix# 642, =1682.3nm,

0deg Field, 6mm Focus

Pix# 892, =1682.3nm,

8deg Field, 6mm Focus

Pix# 1142, =1682.3nm,

15.9deg Field, 6mm Focus

-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.50

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Frame #

No

rm. R

esp

on

se

Fu

nctio

n

=501nm

FWHM=1.05

=1118nm

FWHM=1

=1801nm

FWHM=0.986

=2329nm

FWHM=1.04

Sample Reflectance Measurements

• Figure (top) shows measured spectra

of several minerals [Kaolinite,

Pyroclast, Laterite, Hematite and

Gypsum].

• The reflectance curve has been

processed using a Richardson-Lucy

algorithm, in order to correct for

spectral response.

• Figure (bottom) shows measured

spectra of several organic samples

[Snow, Maple Leaves, Ice Plant

Succulent, Dead Sycamore Leaf, Bark

and Oak].

• Concurrent spectra were take with an

ASD spectrometer. Both show nearly

identical results.

400 600 800 1000 1200 1400 1600 1800 2000 2200 24000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Wavelength [nm]

Re

flecta

nce

Kaolinite

Pyroclaste

LateriteRock

CarbonatiousRocks

HematiteSilicateSand

GypsumSand

400 600 800 1000 1200 1400 1600 1800 2000 2200 24000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Wavelength [nm]

Re

flecta

nce

Snow

MapleLeaves [7]

MapleLeaf [one]

IcePlant

DeadSycamoreLeaf

Bark

Oakw filt corr

• To assure viability for a space mission the VSWIR-Dyson was subjected

to vibration testing in the Autumn of 2016.

• The integrated VSWIR-Dyson on the vibration table during vibration

testing to GEVS levels. Fifteen accelerometers and six force transducers

capturing input and response levels at 13.5 and14grms.

• Following vibration testing the VSWIR-Dyson was returned to the

laboratory for alignment and calibration testing. The alignment remained

within specification.

Vibration Testing

Example vibration testing

• A VSWIR-Dyson imaging spectrometer has been designed to support a

range of potential NASA space missions including HyspIRI.

• The instrument measures the spectral range from 380 to 2510 nm with

F/1.8 optical throughput and high uniformity (≥95%) and accommodates a

wide swath of up to 1600 samples. An array with 1280 samples has been

used for testing. A scaled dual VSWIR-Dyson can support > 6000

samples.

• The VSWIR-Dyson is currently equipped with a laboratory test telescope

that is expected to be replaced with the appropriate flight telescope as

required.

• The VSWIR-Dyson has been aligned, calibrated and characterized at

cryogenic temperatures.

• To demonstrate viability for space flight, the VSWIR-Dyson was subjected

to random vibration testing in the Autumn of 2016.

• Post vibration testing shows the instrument remained within specifications

and this VSWIR-Dyson design and implementation is suitable for use in a

space mission.

Summary & Conclusions