FreeForm Gratings for Imaging Spectrometers V. Moreau 1 A.Z. Marchi - - PowerPoint PPT Presentation

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FreeForm Gratings for Imaging Spectrometers V. Moreau 1 A.Z. Marchi - - PowerPoint PPT Presentation

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FreeForm Gratings for Imaging Spectrometers V. Moreau 1 A.Z. Marchi 2 B. Borguet 1 Dispersing element for astronomy : New trends and Possibilities Milano, October 9-11, 2017 1 AMOS, Liege Science Park, Rue des Chasseurs Ardennais, 4031 Angleur,

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SLIDE 1

FreeForm Gratings for Imaging Spectrometers

1 AMOS, Liege Science Park, Rue des Chasseurs Ardennais, 4031 Angleur, Belgium, vincent.moreau@amos.be 2 2 ESA/ESTE

TEC, Keplerlaan 1, 2201 AG Noordwijk, The Netherlands,

  • V. Moreau1

A.Z. Marchi2

  • B. Borguet1

Dispersing element for astronomy : New trends and Possibilities Milano, October 9-11, 2017

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SLIDE 2

2

PanSta tarr rrs s Survey ey Telescop escope OAJ: : Javala alambre mbre Observa servatory

  • ry

Turn-key Telescopes

Devast astal al Opt ptic ical al Telesc scop

  • pe

3,6 ,6 m Telesco scope ARIES S Nainital ital 1,8 ,8 m Telesco scope IfA - Hawaii waii 2,5 ,5 m Telesco scope Javalam alambre bre

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SLIDE 3

3

Large opto-mechanical sub-systems

3

Auxil illary ry Telesco scopes Syst stem for VLTi i (ESO) SO) Adap apter ter/Rot

  • tat

ator

  • r for

VLT (ESO) SO) M3 unit t for GTC Interfe rferome rometr tric ic Telesco scopes s for MRO RO Primary mary Mirror ror Cell l for for DKIST Sub-syst ystems ms studie dies for E-ELT LT

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4

Astronomy Optics

  • Polishing capabilities for complex large optics

up to 3 meters

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SLIDE 5

5

Space Optics

Alumin minium ium Mirror rors Tropom pomi (Sentinel 5 precursor

sor)

Silicon icon Carbid rbide Mirror rors GAIA Ceram ramic ic Mirror rors

DM3 Satellite lite (for r SSTL)

These equipment pments s are flying ng on board satellite lites, , probes es or the Space Shutt ttle.

  • le. They are mainly

nly instr trum uments nts, , mirrors

  • rs,

, mounts ts, , telescope

  • pes,

s, struct uctur ures s or mechanis nisms.

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SLIDE 6

6

Free Form Optics

“ Freeform Optics is not just an Evolution, It’s a Revolution”

  • J. Rolland, Director of Center for Freeform Optics, Rochester NY

Freeform Optics = surfaces without rotational symmetry

Coaxial design

  • On-axis
  • Obscuration
  • Narrow Field-of-view

Conical

Coaxial design

  • Off-axis
  • No Obscuration
  • Large Field-of-view

Aspherical

Free orientation of optics Free position of image No obscuration Large Field-of-view 3D-configurations possible Much more compact

Freeform

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SLIDE 7

7

Imaging spectrometers

Front Telescope Slit Dispersive element Optical Relay

Spectral axis

2D Detector

Original question (ESA 2013) subsidiary issue

“The technology of ruling a grating on a convex freeform surface has not yet been demonstrated to our knowledge, but is an active research area”.

In Light: Science & Applications (July 2017) “Freeform spectrometer enabling increased compactness”, J. Reimers, A. Bauer, K. P. Thompson and J. P. Rolland.

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SLIDE 8

8

Freeform Grating Spectrometer

Slit Concave Spherical mirror Convex Spherical Grating Sensor

Freeform design

  • 2->1 demagnification
  • Larger Slit = 30 µm x 60 mm
  • Compatible with 15 µm pix detector
  • F# = 2.5 at focal plane -> high SNR
  • Factor 4 reduction in volume
  • Really Compact design : A5 format

Original Offner Spectrometer ELOIS

(Enhanced Light Offner Imaging Spectrometer) 150mm 150mm

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SLIDE 9

9

Free Form Grating = Significant improvement of performances

Optical performance ELOIS spectrometer Image F# 2.5 Entrance slit 60 mm x 30 µm Image 30 mm (spatial) x 2.7 mm (spectral) Grating frequency 104 lp/mm Spectral range 400-1000 nm Spectral sampling 2.5 nm Keystone 1.1 µm Smile 1.9 µm Global size 116 x 145 x 130 mm (with folding)

Usual Spherical Grating Free Form Grating

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10 10

Free Form Grating Manufacturing

Machined on NiP-plated Aluminum blank with a 5 axis ultra-precision lathe using a sharp edge diamond tool. Blaze angle is following the normal to the surface (-6° -> 6°) Nominal Shape Mirror turning : 50 nm rms SFE Grating ruling: 57 nm rms SFE

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SLIDE 11

11 11

Diffraction efficiency

  • Maximum diffraction efficiency of 85% is measured at 633 nm

Simulated curve from measured profile Measures (TE and TM)

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SLIDE 12

12 12

Grating Roughness

  • First trials were…disappointing
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SLIDE 13

13 13

Identification of Ghost origin

11 grooves

  • Analysis of groove to groove spacing on a microscope image of the grating
  • Evidence of a periodic error in groove spacing
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SLIDE 14

14 14

Grating Roughness

  • First trials were…disappointing
  • But perseverance finally paid…

Notable reduction of Ghost (<10-3) and grass (<10-5) Low Scattering (<10-7) -> correspond to Rq=3.5 nm rms

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SLIDE 15

15 15

ELOIS: Breadboard Performances

Integratig sphere Target Object Slit Optical Relay

BREADBOARD TEST SETUP

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SLIDE 16

16 16

First results from breadboard tests

500 1000 1500 2000 2500 3000 3500 4000 4500 820 825 830 835 840 845 850

Nyquist Period

Initial results : Test of the Breadboard in front of a Xenon arc Lamp Image from ELOIS Theoretical spectrum

500 1000 1500 2000 2500 3000 3500 4000 4500 200 400 600 800 1000

Wavelength (nm)

500 1000 1500 2000 2500 3000 3500 4000 4500 200 400 600 800 1000

Titre de l'axe

The resolution of the spectrometer is well achieved !

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SLIDE 17

17 17

Hyperspectral Image acquisition :

Vegetation Samples

1 : Honeysuckle (Lonicera periclymenum) 2 : Bunchgrass (Brachypodium sp) 3 : Blackberry (Rubus fruticosus)

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SLIDE 18

18 18

New perspectives : CHIMA - High Spectral resolution instrument

  • Holographic FreeForm Grating

Spectro-Imager

  • Demagnification factor of 3
  • All Reflective design – Full aluminum -

Athermal

  • Spectral Resolution R~4000 (0.16 nm)
  • Spectral Bandwidth 0.5 nm
  • Long slit (60 mm)
  • Excellent imaging prop. (MTF > 0.5)
  • High SNR (> 1000)
  • Compactness (20x20x40 cm³)

1000 lp/mm Freeform replicated grating

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SLIDE 19

19 19

New perspectives : Multi-blazed Gratings

19

Measured profile Typical roughness ~ 4nm RMS Typical grating SFE ~ 30nm RMS

Optical performance Chandrayaan II gratings Grating frequency 20 lp/mm Spectral range 700-5000 nm Shape Spherical convex Multi-blazed 9 blaze angles

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20 20

New Perspectives : ELOIS VNIR/SWIR

  • Multi-Blazed Freeform grating for combined VNIR/SWIR

Spectrometer with Splitted-orders

VNIR+SWIR Order 1 VNIR Order 2 SWIR Order 1

Diffraction efficiency SNR

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SLIDE 21

21 21

New Perspectives : ELOIS VNIR/SWIR

Requirement Performance Spectral range 400-2450 nm Ground sampling distance (@650 km) 35 m Swath width 70 km Mass 40 kg Volume 550 x 650 x 450 mm³ Number of bands 210 Spectral FWHM <12 nm (uniform over range) MTF >0.3 SNR at 0.3 albedo VIS NIR SWIR > 400 >250 > 100 Radiance accuracy > 95% Polarisation sensitivity <3% absolute, 2% between bands Out of band rejection <1%

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22 22

Conclusions

  • Innovative non-symmetrical Offner Imaging spectrometer

with large demagnification have been successfully designed by introducing Freeform Grating.

  • > Improved SNR
  • > Compact design
  • > Longer Slit (=FoV/Sampling ratio)
  • > Smaller detector pixels
  • Ultra-accurate single point diamond machining is a key

technology for manufacturing FFO systems:

  • For low resolutions grating (<150 lp/mm), it offers new degrees of freedom :,

Complex shapes, Multi-blazed, variable period…

  • Through the tests of a functional breadboard, we

demonstrate the perfect control of the complete process chain for freeform grating and instrument, from design to manufacturing & calibration.

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SLIDE 23

Thank You !

Coralie De Clercq (AMOS) Arnaud Cotel (Horiba-Jobin-Yvon) Luca Maresi (ESA) Atul Deep (ESA) Michael François (ESA) Yvan Stockman (CSL)

Acknowledgments :