freeform optics at esa: an overview - lam · - heritage from 2-mirror telescope of ... for official...

ESA UNCLASSIFIED - For Official Use

Freeform optics at ESA: an overview C.Pachot, A.Zuccaro Marchi, S.Mahalik, M.Miranda, A.Deep, L.Maresi, O.Sqalli, M.Taccola, M.Francois, S.Santandrea European Space Agency ESA, ESTEC, Keplerlaan 1, 2201 AZ Noordwijk ZH, The Netherlands 12-13/10/2017 – Marseille

ESA UNCLASSIFIED - For Official Use Freeform optics at ESA: an overview |Charlotte Pachot | Freeform workshop 12/10/2017 | Slide 2

Outline

Why use freeform optics in space ?

Challenges of Freeform optics

Freeform activities overview

Next steps/Future work


Challenges in space optical instruments

Straylight

Mass/volume Spectral/spatial resolution

Thermal stability

SNR

Image quality


Why use freeform optics in space ?

Freeform optics

Field of View - Larger - Constant performance

Throughput - Less optical elements - Innovative designs

Image quality - More degrees of freedom

Compactness - Decrease mass - Decrease volume


Freeform optics

Design

Manufacturing

Alignment

Metrology

Freeform optics challenges

Surface mathematical definition Optical design optimisation Tolerancing analysis

Technique Machine Complexity Time Material

Non standard measurement techniques Data file formats


TROPOMI = TROPOspheric Monitoring Instrument

Push-broom type imaging spectrometer covering

a spectra range from UV to SWIR.

Successor of OMI and SCHIAMACHY

Instrument main characteristics

- UV-VIS-NIR-SWIR nadir view grating spectrometer

- Spectral range 270-495, 675-775, 2305-2385 nm

- Spatial resolution 7x7 km² at nadir

- Swath width 2600 km

- Spectral resolution 0.25-1.1 nm

Tropomi instrument (1/3)

Launch: 13/10/2017 !

(Courtesy:KNMI)


Tropomi instrument (2/3)

Telescope freeform design

- Heritage from 2-mirror telescope of OMI/SCIAMACHY

- 2 freeform-mirror telescope

Advantages

- Minimize number of optical elements for better performance

- Better spatial resolution (factor 10 to 30)

- Resolution more constant over FoV

Drawbacks

- Longer manufacturing time Tropomi telescope optical design (courtesy:TNO)

Tropomi telescope breadboard set-up (courtesy:TNO)

Paper ICSO: The TROPOMI Telescope Design, Fabrication and Test of a freeform optical system, David Nijkerk, Bart van Venrooy, Peter van Doorn, Rens Henselmans, Folkert Draaisma, André Hoogstrate


Tropomi instrument (3/3) Freeform mirrors design

- Design and analysis using Code V

- 2 toroidal mirrors (size 100mm) -> anamorphic telescope

- Surface shape represented by XY-polynomial

- Tolerance analysis incl. mid-spatial frequencies

- Alignment not necessary post assembly

Freeform mirrors manufacturing

- Prototype of the telescope has been built

- Material: Al6061

- Manufactured by single point diamond turning in slow servo mode (NiP coating to achieve required roughness + manual post polishing) – long process 30 hours

Metrology

- Use of TNO Nanomefos machine (accuracy <15nm rms): non-contact measuring machine with an optical probe

- Volume of the sample up to Diam.500x100 mm

- Best aspherical surface fit deviation up to 5 mm PV

Courtesy: TNO Courtesy: TNO

https://www.tno.nl/en/focus-areas/industry/semiconductor-equipment/industrial-instrumentation/nanomefos-non-contact-measurement-machine-freeform-optics/


ESA-funded R&D activities on freeform optics

TRP/GSTP activities

Feasibility of Diffraction Grating on Free Form Surfaces

Linear variable based hyperspectral instrument - Elois spectrometer

Development of the EQM of a Compact Spectrometer Based on a Free Form Grating

Free-Form Diffraction Grating and Mirrors – Breadboarding

Design Tools & Alignment Concepts for Free-Form Optics


Feasibility of Diffraction Grating on Free Form Surfaces (1/2)

Objectives

• Demonstrate the feasibility of gratings on freeform substrates

• Assess the achievable performance of freeform gratings

• Identify a design concept of hyperspectral imager which can benefit from a freeform

grating

Overview

• TRP contract

• Activity started in 2012, successfully completed in 2015

• Budget 200k€

• Contractor AMOS (BE)

Achievements

• Grating design: 104 lp/mm, on NiP Aluminium 6061T6 alloy

• Freeform substrate machined by single point diamond turning

• Grating grooves engraved directly on the surface by diamond tool

Microscope view of the grating grooves (Courtesy: AMOS)

Manufactured freeform grating (Courtesy: AMOS)

57nm SFE after ruling


Feasibility of Diffraction Grating on Free Form Surfaces (2/2)

Hyperspectral imager concept

- Starting point

- Sentinel 5p Tropomi instrument requirement specification

- Offner spectrometer with 1:1 magnification

- Concept

- 2:1 magnification Offner spectrometer using a freeform

grating on a convex mirror surface

- Achievements

- Increase of the SNR

- Minimisation of the volume

- Good image quality over FoV


Application: Elois spectrometer

Freeform grating main benefit =

Compactness at equal performance

Elois spectrometer optical design (Courtesy: AMOS)

Elois spectrometer Breadboard (Courtesy: AMOS)

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 450-900 nm

Spectral sampling 2.5 nm

Keystone 1.1 µm

Smile 1.9 µm

Global size 116 x 145 x 130 mm (with folding)

Elois spectrometer Breadboard (Courtesy: AMOS)

C. DeClercq, V. Moreau, J.-F. Jamoye, A. Zuccaro Marchi, P. Gloesener 2015, “ELOIS: an innovative spectrometer design using a free-form grating”, Proc. SPIE 9626


Development of the EQM of a Compact Spectrometer Based on a Free Form Grating Motivation

Freeform grating Offner topology provides a very compact and lightweight solution for implementation of a high performing spectrometer

Develop a full instrument capitalizing on the ELOIS spectrometer breadboarding experience

Objectives

• Extend and demonstrate the freeform grating offner spectrometer concept to the VNIR and SWIR wavelengths within one compact instrument covering the following spectral range: 400 nm to 2450 nm.

• Design, manufacturing, testing of an EQM of the complete payload

Overview

GSTP activity

Started in December 2016, 2.5 years activity

Contractor SpaceBel, AMOS (+Caeleste, CSL, DeltaTec)


Development of the EQM of a Compact Spectrometer Based on a Free Form Grating Main characteristics

- Overall payload designed to be compatible with Proba-Next platform (mass, volume, on-board resources)

- Design envelope Sun Synchronous orbit:

- LTDN/LTAN: 10:00 AM – 12:00 AM

- Altitude: 590km - 670km

- Swath width: 70 km (@ Nadir) @642 km altitude

- Capability to image with an +/- 30 deg across track off-pointing

- Optical telescope (front-end optics) single TMA

- Beam-Splitter to separate VNIR and SWIR channels (@ 900 nm)


Free-Form Diffraction Gratings and Mirrors – Breadboarding

Motivation

• Encouraging results from TRP on freeform grating

• Objective to increase the groove density of the grating

Objectives

• Design, alignment, manufacturing and testing of a freeform grating spectrometer according ESA requirements

• Freeform grating > 1000gr/mm manufacturing and testing

Overview

• TRP activity

• Activity started in Sept 2016

• Budget 500 k€

• Contractor AMOS/HORIBA/CSL



Optical design - High spectral resolution and SNR - Compactness Freeform design Freeform grating Freeform Blazed Holographic grating of 1000 gr/mm (± 2 mm deviation to BFS) Sketch of the Optical design (Courtesy: AMOS)

Payload opto-mechanical design (Courtesy: AMOS)



Achievements

- Activity on-going

- Manufacturing starting in Fall 2017

- Assembly, Integration, Verification in mid 2018

- Full Focal plane characterization through Summer 2018 (CSL)



Motivation

Existing ray-tracing software optimisation algorithms (Code V, Zemax) are not optimised for freeform surfaces

• Not converging in the optimisation process

• Optimisation phase of optical designs involving freeform slow

Objectives

• Define the baseline mathematical description of free-form surfaces;

• Define procedure and best practices for optical design using free-form components;

• Demonstrate advantage of the developed free-form design tools on two case studies;

• Develop alignment strategy and test methods with accuracy compatible with the two case studies.

Overview

• TRP activity

• Activity started in Sept 2016, Final Review planned Dec. 2017

• Budget 350 k€

• Contractor TNO (NL)

Zernike polynomials



Achievements

- Activity on-going

- Optimisation tool developed in Python, delivery as a plug-in to existing optical software Code V

- Work with any definition of freeform surfaces: Q-polynomials, NURBS, Zernike… but optimisation tool will be tested on 2 mathematical descriptions (from in-built capabilities of Code V) in the frame of this activity

•Requirements •Mathematical description

Concept

• Optimisation • Merit function

Nominal •Opto-mechanical design

• Tolerancing • MAIT

Final


Next steps/future work

• Analytical part

• Straylight from freeform surfaces

• Large scale manufacturing and metrology tools

• Alignment


Upcoming events

https://atpi.eventsair.com/QuickEventWebsitePortal/space-engineering-and-technology-final-presentations-days/set-pfds


Upcoming events

https://atpi.eventsair.com/QuickEventWebsitePortal/workshop-on-innovative-technologies-for-space-optics/workshop-on-innovative-technologies-for-space-optics

ESA UNCLASSIFIED - For Official Use

Thank you for your attention !

Acknowledgement: SpaceBel, AMOS, TNO, Horiba, CSL

For further information and/or contact: [email protected]

freeform optics at esa: an overview - lam · - heritage from 2-mirror telescope of ... for official...

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