Compact Multispectral and Hyperspectral Imagers based on a Wide Field of View TMA

Transcription

1 Compact Multispectral and Hyperspectral Imagers based on a Wide Field of View TMA M. Taccola (AOES),S. Grabarnik (AOES), L. Maresi (ESA/ESTEC), V. Moreau (AMOS), L. de Vos (OIP), Y. Versluys (OIP), G. Gubbels (TNO) Rhodes, October 4 th 8 th 2010

2 Introduction - TMA in space The aim of this presentation is to provide a brief description of the recent activities done at ESA and focussed on the use of compact wide field of view TMA telescopes 1) GSTP Development of TMA telescope for Proba V (OIP,AMOS) 2) GSTP Single Point Diamond Turning of aluminium alloys (TNO Delft) 3) Compact Hyperspectral Breadboard (AMOS) 4) TMA based instruments for CubeSat (ESA) 2

3 Introduction TMA design Optical Design Form All reflective Cooke triplet Wide field of view: 5 deg x 35 deg (wider FOV in one direction is good for pushbroom imagers) Non-relayed (no intermediate image) Aperture stop on secondary mirror (F# from 5 to 7) Near telecentric design (< 1.0 deg non-parallel chief rays) for spectral filters or spectrometer interface Flat image plane Low distortion ( < 1.5 %) Excellent image quality (MTF > lp/mm) Optical Components Primary Mirror: Concave general asphere Secondary Mirror: Convex sphere Tertiary Mirror: Concave general asphere Material Aluminium Mirror manufacturing 3 Single Point Diamond Turning

4 Proba V Mission Data continuity with SPOT VGT Measurement of Vegetation index (NDVI) Design based on wide FOV telescope. Compactness was the design driver for platform constraints 3 telescopes to cover 102 deg (2250 Km swath) for daily coverage Prime contractor QinetiQ Space Instrument design OIP Telescope manufacturing and alignment AMOS Parameter Spot VGT Available for ProbaV Power consumption 200W ~30W Volume 700x1000x1000 mm3 200x800x500 mm3 Mass 152 Kg ~25Kg 4 Optical configuration Dioptric design (fish eye) Reflective design (Wide FOV TMA)

5 Proba V telescope layout Focal length: 110 mm FOV: 34.5 deg x 5.5 deg F#: 7 (Good optical quality up to F# 5) MTF larger than 0.55@37.5 lp/mm Telecentric layout with 2 focal planes to accommodate 3 VNIR and 1 SWIR channel Mirrors and structure in aluminium (athermal design) Alignment concept based on ultra precise SPDT of mirrors and structure 5

6 Proba V TMA breadboard Breadboard telescope has been manufactured and aligned (AMOS) Optical quality better than predicted from tolerances analysis Telescope aligned in 2 days Blue Channel GSTP TMA 2 sigma Rms WFE in wave Wavefront Error Normalized Image Height 6

7 Improvements of mirror roughness for aluminium mirrors Study in progress with TNO to evaluate possible improvements of SPDT of aluminium alloys (surface roughness): 1)Definition of the aluminium alloys Chemical composition Thermal treatment Alumiplate vs Rapid solidified alloys 2)Optimisation of SPDT process Feed rate Tool angle Vibrations Best results obtained with Rapid solifidified alloys Technology developed by a dutch company RSP Technology B.V. in Delfzijl 7

8 Improvements of mirror roughness for aluminium mirrors Flat samples! AA6061: rms 5 to 10 nm rms RSP6061: rms 0.9 nm rms Results obtained are very positive Microroughness lower than 1 nm rms (on flat samples) Surface finish comparable with Nickel Plated mirrors - Much simpler process (time and costs) - No bimetallic effects (cryogenic applications) 8

9 Further developments Hyperspectral imaging Hyperspectral imagers based on TMA are not new Narrow FOV instruments (grating or prisms) with high resolution on ground: CHRIS and Hyperion operating Prisma, EnMap and HyspIRI in development Compact TMA combined with: Large format 2D photodetector array Linear Variable Filter (LVF) is a new class of relatively inexpensive and compact instruments with medium ground resolution 9

10 Further developments Compact Hyperspectral Imager Breadboard The Compact Hyperspectral Image Breadboard is being developed by AMOS Breadboard done with already existing hardware Breadboard is complete. TRR passed successfully 4/10/ Waiting for the first images!

11 Further developments Compact Hyperspectral Imager Breadboard Detector 10000x2400 pixels CMOS detector developed by Cypress for Medusa (ESA airborne imager) 2 butted chips 10000x1200 (RGB and panchromatic) Only panchromatic for the BB Parameter Specifications Pixel Architecture 6 transistor pixel Pixel Size 5.5 µm x 5.5 µm Format x 1200 (each of the batted chips) FWC > e- 11 Dark Current C

12 Further developments Compact Hyperspectral Imager Breadboard LVF with the gradient of 60 nm/mm is fabricated by Selex-Galileo Centre Wavelength (nm) Max Transmissi on (%) FWHM (nm) , , , , , , , , , Mean Values 60,81 11

13 Further developments Compact Hyperspectral Imager Breadboard Radiometric analysis S sig 1 0 int egration optics 2, 4F# hc 0 2 t L T p T QE 0 0 d Read-out noise, (e-) 100 Pixel size, um 5.5 F# 6 Transmission of optics 0.91 Typical Spectral Radiance, (W/m2/sr/um) SNR between 30 and 40 for integration time of 10 ms This is a limitation for several scientific applications

14 Further developments Compact Hyperspectral Imager Breadboard Hyperspectral Imager Main results Compact and low cost instrument Large FOV for a large swath on ground (daily coverage with 3 telescopes as for Proba V mission) Spectral resolution between 10 nm and 15 nm FWHM SNR is limited for many applications but it is still good for early warning applications 14

15 Further developments - miniaturization The recent developments in small satellites like Cubesats (10 cm x 10 cm x 10 cm) indicate a need for payloads that could fit in such a small volume. An extremely compact hyperspectral/multispectral imager can be a potential candidate for Cubesat missions Spot Vegetation Proba V Spectral imager on CubeSat? 15

16 Further developments - miniaturization Instrument concept based on the miniaturisation of the Proba V design Design optimised to obtain very large FOV along track (increased to 50 ) to ensure a large swath coverage (565 Km from 600 Km altitude) Focal length has been reduced to 40 mm to fit in CubeSat 16

17 Further developments - miniaturization Optical layout The optical design is based on a Three Mirror Anastigmat concept (TMA) All reflective design gives flexibility to adapt the instrument in the visible and infrared region of the spectrum LVF or line filters mounted directly on the focal plane assembly M1 and M3 are aspherical M2 is spherical 17 Pupil stop on M2 to ensure telecentric design

18 Further developments - miniaturization Instrument on the CubeSat Preliminary study has been done to evaluate the mechanical accommodation on a CubeSat platform Allocation of the instrument in the CubeSat volume leaves room for AOCS and satellite subsystems Possibility to use 3 cube units with 1 cube for the optical payload 18

19 Further developments - miniaturization Linear Variable Filter Spectral selection achieved with LVF mounted on the focal plane assembly Filter design based on 2 Linear Variable Edge Filter (High pass and low pass) The filters are coated on the 2 sides of a fused silica substrate 19

20 Further developments - miniaturization Linear Variable Filter High pass filter and Low pass filter in cascade generate linear variable bandpass filter Peak transmission 80% Out of band rejection: average transmission lower than 0.01% Spectral resolution : FWHM 15 nm (can be reduced to ~10 nm) Spectral range: 450 nm 900 nm 20

21 Further developments - miniaturization Image quality - WFE WFE increase is very limited up to 20 Steep increase between 20 and 25 but optical quality at the edge is still very good 70 nm rms (λ/10 rms) Wavefront map for the axial FOV: Nominal design WFE value is 19 nm rms 21

22 Further developments - miniaturization Tolerance analysis Sensitivity analysis has been performed to assess: Mirror manufacturability Feasibility of telescope alignment Based on Proba V experience and Amos background the instrument design is considered challenging but feasible Sensitivity table Results in terms of WFE 22

23 Conclusions 1) ProbaV TMA breadboard proved the manufacturability of the mirrors and the alignment concept of wide FOV telescopes (FOV 35 deg) 2) GSTP SPDT of aluminium alloys indicates the possibility to produce aluminium mirrors with low microroughness (Rq 1 nm rms) without Nickel plating and postpolishing 3) Breadboard assembled by AMOS indicates the feasibility of hyperspectral imagers based on wide FOV telescope 4) ESA internal study (with AMOS,Selex Galileo and OIP inputs) assessed the feasibility of miniaturised telescopes (aimed to CubeSat) The technology for compact low cost and medium spatial resolution hyperspectral / multispectral imagers based on wide FOV TMA is mature 23

24 24 Thank you!