why stray-light in optical instruments matters and what to do … stray... · 2019. 2. 19. · why...
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Why stray-light in optical instruments matters and what to do about it?
L. Clermont
Centre Spatial de Liège
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Added values from space activities are crucial(earth observation, science, etc.)
Just in case you didn’t knew it already …
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Optics is at the core of space instruments
OpticsAnother
lesser field
A lesser field
An even worst lesser field
… says a totally unbiased optical guy
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Optical designers build optical instruments by finding a configuration of optical elements which minimizes aberrations
Optical elements (lenses, mirrors, diffractive elements) can bend rays to form an image
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Minimizing aberrations is great …but that’s not the only driver of optical performance
stray-light is a critical concern for space optical instruments
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Stray-light is light reaching the detector by following different paths or processes than the nominal beam
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Nominal rays sequence
Ghost reflections
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- Broadening of the point spread function (PSF)- Addition of unwanted features on the detector- Decrease of the signal to noise ratio (SNR)- Saturation of the detector
Stray-light decreases the image quality and hence the added-values of spaceborne observations
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The ingredients of stray-light:
Source of lightExternal sourceThermal self-emissionFluorescence
Non-nominal processGhost reflectionScatteringDiffractionRogue paths
Nominal process
Stray-light
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Stray-light even in the greatest …
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Nomex fibers sticking out of the sun shield
Rogue rays from edge of sun shield can enter
Stray-light even in the greatest …
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What to do against stray-light?
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What to do against stray-light?
Design the instrument against stray-light (control)
Predict the origin and level of stray-light (analysis)
Verify the prediction experimentally (measurement)
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Some examples of stray-light reduction tricks …
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The stray-light engineering process is iterative and works as a feedback loop, it must consider also other aspects than optics
Optical designEfficient against aberrations
System designEfficient against stray-light
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Experimental characterization requires dedicated facilities
FOCAL 3 facility @ CSL
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Stray-light requirements are always more stringent
Hardware optimization is not enough !
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For METOP-3MI, we have developed a stray-light correction algorithm, which works together with an on-groundstray-light characterization
3D view of the payload
Optical design
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What to remember?
Stray-light = critical concern (performance driver)
Need to be considered from early phase of mission @system level
Hardware is the first cure, post-processing is the second