The knowledge of the bi-directional scattering distribution function (BSDF) of an optical component is an import requirement for the design and assessment of high-performance optical instruments. However, precise BSDF measurement with high resolution close to the specular beam can be very challenging and require sophisticated instrumentation. In this paper, we present a newly developed scatterometer, the “Enhanced Resolution Light Scattering Analyzer for Curved Gratings (single detector axis)” – ELSA/CG-S which is designed specifically to measure the BSDF of curved optical components with a very high resolution not only close to the specular direction, but throughout the whole angular measurement range with an instrument signature that can compete with the top of the class of current commercially available instruments. The distinguishing feature of the instrument is the use of a high-resolution silicon sCMOS imaging detector which enables fast acquisition times and provides access to a two-dimensional section of the BSDF around the main detection plane of the instrument with an out-of-plane FoV of about ±0.6°. In the following, we will describe the general design of the instrument and explain the measures that have been taken to enable a very low stray light signature with the chosen detection scheme. After this, we will assess the instruments capabilities and present measurements of the instrument signature and BRDF measurements of plane and curved diffraction gratings with high groove densities. These measurements will also demonstrate the additional value that is provided by using an imaging detector. All measurements will be compared to results obtained with ESTEC’s commercial CASI scatterometer from The Scatter Works, that represent the current state of the art.
We demonstrate that full-field deflectometry is a viable alternative to interferometry for the characterization of free-form mirrors. Deflectometry does not require the use of a CGH. Instead of measuring the surface height map, the deflectometer measures the surface slopes in two orthogonal directions using the phase-shifting Schlieren method [1]. The surface height map is then reconstructed by integration of the slope maps. We present two instruments. The first one can be mounted in the lathe for in situ measurement. The second is adapted for the characterization of large concave mirrors.
Recent Earth Observation instruments require a highly accurate knowledge of their Instrument Spectral Response Function (ISRF). This translates into lengthy and costly characterisation programs during the Assembly, Integration, and Test phase of the instrument. In addition, potential changes of ISRF after launch suggests to use an on-board equipment dedicated to this accurate characterisation in flight. This topic has been studied by Thales Alenia Space and TNO in the frame of a TRP study funded by ESA during years 2019 and 2020. This paper first recalls the potential application cases and the related target performances of the study. Then we identify conceptual solutions, such as operating tuneable laser diodes, Fabry-Perot interferometer, microresonators. Next, we describe the principle of the selected solution: a Fourier Transform Spectrometer (FTS), used as a calibration light source. Indeed, a single FTS can cover a very large spectral range with an extremely high spectral accuracy. But FTS are known as complex and sensitive devices: we have designed a simplified solution, well adapted to space applications. In a second step, we describe the flight hardware designed by Thales Alenia Space, targeting optimised manufacturing and implementation. The main performances, being spectral resolution and signal to noise ratio (SNR), are discussed. Breadboard activities also took place in TNO in order to validate the main hypotheses. Innovative data processing has been tested, that only makes use of interferogram data, while maintaining a high accuracy. Description of the breadboard as well as the first test results are presented.
The measurement of the bi-directional scatter distribution function (BSDF) is a well established process on reflective surfaces and largely also on transmissive optical elements. Such measurements are crucial and frequently used as input to assess the straylight performance of optical space instruments. The straylight performance of grating based spectrometers, such as used in Sentinel-5 or FLEX, is to a large extent driven by the grating itself. Thus, a BSDF characterization of gratings for such spectrometers is necessary early in the spectrometer development. However, performing such measurements on gratings turn out to be challenging for a number of reasons that will be presented and addressed in this paper. In this paper we address some of the challenges experienced when measuring BSDF at different wavelengths from UV up to SWIR for several type of gratings. Scatterometers are usually designed to measure the BSDF of a single flat optical surface. The particular form or construction of such gratings, for example transmissive gratings with at least two optical interfaces or gratings with optical power, requires a reconfiguration of the classical measurement set-up to minimise the errors in the BSDF characterization. Additionally, there is a difficulty of measuring the near-specular scatter of those components due to the inherent optical aberrations and potentially their curvature. The dispersive property of gratings imposes the use of a very stable and spectrally pure light source for the measurement. We suggest some strategies and configurations to mitigate the above-mentioned difficulties. Some BSDF measurements on curved and immersed gratings are presented in this paper for illustration.
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