FLORIS (FLuorescence Imaging Spectrometer) is the single High-Resolution Spectrometer instrument of the FLEX (FLuorescence EXplorer) mission, currently under development by the European Space Agency as the eighth Earth Explorer Mission. The goal of the mission is the monitoring of the chlorophyll fluorescence of plants giving information about their photosynthetic activity.
Leonardo Avionics & Space System Division is the prime contractor for the FLORIS Instrument for which Media Lario is manufacturing the QM unit of the spherical mirror included in the High-Resolution Spectrometer (HRSPE), hereafter called HRM mirror.
The High-Resolution Mirror is a 250-mm diameter spherical mirror with a radius of curvature of approximately 440 mm. For the mirror substrate, Leonardo has selected the Aluminium alloy AlSi40, a special alloy with 40% Silicon content, coated with a hard polishing layer of Nickel Phosphorus (NiP), deposited by electroless chemical process. The Silicon content allows this special Aluminium alloy to have the same coefficient of thermal expansion (CTE) of the NiP layer, therefore preventing thermal deformations deriving from the bimetallic effect. The mirror structure is light-weighted to approximately 2.8 kg. The required wave-front error of the mirror is better than 0.5 fringes PV, while the surface microroughness has been specified at 0.5 nm RMS due to stringent straylight requirements of the FLORIS instrument.
Media Lario has been selected for the mirror development phase because of their experience in the design and manufacturing of AlSi/NiP mirrors demonstrated in the development of the Earth Observation optical payload for small satellites (called STREEGO), based on an AlSi40 TMA telescope. The manufacturing process includes precision diamond turning, optical figuring and super-polishing. The optical coating will be done by Leonardo at their thin-films facility of Carsoli, Italy. Since the recipe prescribes to pre-heat the mirror surface at 100° C, Media Lario will qualify the mirror substrate with -25/+110°C thermal cycles to ensure adequate thermal stability for the coating process.
Beside homogeneous filter coatings a coating can also be applied with a linear gradient. Linear gradient or linear variable filters show a gradient of a band edge or central wavelength depending on the filter type in spectral direction and they are homogeneous in spatial direction. In this paper, we present a linear variable narrow band pass filter with full width half maximum of about 8 nm and a transmittance of more than 98% in the wavelength range of 670 nm to 780 nm. The target for the gradient is 3.3. nm/mm. Due to the need of transmittance filter and AR coating are manufactured by means of Plasma Enhanced Magnetron Sputtering (PARMS). Additionally, the linear variable filter for FLEX mission requires a black mask to separate between HR1 and HR2 channel. This mask is also applied by OBJ by means of the PARMS process and a Ti based layer stack. Here, a reflectance of <1.5% in the range of 400 nm to 800 nm can be demonstrated. The definition of black mask was done by means of photolithography.
The PRISMA (PRecursore IperSpettrale della Missione Applicativa) Programme is an ASI (Agenzia Spaziale Italiana)
hyperspectral mission for Earth observation based on a mono-payload single satellite: an Italian Consortium is in charge
to realize the mission; Selex ES has the full responsibility of the hyperspectral payload composed by a high spectral
resolution spectrometer optically integrated with a medium resolution panchromatic camera.
The optical design permits to cover the wavelength range from 400 to 2500 nm and it is based on high transmittance
optical assemblies, including a reflective common telescope in Three-Mirror Anastigmat (TMA) configuration, a single
slit aperture, a panchromatic camera (700-900 nm) and a spectrometer having two channels (VNIR and SWIR), each one
using an suitable prism configuration and spectrally separated by a beam splitter, conceived to minimize the number of
optical elements. High performance MCT-based detectors represent the core of the instrument. To provide the required
data quality for the entire mission lifetime (5 years), an accurate and stable calibration unit (radiometric and spectral) is
integrated, for the in-flight instrument calibration. The thermal design has been based on a passive cooling system: a
double stage radiator, suitable oriented and protected from unwanted heat fluxes, high performance heat pipes and an
operational heaters network represent the solution adopted to achieve the required thermal stability.