Preparing optical fabrication for microcarb, first satellite based on full freefrom optics

In dec 2015, during COP21, France has initiated the development of the MicroCarb satellite dedicated to better understand the carbon cycle within our atmosphere and to predict its evolution. CNES selected Airbus Defense & Space for the development of the instrument and Safran Reosc has been selected for the optical polishing of the key optical components of this innovative compact spectrometer, the first in EU of its class entirely based on precision freeform optics. The benefits of freeform optics will be highlighted before introducing to the MicroCarb satellite and instrument optical design. Our joint design efforts with Airbus Defense & Space towards ‘feasible’ optics will be presented with the latest technology development at Safran Reosc on freeform optics polishing technology. 1. Benefits of freeform optics 1.1. For imaging instruments We previously showed to the community (2) the benefits of using freeform optics within high resolution space camera in term of compactnes and/or increase of design performance (image quality and/or field of view) with some academic design variations of the of Pleiades payload summerized in figure 1 below. Figure 1. Pleiades WFE RMS vs DM1-M2 and freeform We showed how the design performance (the highest WFE RMS residual through the telescope field fo view) is degrading when one reduces the DM1-M2 distance separating the two main mirrors, but also how it improves when smart or more aggressive freeform surfaces are used in the design instead of conventional aspheric off-axis. Freeform surfaces clearly open new horizons in term of lens design for more compact and/or more performance optical system. Other considerations of manufacturability and error budget are not included in this exercise we conducted. 1.2. For spectrometer instruments A similar exercise has been made recenty by some researchers from the University of Rochester, Institute of Optics but on spectrometer instrument instead of imager. The result is exaclty the same, i.e. the introduction of freeform optics enable higher performances in the same volume or similar performances in a smaller volume. Figure 2. Benefits of freeform within a spectrometer Better performance in same volume (See ref (3)) The introduction of freeform within such type of optical imaging of spectrographic instrumentation is generally done with the addition of polynomial terms beyond the conventional aspheric, onor offaxis, optical prescription. The most used functions are X,Y, Zernike, Forbes polynomials which remain ‘close’ to the natural third and higher order aberration terms. This consideration is no more valid for special applications like illumination which require much more strong deviations from conventional optics and make use of a much wider diversity of surface representations functions. 2. MicroCarb, the CO2 cartographer