The MicroCarb mission was initiated in Dec 2015 during the COP21 climate conference. Its objective is a better understanding of the carbon cycle within our atmosphere and thus enabling the prediction of its evolution. The French space agency CNES selected Airbus Defense & Space for the development of the satellite and its disruptive instrument with an original concept based on multi freeform mirror design. Safran Reosc was selected by Airbus for the optical manufacturing of all the key optical components of this innovative compact spectrometer, the first in EU of its class entirely based on precision freeform optics. We will first recall the benefits of using freeform optics in an imaging or spectral instrument. Then we will present the MicroCarb satellite and its instrument optical design. Joint design analysis was spent with Airbus towards ‘feasible’ optics despite the fact that this project constituted a unique opportunity for Safran Reosc to make a quantum leap in optical manufacturing and testing technology for precision freeform optics in Silicon Carbide (SiC).
The introduction of freefrom optical surfaces in a space instrument offers the possibility to improve its performance, its volume and weight or a combination of both. This is shown to be valid for either imaging systems or spectroscopic systems. At Safran Reosc we master freeform robotic polishing technology since more than 25 years and we will show how we continue to improve it in term of quality, productivity and adequacy to ceramic materials like Silicon Carbide. Surprisingly, the conservative space industry evolved slowly from spherical to rotationnally symmetric aspherical and off-axis optics during the last decades. This is no more valid today and we are proud to contribute to MicroCarb and IASI NG, two recent innovative projects taking benefit of such freeform optics designs enabling more cost-effective advanced missions. These projects will be briefly presented with their status of progess of work at the company. We will also take this opportunity to present our work on the extreme off-axis optics used within the MERLIN lidar project, produced with the same freeform technology.
The Spectral Separation Assembly (SSA) is a key component of the Flexible Combined Imager (FCI), an instrument that will be onboard Meteosat Third Generation (MTG). It splits the input beam coming from the telescope into five spectral groups, for a total of 16 channels, from 0.4 to 13.3 μm. It comprises a set of four dichroics separators followed by four collimating optics for the infrared spectral groups, which feed the cold imaging optics. To assess the optical performances, a specific multi-wavelength infrared test bench has been designed. The wavefront error can be measured for each channel of each spectral group. Other parameters can also be measured, namely pupil centering, line of sight, pupil diameter and pupil aberrations. This paper will present this test bench and the solutions developed to enable these measurements on a very large spectral range.
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 and 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 and Space towards ‘feasible’ optics will be presented with the latest technology development at Safran Reosc on freeform optics polishing technology.
Meteosat Third Generation is the next ESA Program of Earth Observation dedicated to provide Europe with an operational satellite system able to support accurate prediction of meteorological phenomena until the late 2030s. The satellites will be operating from the Geostationary orbit using a 3 axes stabilized platform. The main instrument is called the Flexible Combined Imager (FCI), currently under development by Thales Alenia Space France. It will continue the successful operation of the Spinning Enhanced Visible and Infrared Imager (SEVIRI) on Meteosat Second Generation (MSG) with improved performance.
This instrument will provide full images of the Earth every 10 minutes in 16 spectral channels between 0.44 and 13.3 μm. The ground resolution is ranging from 0.5 km to 2 km. The FCI is composed of a telescope developed by Kayser-Threde, which includes a Scan mirror for the full Earth coverage, and a calibration mechanism with an embedded black body dedicated to accurate in-flight IR radiometric calibration. The image produced by the telescope is split into several spectral groups by a spectral separation assembly (SSA) thanks to dichroïc beamsplitters. The output beams are collimated to ease the instrument integration before reaching the cryostat. Inside, the cold optics (CO-I) focalize the optical beams onto the IR detectors. The cold optics and IR detectors are accurately positioned inside a common cold plate to improve registration between spectral channels. Spectral filters are integrated on top of the detectors in order to achieve the required spectral selection.
This article describes the FCI optical design and performances. We will focus on the image quality needs, the high line-of-sight stability required, the spectral transmittance performance, and the stray-light rejection. The FCI currently under development will exhibit a significant improvement of performances with respect to MSG.
The Spectral Separation Assembly is a key component of the Flexible Combined Imager, an instrument that will be on-board Meteosat Third Generation. It splits the input beam coming from the telescope into five spectral groups, for a total of 16 channels, from 0.4 to 13.3 μm. It comprises a set of four dichroics separators followed by four collimating optics for the infrared spectral groups, which feed the cold imaging optics. The visible spectral group is directly imaged on a detector. This paper presents the optical design of the assembly, the mechanical mounting of the optical components, and the coatings developed for the dichroics, mirrors and lenses.
PICARD, a Sun observing satellite, has produced more than one million images during its 4-year mission. SODISM is one of three instruments on-board, whose main goal is to measure the solar limb and its spectral dependence from the middle ultraviolet to the near infrared. The very high accuracy (a few milli-arcseconds) needed to measure the solar limb with its spatial and temporal variations makes the instrument very sensitive to small aberrations. In this paper, we will present the impact of various parameters on the solar limb measurement, from simple displacements of mirrors to complex mirror deformations and thermal gradients. A complete scenario has been constructed from these simulations, leading to a model that describes the actual limbs obtained with SODISM. All these simulations will help improving future missions, by assessing the critical parameters affecting the measurement accuracies of such instruments.
The near-infrared spectrograph (NIRSpec) is a complex instrument that will be launched on board the James
Webb Space Telescope (JWST). It is composed of three three-mirror anastigmats (TMAs) made of silicon carbide
(SiC). Sagem REOSC has been in charge of the mirror polishing, coating, alignment and testing, as well as
cryogenic testing. The performance level and the alignment constraints, along with the polishing and alignment
processes, have led to the set up of a model to accurately predict the final performances of each TMA, and
minimize the risk of vignetting. The model has then been fitted to the measured parameters obtained after
alignment (wavefront error, magnification or focal length...) to get an accurate modelization of the actual
performances, and allow their evaluation on the full field of view. The model has been finally delivered with
each TMA, as a basis for the instrument performance simulator. We will show a good correlation between the
predicted performance (before alignment, obtained from individual mirror data) and the final performance (after
alignment), as well as a very good fit between the as-built models and the actual TMAs.