David Laubier

Feasibility demonstration of a high performance compact telescope

Abstract In order to demonstrate the feasibility of a compact telescope with both wide field of view and high resolution, a demonstration model has been built and tested at CNES. This 1.08-m focal length, f/6 three-mirror anastigmat telescope has excellent image quality over its whole 8.4 ° × 1.4 ° field of view: modulation transfer function is about 0.54 @ 77 1p.mm −1 (detector Nyquist frequency). The focal plane comprises two channels, one panchromatic with 24 000 pixels and one multispectral with 12 000 pixels in each of its 3 spectral bands. The whole telescope (mirrors, structure, baffles and focal plane) weighs less than 45 kg. The conception has been done in order to favor structure simplicity and short alignment time while retaining high stability and good image quality. The first part of this paper will focus on the instrument design, and the second part on the manufacturing and test results.

Comparative theoretical and experimental study of a Shack-Hartmann and a Phase Diversity SENSOR, for high-precision wavefront sensing dedicated to Space Active Optics

Earth-imaging or Universe Science satellites are always in need of higher spatial resolutions, in order to discern finer and finer details in images. This means that every new generation of satellites must have a larger main mirror than the previous one, because of the diffraction. Since it allows the use of larger mirrors, active optics is presently studied for the next generation of satellites. To measure the aberrations of such an active telescope, the Shack-Hartmann (SH), and the phase-diversity (PD) are the two wavefront sensors (WFS) considered preferentially because they are able to work with an extended source like the Earths surface, as well as point sources like stars. The RASCASSE project was commissioned by the French spatial agency (CNES) to study the SH and PD sensors for high-performance wavefront sensing. It involved ONERA and Thales Alenia Space (TAS), and LAM. Papers by TAS and LAM on the same project are available in this conference, too [1,2]. The purpose of our work at ONERA was to explore what the best performance both wavefront sensors can achieve in a space optics context. So we first performed a theoretical study in order to identify the main sources of errors and quantify them — then we validated those results experimentally. The outline of this paper follows this approach: we first discuss phase diversity theoretical results, then Shack-Hartmann’s, then experimental results — to finally conclude on each sensor’s performance, and compare their weak and strong points.

Tests of a high-resolution three-mirror anastigmat telescope

With the 3S program (the French acronym for SPOT System Follow-on), CNES intends to continue the SPOT Earth Observation mission with the purpose of achieving severe costs reduction. This increases the need for new, lighter, more compact technologies for the payload. Therefore, CNES has launched preliminary studies in some critical payload domains. The paper deals with the work done in order to demonstrate the feasibility of a small telescope that can be used onboard a mini-satellite. A Three-Mirror Anastigmat (TMA) telescope breadboard has been manufactured using the same technologies that would be required to provide in-orbit stability. The TMA has excellent image quality over its whole 8.4 X 1.4 degree FOV and an intrinsic compactness that makes it much smaller than its focal length: as a result, the whole telescope weighs about 40 kg. The paper particularly focuses on the telescope design, the alignment method and the optical performance under stable laboratory environment. The behavior of the telescope under space environment is described, as well as the tests conducted to validate the computed optical performance under thermal variations and vibrations conditions.

Wave-front sensing for space active optics: Rascasse project

The payloads for Earth Observation and Universe Science are currently based on very stiff opto-mechanical structures with very tight tolerances. The introduction of active optics in such an instrument would relax the constraints on the thermo-mechanical architecture and on the mirrors polishing. A reduction of the global mass/cost of the telescope is therefore expected. Active optics is based on two key-components: the wave-front sensor and the wave-front corrector.

New statistical approach for design optimization and performance assessment of spaceborne optical systems

The spot follow-on system is anew CNES Earth Observation program with reduced development cost as a major objective. In order to reach this goal, new methods for design optimization and performances assessment will have to be taken into account, with the aim to decreasing costly design safety margins. Present paper describes an original approach for rational assessment of image quality of spaceborne observation optical systems. This approach is based upon an overall end to end thermal, mechanical, optical modelization of the instrument which is submitted to both ground operations and space environment. Potential presence of ground and/or on-board active control loops can also be taken into account in that global chain. Based on this approach, a simulation tool has been developed, which allows statistical evaluation of the optical performance expressed as the RMS wave front error (WFE) using the Monte-Carlo method. Instrument modulation transfer function statistical budgets, including detector convolution effect, can be finally obtained as a simple post-processing of preceding WFE calculation step. Validation results and application to the 3S TMA instrument will be presented.

IS HIGH RESOLUTION POSSIBLE ABOARD MINISATELLITES

Abstract To achieve the size reductions needed to lower the costs of space imaging, every part of a satellite (telescope, detector, digitization, image compression, memory, telemetry, and of course satellite platform) has to be improved. With regard to the instrument, new dimensioning rules, new optical designs, and new detection systems allow significant gains. Better balance between detector and optics modulation transfer function (MTF) performances can be achieved. Though the involved solutions raise the question of signal level, the latter can be solved with the use of TDI (Time Delay and Integration) detection. The instrumental choices, the optical system under investigation and how the above techniques are allowed for in the next generation SPOT satellites development studies led by CNES will be presented in this paper, showing how they can achieve the goal of weight reductions.

The LAM space active optics facility

The next generation of large lightweight space telescopes will require the use of active optics systems to enhance the performance and increase the spatial resolution. Since almost 10 years now, LAM, CNES, THALES and ONERA conjugate their experience and efforts for the development of space active optics through the validation of key technological building blocks: correcting devices, metrology components and control strategies. This article presents the work done so far on active correcting mirrors and wave front sensing, as well as all the facilities implemented. The last part of this paper focuses on the merging of the MADRAS and RASCASSE test-set up. This unique combination will provide to the active optics community an automated, flexible and versatile facility able to feed and characterise space active optics components.

New instrument concept to follow up and improve the SPOT program success

The SPOT follow-on system whose French acronym is 3S (Suite du Systeme Spot) is a new CNES (French Space Agency) Earth Observation program with reduced development cost as a major objective. In order to reach this goal, the new satellite architecture must lead to an important reduction of the mass, the target being around 500 kg. The reduced mass of the new instrument, that is the essential part with regard to the image quality performances, will have to be taken into account with the aim of using as much as possible the R&D studies that are in progress. The present paper describes a new concept of instrument. The optical system based on a TMA (Three-Mirror Anastigmat) telescope with 3.8 degree Field Of View (FOV) allows the compact architecture necessary to the mass reduction while keeping high performances. The mechanical architecture realizes the deportment of the instrument parts that are submitted to the ground and flight environment while keeping stiffness and stability. The pointing system modifies the direction of the image acquisitions within a plus or minus 30 degree range along track and plus or minus 45 degrees across. The focal plane is composed of two sensors for the acquisition in one panchromatic and four multispectral bands. The processing electronics converts with a 6 Mpixels/s rate the data to 10 bit words in the entire signal dynamic. Finally, the main performances are shown.