Future high-resolution space telescopes will use discontinuous apertures, either primary segments ("Fizeau" case) or sub-telescopes (" Michelson" case) . One of the most critical points in the operation of such instruments will be the cophasing of the sub-apertures. Cophasing sensors are available on ground interferometers, allowing sub-aperture piston/tip/tilt measurement on unresolved or slightly resolved stars. But when observing a very extended object, such as the Earth as seen from space, no reference star can be found in the field. In this case, the cophasing measurement must be derived from the observed object itself, which is a major issue for extended objects.Phase diversity is one of the very few solutions to this problem. Phase diversity consists in the joint estimation of the object and the instrument aberrations from the analysis of several images obtained with different but perfectly known aberrations, for example the focal image and a slightly defocused image. Theoretical analysis and numerical simulations were carried out to investigate how our phase diversity algorithm behaves when estimating sub-aperture piston and tip/tilt in various conditions. Our study shows that the aperture configuration has a major impact on performance. For diluted apertures, when the optical transfer function has zeros within the frequency domain of interest, it can be shown that at least two possible solutions can be derived by phase diversity. For redundant apertures, when several pairs of subaperture contribute to the same spatial frequency, the piston or tip/tilt estimation is degraded for sub-apertures contributing to redundant frequencies.
High resolution observing systems need bigger and bigger telescopes. The design of such telescopes is a key element for the satellite design. In order to improve the imaging resolution with minimum impact on the satellite, a big step must be made to improve the compactness of the telescope.This paper describes the comparative study of several compact optical designs. Different apertures, from F/5 to F/20, and different concepts, TMA (Three Mirror Anastigmat), Ritchey-Chrétien concept and Korsch concept have been studied. We will point out advantages and disadvantages of each design. We will show how compact a high resolution telescope can be. A diffraction limited telescope can be less than ten times shorter than its focal length. A constraining consequence of this compactness is the huge increase of the sensitivity factors. The impacts on the optics and on the opto-mechanical tolerances have been analysed. The need to implement active optics raises. A first active optics experiment is presented
Because of its nicely chromatic behavior, Calcium Fluoride (CaF 2 ) is a nice choice for an optical designer as it can easily solve a number of issues, giving the right extra degree of freedom in the optical design tuning. However, switching from tablet screens to real life, the scarcity of information -and sometimes the bad reputation in term of fragility-about this material makes an overall test much more than a "display determination" experiment. We describe the extensive tests performed in ambient temperature and in thermo-vacuum of a prototype, consistent with flight CTEs, of a 200mm class camera envisaged for the PLATO (PLAnetary Transit and Oscillations of Stars) mission. We show how the CaF 2 lens uneventfully succeeded to all the tests and handling procedures, and discuss the main results of the very intensive test campaign of the PLATO Telescope Optical Unit prototype.
This paper describes the activities managed by CNES (French National Space Agency) for the development of focal planes for next generation of optical high resolution Earth observation satellites, in low sun-synchronous orbit. CNES has launched a new programme named OTOS, to increase the level of readiness (TRL) of several key technologies for high resolution Earth observation satellites. The OTOS programme includes several actions in the field of detection and focal planes: a new generation of CCD and CMOS image sensors, updated analog front-end electronics and analog-to-digital converters. The main features that must be achieved on focal planes for high resolution Earth Observation, are: readout speed, signal to noise ratio at low light level, anti-blooming efficiency, geometric stability, MTF and line of sight stability. The next steps targeted are presented in comparison to the in-flight measured performance of the PLEIADES satellites launched in 2011 and 2012. The high resolution panchromatic channel is still based upon Backside illuminated (BSI) CCDs operated in Time Delay Integration (TDI). For the multispectral channel, the main evolution consists in moving to TDI mode and the competition is open with the concurrent development of a CCD solution versus a CMOS solution. New CCDs will be based upon several process blocks under evaluation on the e2v 6 inches BSI wafer manufacturing line. The OTOS strategy for CMOS image sensors investigates on one hand custom TDI solutions within a similar approach to CCDs, and, on the other hand, investigates ways to take advantage of existing performance of off-the-shelf 2D arrays CMOS image sensors. We present the characterization results obtained from test vehicles designed for custom TDI operation on several CIS technologies and results obtained before and after radiation on snapshot 2D arrays from the CMOSIS CMV family.
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