The ESA M size mission PLATO (PLAnetary Transits and Oscillation of stars) is planned to be launched in the 2026, with the aim of discover exoplanets that will be characterized with unprecedented precision. The optical elements of PLATO are 26 small telescopes, the TOUs (Telescope Optical Units), that using partially overlapping Fields of View will permit instantaneous sky coverage larger than 2100 square degrees. Each TOU has an aperture of 120 mm diameter assured by an internal stop, and it is composed by 6 lenses, the frontal one having an aspherical surface and the last acting as field flattener. The mechanical structure is realized mainly in AlBeMet. We here describe the optical design, summarizing several optical properties (materials, coatings, etc.), and report on nominal performances of the TOU system.
KEYWORDS: Sensors, Modulation transfer functions, Point spread functions, Stars, Telescopes, Detection and tracking algorithms, Cameras, Imaging systems, Exoplanets, Data modeling
The ESA M-size mission PLATO (PLAnetary Transits and Oscillation of stars) is scheduled for launch in 2026. During its 4-year mission orbiting Sun-Earth L2 is aimed at the discovery of exo-planets in nearby star systems. Its 26 cameras will be looking toward deep space, covering a wide area of the sky. Each camera is based on a fully dioptric design. Each camera is composed of a telescope optical unit (TOU) and a focal plane array (FPA). Here the procedure for the characterization of the best imaging plane (BIP) for each TOU is described. Camera manufacturing activity faces the challenge, by design, of locating the BIP for each TOU with high precision (±30 μm). Strict tolerances for accommodating the FPA are in fact in place in terms of inter-distances with respect to the mechanical mounts and tilts. Localization of the BIP will be carried out by mathematical optimization of the enclosed energy performances that will be evaluated at a pre-defined set of 45 field positions. Given the relatively big field-of-view of each camera (1037 deg2 for 24 normal-type and 610 deg2 for the 2 fast-type), this poses a challenge for a single service detector. Instead, an approach based on stitching single views of a smaller active area detector will be carried out. To circumvent performance limitations of detectors, for example, image degradation due to finite-size pixel sampling, dithering will be performed by leveraging a very precise mechanical positioner (hexapods) on which the detector is mounted.
his paper focuses on depth of field (DOF) extension through polarization aberrations. The addition of polarizing elements into an optical system allows to exploit the polarization of the incoming light as an additional degree of freedom in the optical system design. Two optical systems have been studied: the first characterized by the presence of polarizing thin film coated optical surfaces, the second based on the addition of an anisotropic birefringent waveplate into the path of light rays of an optical system. The polarization dependent DOF of these two systems are compared. It is shown that the effect of polarizing elements is similar to a polarization dependent apodization of the pupil.
A Focused Plenoptic Camera in Galilean configuration is studied and its aberrations behavior is interpreted with the Nodal Aberration Theory (NAT). Sequential ray tracing is applied to individual optical channels constituted by the camera objective and individual decentered microlenses. The wavefront aberration field is retrieved at the exit pupil of the optical channels and is analyzed through the Zernike Fringe decomposition technique. Decentered optical channels show nodes in the field-dependence of different Zernike coefficients approximating the wavefront aberration field. The nodal behavior is a consequence of the loss of rotational symmetry in a decentered optical channel due to the displacement of a microlens with respect to the mechanical axis of the camera.
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