To image nearby Earth-like exoplanets (located 10 parsecs from us) with a spatial resolution of less than 1 microarcsecond, which would allow us to distinguish surface environments such as oceans, land, plants, and volcanoes, we would need an optical space telescope with an aperture exceeding 100 km. Given the unprecedented scale and technical difficulties of such a telescope, it is necessary to think beyond the constraints of existing telescopes. Accordingly, we have initiated a study to assess the feasibility of a novel telescope concept. This concept employs a combination of “magnetic formation flight of ultra-small satellites” and a “diffractive optical system”. The advantages of the diffractive optical system include its lightweight focusing optical system, low installation accuracy requirements, low manufacturing cost, and high storability when loaded in a rocket. However, there is no precedent for a telescope with multiple apertures using Diffractive Optical Elements (DOEs). Therefore, for a ground-based demonstration, we have fabricated two types of DOEs: binary amplitude and 4-level phase. The 4-level phase type exhibits significantly higher diffraction efficiency compared to the binary amplitude type, enabling high throughput of the optics. Each DOE is a 30 mm square and part of 100 mm diameter optics with a focal length of 1000 mm. We describe the experimental results of their point spread functions, wavefront errors, and transmittances.
In order to spatially resolve the surface environment of the second Earth within 10 parsec from us and theoretically predicted gaseous accretion disk around the first stars at around z = 20, we require an optical infrared space telescope with an aperture of 100 km to achieve a spatial resolution of 1 microarcsecond and a sensitivity of 34 magnitudes. However, the realization of such an extremely large space telescope is technically challenging and requires a breakthrough beyond existing telescope ideas. Here, we propose a new telescope concept with a combination of multiple diffractive optical elements and electromagnetic formation flight of 1 - 100 million ultra-small satellites in Sun-Earth L2 halo orbit. To determine the telescope concept, we performed an alignment error analysis of a segmented optical system with many optical elements. As a result, we selected multiple diffractive optical elements, which have huge advantages over lenses and mirrors in terms of required alignment accuracy as well as volume / mass. We have started feasibility studies of this new concept to check if the required spatial resolution and sensitivity can be achieved with multiple diffractive optical elements, by comparing numerical predictions and ground experiments. In this presentation, we introduce the science cases, science requirements, and telescope concept.
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