MOIRE: Ground Test Bed Results for a Large Membrane Telescope

Abstract

The Membrane Optical Imager Real-time Exploitation (MOIRE) program is developing technology to reduce the mass of large optical space telescopes through the use of a membrane primary optical element. Applications in astronomy and Earth observation envision apertures in excess of 10 m in diameter, which are too massive to launch even with the best current lightweight mirror designs. The primary aperture of the MOIRE telescope is a transmissive membrane etched with a diffraction pattern that achieves as much as a factor of 7 in mass savings per unit aperture area compared to lightweight mirrors. The transmissive primary significantly reduces the sensitivities to out of plane motion as compared to reflective systems while at the same time reducing the manufacturing time and costs. This paper focuses on the ground demonstration of the MOIRE telescope concept that traces to the design of a geosynchronous space-based demonstration system with a 10 m primary aperture. The primary purpose of the ground demonstration, or Brassboard, is to prove the ability to capture a high-quality scene image using diffractive membrane collection optics with narrowband incoherent spectral illumination. The Brassboard demonstrates the manufacturability and efficacy of the optical train, in particular the segmented diffractive optical elements of the membrane primary and the glass chromatic dispersion corrector. While the initial goal for the broadband optical image quality was to show tracability to an equivalent NIIRS 3.5 rating, the test bed setup at the time of this writing permits only a single diffractive element to be used for imaging, resulting in an expected equivalent NIIRS performance of 2.8. Images taken with the test bed yield a NIIRS value of 2.3, where the 0.5 knockdown in performance is a result of laboratory humidity and atmospheric turbulence. The MOIRE ground demonstration test bed establishes the ability to capture images using a diffractive telescope with a membrane primary, proving that a larger, lighter, and cheaper telescope can be manufactured and used to provide imagery of interest.