Jeanette L. Domber

MOIRE: initial demonstration of a transmissive diffractive membrane optic for large lightweight optical telescopes

The desire to field space-based telescopes with apertures in excess of 10 meter diameter is forcing the development of extreme lightweighted large optics. Sparse apertures, shell optics, and membrane optics are a few of the approaches that have been investigated and demonstrated. Membrane optics in particular have been investigated for many years. The majority of the effort in membrane telescopes has been devoted to using reflective membrane optics with a fair level of success being realized for small laboratory level systems; however, extending this approach to large aperture systems has been problematic. An alternative approach in which the membrane is used as a diffractive transmission element has been previously proposed, offering a significant relaxation in the control requirements on the membrane surface figure. The general imaging principle has been demonstrated in 50-cm-scale laboratory systems using thin glass and replicated membranes at long f-number (f/50). In addition, a 5-meter diameter f/50 transmissive diffractive optic has been demonstrated, using 50-cm scale segments arrayed in a foldable origami pattern. In this paper we discuss Membrane Optical Imager Real-time Exploitation (MOIRE) Phase 1 developments that culminated in the development and demonstration of an 80 cm diameter, off-axis, F/6.5 phase diffractive transmissive membrane optic. This is a precursor for an optic envisioned as one segment of a 10 meter diameter telescope. This paper presents the demonstrated imaging wavefront performance and collection efficiency of an 80 cm membrane optic that would be used in an F/6.5 primary, discusses the anticipated areal density in relation to existing space telescopes, and identifies how such a component would be used in previously described optical system architectures.

MOIRE: Ground Test Bed Results for a Large Membrane Telescope

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.

MOIRE: ground demonstration of a large aperture diffractive transmissive telescope

The desire to field space-based telescopes with apertures in excess of 10 meter diameter is forcing the development of extreme lightweighted large optomechanical structures. Sparse apertures, shell optics, and membrane optics are a few of the approaches that have been investigated and demonstrated. Membrane optics in particular have been investigated for many years. The MOIRE approach in which the membrane is used as a transmissive diffractive optical element (DOE) offers a significant relaxation in the control requirements on the membrane surface figure, supports extreme lightweighting of the primary collecting optic, and provides a path for rapid low cost production of the primary optical elements. Successful development of a powered meter-scale transmissive membrane DOE was reported in 2012. This paper presents initial imaging results from integrating meter-scale transmissive DOEs into the primary element of a 5- meter diameter telescope architecture. The brassboard telescope successfully demonstrates the ability to collect polychromatic high resolution imagery over a representative object using the transmissive DOE technology. The telescope includes multiple segments of a 5-meter diameter telescope primary with an overall length of 27 meters. The object scene used for the demonstration represents a 1.5 km square complex ground scene. Imaging is accomplished in a standard laboratory environment using a 40 nm spectral bandwidth centered on 650 nm. Theoretical imaging quality for the tested configuration is NIIRS 2.8, with the demonstration achieving NIIRS 2.3 under laboratory seeing conditions. Design characteristics, hardware implementation, laboratory environmental impacts on imagery, image quality metrics, and ongoing developments will be presented.

Large-aperture fast multilevel Fresnel zone lenses in glass and ultrathin polymer films for visible and near-infrared imaging applications.

The ability to fabricate 4-level diffractive structures with 1 µm critical dimensions has been demonstrated for the creation of fast (∼f/3.1 at 633 nm) Fresnel zone lenses (FZLs) with >60% diffraction efficiency into the -1 focusing order and nearly complete suppression of 0 and +1 orders. This is done using tooling capable of producing optics with 800 mm apertures. A 4-level grating fabricated in glass at 300 mm aperture is shown to have <15  nm rms holographic phase error. Glass FZLs have also been used as mandrels for casting zero-thermal-expansion, 20 µm thick polymer films created with the 4-level structure as a route to mass replication of efficient diffractive membranes for ultralight segmented space-based telescope applications.

MOIRE Gossamer Space Telescope - Structural Challenges and Solutions

The MOIRE optical space system, being designed by Ball Aerospace and its partners for DARPA, is a gossamer structure featuring a 10 meter diameter membrane optical element at a distance 50 meters away from the spacecraft bus, with traceability to a 20 meter primary optic diameter system. During launch, the system is tightly packed to fit within the payload fairing and, once on station, it relies on intricate mechanism design to deploy to its final configuration. Pointing control, optical stability, weight, and stiffness requirements drive efforts in finely optimized structural design. This paper reviews these challenges and focuses on those pertaining to membrane stability and survivability and the novel solutions developed to meet them.

MOIRE Primary Diffractive Optical Element Structure Deployment Testing

The Membrane Optical Imager Real-time Exploitation (MOIRE) program, being developed by Ball Aerospace and its partners for the Defense Advanced Research Projects Agency (DARPA), seeks to enable technologies that would make orbital telescopes much lighter, more transportable and more cost-effective. MOIRE intends to design and develop a geosynchronous imager featuring a 10-meter diameter membrane optical element system at a distance 50 meters away from the spacecraft bus, with traceability to a future system with a 20-meter diameter primary optic. The program is preparing for a potential future space-based mission through large-scale, ground-based testing. Full-scale deployment testing of two petal segments combined with quarter-scale testing of a full system demonstrated feasibility of the 10-meter primary diameter design. This paper discusses the design, analysis and testing of the primary optic’s structural elements.

MOIRE Strongback Thermal Stability Analysis and Test Results

The Membrane Optical Imager Real-time Exploitation (MOIRE) program, being developed by Ball Aerospace and its partners for the Defense Advanced Research Projects Agency (DARPA), seeks to enable technologies that would make orbital telescopes much lighter, more transportable and more cost-effective. MOIRE intends to design and develop a geosynchronous imager featuring a 10-meter diameter membrane optical element system at a distance 50 meters away from the spacecraft bus, with traceability to a future system with a 20-meter diameter primary optic. The program is preparing for a potential future space-based mission through large-scale, ground-based testing. Due to the overall system size, subsystem and component level testing is necessary to capture data for incorporation into larger analysis models. Stability testing of two full-scale composite strongback segments was performed to correlate preliminary models of the primary optical structure. This paper discusses the analysis and observed test data.

MOIRE: Membrane Material Property Characterizations, Testing and Lessons Learned

The Membrane Optical Imager (for) Real-time Exploitation (MOIRE) programs’ primary optics are constructed using a membrane material. Using a membrane enables the goal of the MOIRE program to launch large, greater than 10m aperture, optics while simultaneously reducing the weight of the optic. Achieving the desired performance of the membrane as an optical material is highly dependent upon the ability to control that membrane through the design and fabrication process via an understanding of the material properties of the membrane. This paper will cover the material property characterization, testing performed and lessons learned about the unique attributes of membranes, both as a material and as an optic.