Disruptive space telescope concepts, designs, and developments: OASIS and Nautilus -INVITED

Two disruptive space telescope concepts are being designed and developed at the University of Arizona; these are the 20-meter OASIS (Orbiting Astronomical Satellite for Investigating Stellar Systems) and 8.5-meter Nautilus. OASIS combines break-through inflatable aperture and adaptive optics techniques to realize the dream of a 20+ meter class spaceborne terahertz/far-infrared telescope. In the Nautilus visible/nearinfrared telescope concept, conventional primary mirrors are replaced by an ~8.5-meter MODE (Multi-order diffractive engineered) lens with 10 times lower areal density and up to 100 times lower mis-alignment sensitivity over traditional systems, enabling large-diameter optical space telescopes. The OASIS and Nautilus concepts have the potential to greatly reduce mission costs and risks compared to the current state of the art.


Introduction
The realization of a very large, space-based telescope for far-infrared/terahertz studies has long been a goal to study the origins of stars, planets, molecular clouds, and galaxies by providing an essential means of following-up on tantalizing results from recent successful missions such as Spitzer and SOFIA. The OASIS concept combines break-through technologies utilizing inflatable spherical reflectors and adaptive optics to realize a 20m terahertz space telescope [1,2]. In visible and near-infrared astronomy, an outstanding goal is to study the diversity of potentially Earth-like planets. Nautilus is a space telescope concept that builds on an engineered material diffractive-transmissive optical element [3,4]. The primary mirrors typical of current space telescopes are replaced by an ~8.5-meter MODE (multi-order diffractive engineered) lens with 10 times lower areal density, thereby enabling a lightweight structure. MODE lens telescopes also have up to 100 times relaxed alignment tolerances, compared to mirrors. The OASIS and Nautilus concepts have the potential to greatly reduce mission costs and risks compared to current space telescope paradigms through light-weighted optical design and technology.

Disruptive telescope concepts
Two paradigm-breaking space telescopes, OASIS and Nautilus, are summarized in Table 1.

20-meter class inflatable OASIS
The science objectives of OASIS are met by utilizing a 20-meter inflatable aperture with heritage from the Inflatable Aperture Experiment (IAE) mission, which demonstrated in-orbit deployment of a 14-meter aperture system shown in Figure 1. OASIS will utilize a Hencky reflector geometry together with proven adaptive optics techniques to yield a wide-field-of-view inflatable Mylar mirror operating at submillimeter wavelengths (i.e., terahertz frequencies). Mylar is a polyester film made from stretched polyethylene terephthalate called BoPET (Biaxially-oriented polyethylene terephthalate).

8.5-meter class replicable Nautilus
The baseline concept for the Nautilus unit telescope in compact launch and deployed configuration is presented in Figure 2. A cross sectional view of a deployed Nautilus unit telescope is shown in Figure 2(left). The telescopes use an 8.5 m diameter MODE lens as the light-collecting element for exoplanet transit spectroscopy observations. Each Nautilus unit will be a stand-alone telescope equipped with two visual/near-infrared band detectors and a low-resolution spectrograph (e.g., R ~ 200) optimized for the ~0.45-1.6 μm wavelength range.

Fig. 2.
A unit Nautilus telescope is launched in a compact configuration and inflated in orbit (left). Due to the compact launch configuration, multiple Nautilus units could be launched simultaneously in next-generation rocket fairings (right). [3,4] 3 Technology developments

1-meter Mylar prototype for OASIS
The 1-meter class OASIS Mylar reflector demonstration setup has been successfully manufactured, inflated, and tested as shown in Figure 3. The Hencky surface shape and its stability as a function of time and balloon pressure has been measured using a non-null deflectometry setup.

0.25-meter MODE lens camera for Nautilus
The MODE lens has been designed considering both the diffractive and refractive nature of light and fabricated as small-scale prototypes. The prototype lenses are small (~20 -250 mm in diameter in Figure 4) and ideal for understanding the details of the fabrication trade-offs and evaluating as-manufactured optical performance. An advanced optical metrology environment has been assembled to demonstrate quantitative imaging performance.

Concluding remarks
OASIS utilizes a space-based inflatable reflector and has the potential of making the dream of space-based 20meter class telescopes a reality. Nautilus will transform astrophysics through its paradigm-breaking, scalable, and ultralight MODE lens technology. The >10 fold increase in sensitivity OASIS provides will enable unprecedented access to the physical conditions of planetary bodies within our own solar system, as well as those surrounding distant stars. Large aperture visible/near-infrared wavelength spectroscopy with Nautilus will address one of the most fundamental questions in modern astrophysics by providing the first comprehensive, high-quality spectral atlas of small exoplanet atmospheres. Disruptive optics technology enables new discoveries.