The Single Aperture Large Telescope for Universe Studies (SALTUS) is a mission concept for a far-infrared observatory developed under the recent Astrophysics Probe Explorer opportunity from the National Aeronautics and Space Administration. The enabling element of the program is a 14-m diameter inflatable primary mirror, M1. Due to its importance to SALTUS and potentially other space observatories, we focus entirely on M1. We present a historical overview of inflatable systems, illustrating that M1 is the logical next step in the evolution of such systems. The process of design and manufacture is addressed. We examine how M1 performs in its environment in terms of the operating temperature, interaction with the solar wind, and shape change due to non-penetrating particles. We investigate the longevity of the inflatant in detail, show that it meets mission lifetime requirements with ample margin, and discuss the development and testing to realize the flight M1.
OASIS (Orbiting Astronomical Satellite for Investigating Stellar Systems) is a space-based, MIDEX mission concept that employs a 14 meter inflatable aperture and cryogenic heterodyne receivers to perform high resolution (R<106) observations at terahertz frequencies. OASIS targets far-infrared transitions of H2O and its isotopologues, as well as HD and other molecular species from 660 to 63 μm that are otherwise obscured by the Earth’s atmosphere. OASIS will have <10x the collecting area and <4x the angular resolution of Herschel and complements the short wavelength capabilities of JWST. With its large collecting area and suite of terahertz heterodyne receivers, OASIS will have the sensitivity to follow the water trail from galaxies to oceans. OASIS represents a paradigm shift in the realization of large space apertures. Our paper will focus on how the development work for OASIS can be leveraged to realize a new generation of space telescopes.
The Orbiting Astronomical Satellite for Investigating Stellar Systems (OASIS) is a proposed space telescope with a 14-m inflatable primary reflector that will perform high spectral resolution observations at terahertz frequencies with heterodyne receivers. The telescope consists of an inflatable metallized polymer membrane that serves as the primary antenna, followed by aberration correction optics, and a scanner that enables a 0.1-deg field of regards while achieving diffraction-limited performance over wavelength range from 63 to 660 μm. Here, the parametric solution space of the OASIS inflatable telescope design is systematically investigated by establishing analytical relations among figure of merits including first-order geometrical photon collection area and the size of correction optics. The first-order solution was further optimized by ray-trace code by incorporating numerically calculated mirror shape with preformed membrane gores. Design study shows that a space-based telescope with an effective photon collection area of over 90 m2 can be achieved by utilizing a 14-m inflatable aperture.
KEYWORDS: Particles, Solar processes, Space operations, Reflectors, Sun, Data modeling, Telescopes, James Webb Space Telescope, Tolerancing, Solar radiation
This paper discusses pressure control for the OASIS primary antenna element, A1. This discussion is centered around the evaluation of pressure changes and what might drive them. A1 is created from thin polyimide film and from its orbital position near Sun-Earth L1, is subject to many environmental effects, the solar wind, radiation pressure, charging and micrometeoroids. This paper begins by describing the architecture of the pressure control system. We show that the solar wind and radiation pressure are too small to impact A1’s performance. We also discuss the need to connect the A1 to system ground for solid technical and programmatic reasons. A large section discusses the micrometeoroid environment and how recent mission data shows that the flux faced by OASIS is likely larger by factor of ~3 than might be expected from naïve application of the traditional models.
Inflatable membrane primary optics for space telescopes are a smart approach in the context of saving flight payload weight and volume. The Orbiting Astronomical Satellite for Investigating Stellar systems (OASIS) adopted the membrane architecture for primary optics (primary antenna, A1) to have 20 meter diameter collection area with operation bands at the terahertz frequency. The membrane is made of Kapton or Mylar film with an aluminized surface, and the balloon (transparent surface + aluminized surface) is inflated to work as the convex mirror. In order to leverage the carrying volume advantage of inflatable optics, it must be folded during launch and deployed in orbit. The thin membrane film can crumple easily when it is folded, and it should be ironed out when the telescope is deployed for observation. We studied the microroughness and mid-to-high spatial frequency characteristics of the membrane via optical metrology to evaluate the surface properties. Because it is not of traditional shape and material, it is impossible to test with an offthe- shelf interferometer and profilometer. Moreover, the defect spatial frequency of interest is a few hundred microns to millimeters range, so the measurable field and dynamic range need to be in range of a few centimeters with microns resolution. To meet those requirements for metrology, we developed a flexible optics testbed utilizing deflectometry. The microroughness and mid-to-high frequency properties are measured with a white light interferometer and proposed methodology. The test results show that the candidate membrane is suitable for OASIS and this reliable test will guide the further design study of A1 assembly and optical system error budget.
The Orbiting Astronomical Satellite for Investigating Stellar Systems (OASIS) is a 20-meter class proposed space terahertz observatory supported by an inflatable membrane architecture. To measure 150 mm and 1m models of the A1 reflective membrane antenna, two deflectometry configurations were designed. The smaller assembly and its corresponding deflectometer were simulated, built in our laboratory, and produce a reconstructable signal for clocked measurements of the highly-sloped pneumatic surface. We use non-sequential raytracing simulation to bound the maximum contributions of all shape errors and suggest the N-Rotations algorithm to remove the remaining radially asymmetric errors. Then, the 1m prototype assembly was tested inside a thermal vacuum chamber (TVAC). Differential deflectometry measurements tracked the 1m surface shape changes as it was subjected to a variety of environmental setpoints, cycled between three inflation gases, and also during controlled puncture. We summarize our development and results for absolute measurements as well as from TVAC testing.
The uninflated shape configurations of parabolic and spherical membrane mirrors were calculated by solving the inverse problem, i.e., given the design inflation pressure, the membrane material and geometric properties, what must be the initial uninflated shape such that on inflation to the design pressure, the exact desired surface of revolution is obtained. The resulting first order nonlinear differential equation was numerically integrated using the boundary conditions. The initial uninflated shape was then subjected to a forward transformation using FAIM, a proprietary geometric nonlinear membrane finite element code. FAIM has been validated against exact analytical solutions for both small and extremely large deformations that are up to eight orders of magnitude larger compared with the starting undeflected shape. Simulations reveal that to fabricate a very accurate and precise inflated membrane mirror relative to the design parameters, one must not only accurately measure and input the moduli in both meridional and hoop directions but an accurately measured Poisson’s ratio as well. The code was used to guide the membrane mirror design. For very small aperture diameters, the initial uninflated shape may be fabricated by thermo-forming the membrane. For aperture diameters exceeding one meter however, the membrane mirror is built with discrete gores that are joined together with tapes at the seams. This provided the impetus to write a companion computer code FLATE, to calculate the gore shapes using a slight modification of the solution to the inverse transformation equation to account for the presence of the seam tapes. After the gores were determined, the resulting final inflated shape was calculated and verified using FAIM. Sensitivity analyses can now be carried out to determine the resulting surface shape as a function of the different sources of error: gore width, gore length, perimeter attachment uncertainties, thermal effects, variation of material properties over the membrane continuum and inflation pressure changes. The code has been shown to be more robust than equivalent commercial analytical packages in so far as membrane, cable and space-frame element combinations are concerned. In particular, the analytical and finite element codes were used in the preliminary assessment of a membrane optic for the OASIS Mission (Orbiting Astronomical Satellite for Investigating Stellar Systems) [1]. The OASIS is a 20-meter class space observatory operating at high spectral resolution in the terahertz frequencies. Over its nominal 2-year mission it will probe conditions and search for biogenic molecules on hundreds of protoplanetary disks and other solar system objects.
Deformable reflector technology has mainly been used for observations at visible and infrared wavelengths but has yet to be utilized for terahertz wavefront correction. We present an actuator for deformable reflectors that overcomes challenges particular to this wavelength such as a millimeter-scale stroke requirement. Bending moment actuators are used in both the radial and tangential directions to correct low-order wavefront aberrations. Strong and flexible materials such as Delrin are used for the reflector material. Such a deformable antenna can be used to correct wavefronts on future large radio antennae such as the Orbiting Astronomical Satellite for Investigating Stellar Systems (OASIS). This antenna uses a 20-meter thin membrane as its primary radio wave collector. A deformable reflector may be added to this system to allow for looser tolerances on the primary antenna shape and correct for wavefront errors inherent in an inflatable optic. To predict the wavefront errors that may be expected when using this type of thin membrane primary reflector, TVAC (Thermal Vacuum Chamber) test methods are also presented in these proceedings.
Orbiting Astronomical Satellite for Investigating Stellar Systems (OASIS) is a mission concept being developed in preparation for the 2021 MidEX Announcement of Opportunity. This paper describes the key features of the OASIS architecture as they are currently understood. OASIS’s choice of a large inflatable primary reflector results in large collection areas at very high mass efficiency enabling the science mission. We describe the spacecraft bus, based on Northrop Grumman’s LEOstar-2, and the receiver, a heritage design based on the GUSTO balloon heterodyne system. We also discuss the observing strategy and pointing requirements from its planned L1 location. Particular emphasis is placed on challenges to the design, such as momentum management, balancing consumable mass allocations, thermal management, and testing.
OASIS (Orbiting Astronomical Satellite for Investigating Stellar Systems) is a space-based observatory with a large inflatable primary reflector that will perform high spectral resolution observations at terahertz frequencies. An inflatable metallized polymer membrane serves as the primary antenna with large photon collecting area, followed by aberration correction mirror pair that enables a large field of regards of 0.1 degrees while achieving diffraction limited performance over a wide terahertz wavelength ranging from 80 μm to 660 μm. An analytical model is developed to define a solution space based on the profile of primary reflector which is a function of pressure. The photon collecting area, size and weight of the correction mirror pair, and optical aberrations are governed by a 1st order power arrangement of the telescope and is a function of base radius and clear aperture of the primary reflector. Based on the parametric design study, the figure of merit for the profile of the primary reflector is discussed and a baseline design satisfying the scientific and system requirements is proposed.
The ability of space telescopes to see into nascent protostellar systems and even further into our universe is driven by the size of their deployable light collection area. While large monolithic mirrors typically weigh tons, inflatable membrane mirrors present a scalable, ultralightweight alternative. Leveraging decades of advances in adaptive optics technology, the possibility of a well-corrected 20 meter-class space observatory such as the Orbiting Astronomical Satellite for Investigating Stellar Systems (OASIS) is strikingly feasible. However, with great aperture size, comes great metrology requirements. Membrane reflectors are characteristically structured as one transparent and one metallized polymer membrane sealed around a steel tensioning ring. The inflated surface does not naturally conform to a known or prescribed conic but an approximate Hencky surface. Furthermore, multiple internal reflections and polarization interactions between the dielectric and metal layers disturb coherent light that probes it. A non-contact, full-aperture testing method is needed and further, one that can test highly varying membranes after thermoforming too. We present our method in obtaining the absolute shape of thermally formed, inflatable reflectors for space telescopes. Our work measures a 1-meter prototype of the OASIS primary inflatable mirror. Evolving from laser distance scanning to photogrammetry to deflectometry, our survey of metrology techniques for inflatable membrane optics is discussed.
Next generation space telescopes with apertures >10m will require novel technologies to permit lightweight primaries to operate at the diffraction limit in the optical regime. One solution is to construct a telescope from a lightweight, membrane primary, which is holographically corrected for surface distortions, in situ. We have demonstrated the correction of >10,000 waves of error in a 1-m diameter primary having an areal mass of just 17 grams per square meter.
An intensive investigation has been carried out to study the surface profiles obtained as a result of the large deformations of pressurized membranes. The study shows that the inflated membrane shapes may have the requisite surface accuracy for use in future large space apertures. Both analytical and experimental work have been carried out. On the analytical side, the classical work of Hencky on flat circular membranes was extended to eliminate the limitations it imposed; namely a lateral non-follower pressure with no pre-stress. The result is a computer program for the solution of the pressurized circular membrane problem. The reliability of the computer program is demonstrated via verification against FAIM, a nonlinear finite element solver developed primarily for the analysis of inflated membrane shapes. The experimental work includes observations made by Veal on the (W-shaped) deviations between the membrane deflected shape and the predicted profile. More recent measurements have been made of the deformations of pressurized flat circular and parabolic membranes using photogrammetric techniques. The surface error quantification analyses include the effect of material properties, geometric properties, loading uncertainties, and boundary conditions. These effects are very easily handled by the special FEM code FAIM which had recently been enhanced to predict the on-orbit dynamics, RF, and solar concentration characteristics of inflatable parabolic antennas/reflectors such as the IAE that flew off the space shuttle Endeavour in May 1996. The results of measurements have been compared with analyses and their ramifications on precision-shape, large-aperture parabolic space reflectors are discussed. Results show that very large space apertures with surface slope error accuracies on the order to space reflectors are discussed. Results show that very large space apertures with surface slope error accuracies on the order of 1 milliradian or less are feasible. Surface shape accuracies of less than 1 mm RMS have been attained on ground measurements.
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