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1.INTRODUCTIONThe scientific case for very large telescopes has been very well established by various studies over the past decade, most recently and particularly well by the Association of Universities for Research in Astronomy (AURA) “From Cosmic Birth to Living Earths” study1 in which they present and discuss a concept for a High Definition Space Telescope (HDST). Building this observatory in the current and likely future cost constrained environment will be a challenge. Space telescope costing studies, for example Stahl et al2, have developed empirical relationships that indicate that cost of large astronomical telescopes will greatly challenge the existing NASA Astrophysics budget. Telescope, spacecraft, and instrument technology has been advancing at a rapid pace, enabling new approaches to building and operating future large observatories. We as a nation are beginning to expand our in-space infrastructure, further enhancing our options for large space telescopes. In going forward we must examine alternate telescope architectures that reduce cost, shorten schedules, expand performance, and take advantage of new or pending technology and the developing space infrastructure and not be locked into traditional approaches. While there are certainly technical challenges with any new architecture, the greater challenge is more often the culture changes that are required to abandon the comfortable traditional approach and adopt a new path. In this paper we look at two alternate architectures for future large astronomical observatories. An Evolvable Space Telescope (EST) concept that builds the observatory in stages in space utilizing our in-space servicing and assembly infrastructure to keep annual costs relatively low while enabling science return early in the program and producing a very long lived observatory that evolves in both size and science capability to take advantage of the advancing technology and adapting to new science questions as they arise. The EST concept was first presented at SPIE in 2014 (Polidan et al3). The second large telescope architecture is a Rotating Synthetic Aperture (RSA) concept that utilizes a large rectangular aperture and rotates to fill the UV-plane. This concept was initially developed by Raytheon Space and Airborne Systems for non-NASA applications and is being reexamined for application to astrophysics by a combined Northrop/Raytheon team. RSA offers potential cost and complexity reductions while delivering exceptional science performance. 2.EST SCIENCE GOALSThe EST science goals will only very briefly be discussed in this paper, since the science goals for future large observatories are outlined in great detail in the AURA “From Cosmic Birth to Living Earths” study1. The three top-level EST primary science goals will likely be:
Of course there is a wide range of additional science that would be enabled by EST. Stage 1 in particular is designed to be an exceptional UV astrophysics observatory with its large collecting area and high efficiency prime focus instruments. However it is the three top level science goals that carry the most weight in the setting of top level requirements for EST performance. These top level requirements are enumerated in Table 1. Table 1:EST Design Reference Concept Top Level Requirements
3.EST CONCEPT SUMMARYIn 2014 (Polidan et al3) we presented the basic Evolvable Space Telescope concept. The concept had three major measures of merit, most notably in terms of performance and affordability. Three that particularly distinguish the EST mission concept from other approaches are: evolvability, adaptability, and serviceability.
The baseline EST is designed to be a Sun-Earth L2 (SEL2) observatory. It begins with an initial development and launch of a “core” fully functional telescope with instruments. Specifically, this first Stage is a three segment off axis telescope (in 2014 Stage 1 was envisioned as a two element “bow-tie” arrangement). Subsequently, this observatory is augmented with additional mirror segments, new instruments, spacecraft subsystem replacements/upgrades, and other needed infrastructure elements to build a filled aperture telescope. Follow-on augmentations would add additional mirror segments and instruments along with any needed spacecraft and infrastructure replacements/upgrades to build an even larger filled aperture observatory. For simplicity for this study we have adopted a fixed mirror element size of ~4 meters flat-to-flat, or approximately 4.5 meters point-to-point, although these dimensions are purely illustrative. There is nothing in the architecture that requires a specific size for each mirror element and each mirror element can be a single mirror segment (the baseline for this study) or composed of smaller mirror segments that are built up into the basic mirror element. The EST approach can accommodate any segment size, with the telescope size and total collecting scaling with the mirror element size, but the EST architecture functions equally well with smaller or larger mirror segments. This evolution of the EST Primary Mirror Assembly (PMA) is described below and shown pictorially in Figure 1. Illustrations of the full observatory are presented in Figure 2.
4.2015 EST DEVELOPMENTSIn 2015 we continued the development of EST with three primary objectives: 1) to revisit the 2014 Stage 1 approach and architecture vis-à-vis mission science goals and the overarching (3 stage) system design; 2) to explore more thoroughly the prime focus instrument accommodation; and 3) to examine the existing and planned in-space assembly and servicing infrastructure and its application to EST. 4.1Reassessment of EST Stage 1At the end of the 2014 study it was realized that the “bow-tie” EST Stage 1 had less collecting area than desired to meet science goals and that the point spread function from the “bow-tie” was less than optimal. Eventually, a 3 adjacent mirror element architecture was settled upon as the preferred option (Figures 1 and 2). This increased the collecting area by 50% over the bow-tie version and gave a vastly improved point spread function. This change necessitated a modification of the telescope structure which also improved performance by moving EST Stage 1 from an on-axis obscured system to an off axis, unobscured system. It also simplified the transition to EST Stage 2, evened out the fabrication schedule, and made better use of the lifting capacity of the rockets used for the first two stages. The “bowtie” configuration, with its separated elements, made demands on the telescope alignment system that the contiguous half-aperture adjacent segments do not. In addition, looking at both coronagraphic and UV spectroscopy instruments for Stage 1, the prime-focus instrument package was both more practical and provided higher performance than was possible with the “bow-tie” configuration. 4.2EST Optical System Performance4.2.1IntroductionThe EST concept is versatile and flexible to respond to the continuously evolving needs of a diverse astronomical community in the presence of budgetary and technical constraints. By launching the telescope in parts and assembling it in space we have the opportunity to replace aging hardware with more capable hardware without the expense and waste of discarding the telescope and its instruments as will happen to HST and JWST. Preserving our investment in on-orbit hardware will lead to affordable observatories. EST’s primary mirror is segmented. The concept discussed above is to robotically add to or replace segments as needed, in a manner similar to that used by the AAReST4 project at Caltech. The HST has been very successful in large part because of our ability to swap out instruments. But there is no reason that, if we design the system properly we cannot also swap out telescope optical elements to optimize the configuration in-space to address the then current priorities of the astronomical community. Note that because of the rotational symmetry of Stage 1 about the optical axis, the non-recurring engineering costs of Stage 1 are fully applicable to Stage 2. In fact, one can build two identical Stage 1 mirror assemblies, launch the Stage 1 tri-hex system and observe with this off-axis telescope. Then, when sufficient funds are available launch the second unit, rotate it around, and match it to the Stage 1 system to create the 12-meter Stage 2 on-axis, filled aperture telescope. Figure 3 below shows a concept for the EST primary mirror and metering structure. Light, shown as a broken line enters the system from right to left, reflects from the concave primary mirror and converges to a focus near the right end of the metering structure. At the right end of the metering structure is a flange for docking either the Cassegrain secondary or a prime focus instrument assembly (PFIA). Figure 3:The Evolvable Space Telescope core structure. The broken line shows light reflected from the primary mirror converging to a focus near the flange for docking instruments or mirrors. The metering structure shown is notional. The three hex segments of the primary mirror are used in an unobscured telescope.  4.2.2Classical CassegrainFigure 4 below shows a concept for the EST with a classical Cassegrain secondary mirror assembly package docked to the right end of the metering structure shown in Figure 3. The secondary mirror assembly can be undocked and removed in space and replaced with other modules such as a prime-focus instrument assembly or another secondary mirror with an optical figure optimized for a three mirror anastigmat (TMA) wide-field system depending on the science measurement priorities. If the instruments at the Cassegrain focus contain diffraction gratings and fold mirrors that produce Fresnel polarization aberrations, several polarization mitigation approaches are available5,6. Figure 4:A concept for the EST with a classical Cassegrain mirror assembly package docked to the right end of the metering structure shown in Figure 3.  4.2.3Prime FocusIn this section, we introduce the concept of an EST prime focus and examine the performance of a prime focus space-telescope configuration. An instrument at prime focus enables us to minimize scattered light, eliminate a structurally sensitive optical element (the secondary), minimize polarization aberrations, reduce thermal disturbances and maximize transmittance for observing extremely faint objects and for observational UV astronomy. For many years, before the dawn of the age of space telescopes, ground-based astronomers used the Cassegrain configuration for their telescopes. This design approach was chosen to minimize telescope dome structures and place the instruments behind the primary mirror where there was a large volume available. The mass of the instrument was also near the heavy primary mirror and easy to reach by an astronomer peering through the telescope eyepiece. Large telescopes such as the 5-meter Hale telescope and the 3-meter Shane telescope were built with prime focus “cages” where the observer rode along with the science instrument. These cages are less valuable today because mass and volume are constrained. Although several ground based telescopes do have wide field cameras at their prime focus to maximize their sensitivity for faint objects. For space telescopes, the HST is a classical Cassegrain and the JWST is a TMA whose primary and secondary are in classical Cassegrain configuration. Both inherit this tradition from ground-based astronomy. Ground telescopes are constrained to operate at atmospheric temperature, within the Earth’s atmosphere and gravitational field. However, for space telescopes some of these ground-based constraints are missing and a prime focus system may be less expensive to build, and refurbish for a given science return than the Cassegrain. The cost of a telescope depends to a large extent on the number of large optical elements that need to be controlled to nanometer tolerances. The EST gives us an opportunity to conduct trades and analyses and explore the possibility of a new more cost effective space telescope optical configuration. Figure 5 is a sketch of a prime focus telescope system designed for wide-field and coronagraphic imaging by the Stage 1 and 2 twelve-meter EST. Light passes from the right to the left to reflect from the 12m F/# = 2.5 EST primary mirror. The box shown just to the right of the prime focus is the prime focus instrument assembly (PFIA) whose diameter is 3 to 4 meters and length a few meters to hold prime focus instruments such as UV spectrometers, imagers, coronagraphs, wide-field cameras, etc. The focal plane is shared (like HST and JWST) with several instruments contained within the PFIA. A metering structure (see Figure 3 above) separates the primary mirror from the prime focus. A free-flying spacecraft can be attached to the PFIA for unlatching and precision docking of the PFIA from the end of the metering structure for replacement of the entire instrument assembly. Following one of the HST system architecture features, the EST is designed for instrument and mirror upgrades based on new detector developments, new, more efficient optical designs and new, evolving science measurement objectives during the 30-year anticipated lifetime of the primary mirror assembly. Figure 5:Shown is a prime focus concept for the EST system. A prime-focus instrument assembly (PFIA) is shown docked to the flange at the right end of the metering structure, 30m from the vertex of the primary mirror that separates the primary from the PFIA. The metering structure (not shown here) separates the primary mirror from the flange (not shown).  We have selected one example instrument, an imaging system, for discussion here. The prime focus is 30m from the vertex of the primary. Light passes through a field lens and expands to fill the primary of an inverse Schwarzschild7,8 reimaging system mounted just beyond prime focus. The field lens relays an image of each segment of the primary onto the 0.6-meter diameter active segmented number one mirror of the Schwarzschild. Primary mirror wavefront errors (tilt, piston and surface) caused by fabrication and time-dependent thermal, dynamics and structural effects are compensated for at the 60-cm segmented secondary mirror. Light reflects from the secondary to the convex tertiary and then to the focal plane indicated by the circle or to a dispersive element in the case of a spectrometer. The convex curvature on the tertiary combines with the concave powered mirrors 1 and 2 to provide a flat field at the focus. Adjusting the design of the Schwarzschild relay controls the detector sampling frequency. For small fields of view such as those needed for exoplanet coronagraphy and high contrast imaging of stellar neighborhoods, the transparent field lens can be swung out of the way and possibly replaced by a diffractive mask for unprecedented direct control of the unwanted radiation after the first reflection. There are only three reflecting optical surfaces before the detector. There are no fold mirrors in the system, which will minimize polarization aberrations and their deleterious effect on the PSF9,10,11. 4.2.4Point spread functionThe PSF for the prime focus system is shown in Figure 6. As discussed in Section 3, the first Stage of the EST will use three 4-m class segments oriented as shown in Figure 1. Figure 6 shows the point-spread-function (PSF) for the three- hex pupil for 4-m flat-to-flat segments with 5-mm gaps. Geometric aberrations are fully corrected. Polarization aberrations were not calculated, but should be minimal or easily mitigated since the basic three-mirror system has no flat fold elements to add significant Fresnel polarization aberrations. On the left is a Log10 intensity image of the PSF with a Field of view of 4 x 4 arc seconds. On the right is a cross section through the PSF taken at 30 degrees as shown by the solid white line. The horizontal axis is the FOV from −.5 to +. 5 arc seconds and the vertical axis is Log10 intensity from 0 to 10−7. With an effective light gathering area of ~40-m2 and a maximum base line of 12-meters, this Stage 1 system will be a very capable and productive first stage for EST. Redesign of the instruments, in particular the location and shape of the telescope pupil will be necessary for the 12-meter 6-Hex Stage 2 and the eventual 20-meter 18-Hex Stage 3. Figure 6:The point-spread-function (PSF) for the three-hex pupil with 4-meter flat to flat segments and 5-mm gaps. Geometric aberrations are fully corrected. Polarization aberrations were not calculated, but should be minimum or easily mitigated since the basic three-mirror system has no flat fold elements to add significant Fresnel polarization aberrations. On the left is a Log10 intensity image of the PSF with a Field of view of 4 × 4 arc seconds. On the right is a cross section through the PSF taken at 30 degrees as shown by the solid white line. The horizontal axis is the FOV from −.5 to +. 5 arc seconds and the vertical axis is Log10 intensity from 0 to 10-7.  The advantages of the EST and the PFIA system shown in Figure 5 are:
4.3In-Space InfrastructureEssential to the EST concept is the existence of a mature in-space assembly and servicing infrastructure, either purely telerobotic, purely crewed, or most likely a combination of telerobotic and crewed. As stated earlier, leveraging and utilizing in-space assembly and servicing is the key to the success of the Evolvable Space Telescope, particularly given the extended operational lifetime expected of the observatory. Accomplishing the goals of EST will require a NASA in-space infrastructure capable of:
This, in turn, will require a substantial in-space infrastructure designed for maintenance, upgrading, and repair of large space vehicles. This infrastructure does not yet exist, although many aspects have been examined in various degrees of detail and completeness. A skeleton infrastructure has been created, including the International Space Station (ISS), a number of ground-launched single use logistical spacecraft (Orion, Dragon, Cygnus, …), and miniature auxiliaries (SPHERES, Cubesats, and the like). The fact that this is clearly inadequate for a robust development of space, both for science and for manned exploration, implies a clear need for a vigorous infrastructure development and deployment. This need has been recognized and thoroughly discussed in the NASA “On-Orbit Satellite Servicing Project Report” (October 2010)4. Moreover, the value of an infrastructure for assembly and servicing was explicitly recognized by the AURA “From Cosmic Birth to Living Earths” report1: “Finally, on-orbit satellite servicing and on-orbit spacecraft assembly are two key technologies that are beyond the scope of any single project, but that could be of great benefit to HDST and many other future missions, were they to be provided. We encourage NASA to consider whether a new on-orbit servicing infrastructure could be developed, perhaps in concert with other agencies or the commercial space sector. Replacement of failed components extended the life of Hubble, and replacement of its instruments greatly enhanced its scientific value. We would want the same for HDST and other missions as well.” The logical starting point would be the development and deployment of two key first generation systems whose capabilities interlock and permit them to operate in coordination with each other. These are an optionally crewable Deep Space Habitat (DSH – effectively a small space station located in cislunar space) and a smaller, unmanned vehicle (termed MiniServ in this paper) that can be based at the DSH and operated in its vicinity or deeper in space for telerobotic assembly and repair missions. Both systems are considered to be well within the current state-of-the-art, although they will obviously benefit from new technology developments should such become available. Such infrastructure development is the subject of this section, which necessarily assumes certain parameters concerning the telescope itself. The telescope model is, of course, the Evolvable Space Telescope, but many of the same considerations will likely apply to the Rotating Synthetic Aperture (RSA) concept discussed later in this paper and other space telescope architectures. Four assumptions regarding the EST are specifically relevant in the context of that concept:
The EST is only a design concept at this time, so detailed design of the supporting infrastructure is not yet possible. However, based on the objectives and assumptions stated elsewhere in this paper, a few high order conclusions can be reached, and then used as a foundation for more detailed analyses in parallel with the EST design process. In particular, two issues will be addressed here: location of the assembly process, and identification of a set of space vehicles upon which the servicing vehicle can be based and some thoughts on whether it must be manned or can be unmanned. Construction and deployment of the EST is likely to occur on an approximately matching time schedule with buildup of a manned infrastructure in the lunar vicinity (taken to mean anywhere in space between geosynchronous altitude and lunar orbit). Therefore, it would be logical for these two programs to coordinate their activities, both technologically and operationally. This coordination will be assumed. In fact, the existence of at least most of the infrastructure for a large manned program may be a political/economic prerequisite for the astrophysics infrastructure. 4.3.1Assembly LocationGiven that the EST will be located in a SEL2 halo orbit, there will be three basic orbit classes available for assembly: Low Earth Orbit (including both LEO and GEO); the lunar vicinity in the neighborhood of the Earth-Moon libration points EML1, 2 (or a Distant Retrograde Orbit (DRO)) and the neighborhoods of the Sun-Earth libration points, SEL1, 2. With care, a significant portion of the assembly could also be conducted enroute to SEL2, but that will remain for later evaluation.
4.3.2Assembly and Service PlatformsAlthough it is possible for large space telescopes to be designed to assemble themselves in their operational orbits without further outside assistance, once the need for service, repair, or upgrade arises, one or more auxiliary service vehicles will be essential, if only to carry parts, tools, and expendables to the telescope. The International Space Station (ISS) provides an example of a very large platform that could be used to service other space vehicles but, since EST will not be serviceable in LEO, the ISS cannot be considered for other than experimental demonstrations (see below) or as a depot for parts to be carried to a higher servicing orbit. Thus, there are three classes of service platforms, which may be roughly distinguished from each other on a size basis: Macro, Mini, and Micro, each of which has a distinct role to play in establishing and maintaining EST.
4.3.3Nominal Assembly/Servicing Concept of Operations for ESTBased upon the preceding discussions, the following is suggested as a baseline concept of operations for the Evolvable Space Telescope:
4.3.4Concept Validation in Low Earth Orbit (LEO)As noted above, assembly and servicing of an EST-class space telescope in LEO is not an effective approach to the support infrastructure. However, given the current and indefinite availability of the International Space Station (ISS), there are important experiments and demonstrations that can be performed in LEO to provide increased confidence in the design and technology for EST and for its supporting infrastructure. In particular, a concept known as MoDEST (Modular Demonstration of an Evolvable Space Telescope), discussed in Section 4.4, provides a concept for an orbital demonstration of several critical features of EST and of its required supporting infrastructure. It is an example of a possible near term demonstration in support of EST or other concepts for the next large observatory. It is noteworthy that some aspects of a MiniServ system or other MicroService Platforms could also be tested using MoDEST and the current CanadaArm2 telerobotic system on the ISS. In particular, a prototype MiniServ manipulator/tool module could be loosely held by the end of the arm using a tether, moved by the arm into the proximity of a serviceable object, and then allowed to perform autonomous proximity operations near that object. The operations would be closely monitored, and the tether would provide a means of safely aborting them by pulling the module back should a malfunction be indicated. 4.4Modular Demonstration of an Evolvable Space TelescopeThe Modular Demonstration of an Evolvable Space Telescope or MoDEST is an early opportunity to mitigate risks and lower costs of technologies and processes needed for remote autonomous large telescope construction. MoDEST demonstrates: 1) An Evolvable Space Telescope (EST) enabling large (>12 m) HDST-like space telescopes with existing launch systems, 2) An affordable path to large apertures that are grown in stages that alleviates both peak cost per year and overall program cost growth issues, 3) On-orbit assembly of a precision telescope, and 4) Active correction of phase and mirror shape. Working off the International Space Station (ISS) platform, MoDEST will provide an excellent near-earth opportunity to test autonomous and robotic assembly techniques as well as advanced adaptive optics technologies critical for future large telescopes. MODEST (Figure 9) is proposed to be delivered to the International Space Station using standard Express Logistics Carriers (ELCs) and can mounted to one of a variety of ISS pallets. Post-assembly value includes space, ground, and environmental studies, a testbed for new instruments, and a tool for student exploration of space. This demonstration program for next generation mirror technology provides significant risk reduction and demonstrates the technology in a six-mirror phased telescope. Other key features of the demonstration include the use of an active primary optical surface with wavefront feedback control that allows on-orbit optimization and demonstration of precise surface control to meet optical system wavefront and stability requirements. The concept of assembly is in formulation phase and envisions a SpaceX Falcon 9 booster delivering the unassembled elements of the MoDEST telescope to the ISS on a cost effective Dragon capsule. After the capsule docks the robotic arm will retrieve the MoDEST elements placing them into a transport cart which will move them to the standard mission platform for assembly. Once at the assembly area several options are available for the placement of the MoDEST elements onto the platform. The ISS host robotic arm is a candidate for both placement and assembly of the telescope components. Astronauts may play a role in the initial placement as well as observation of the MoDEST assembly processes as they progress. One key area that MoDEST will prove out is the utility of space-based Modal Image Optimization or MIO. With this approach adaptive optics can be assembled in space and algorithmically corrected for optimal optical precision. MIO optimizes deformable mirror segment phase and shape, and provides jitter control. Some benefits of MIO include:
MODEST will also be used to evaluate advances in lightweight mirror and metering structure materials such as Silicon Carbide (SiC) or Carbon Fiber Reinforced Polymer (CFRP) that have excellent mechanical and thermal properties, e.g. high stiffness, high modulus, high thermal conductivity, and low thermal expansion. It has been demonstrated that mirrors built from these materials can be rapidly replicated in a highly cost effective manner, making these materials excellent candidates for a low cost, high performance OTA. A successful MoDEST program will help ensure that future large astrophysics systems are affordable and that exciting emerging missions are achievable. 5.ROTATING SYNTHETIC APERTURERaytheon Space and Airborne Systems initiated and began development of the concept of a rectangular Rotating Synthetic Aperture (RSA) telescope for non-astronomical applications over 20 years ago. The EST team has partnered with Raytheon to explore RSA as an astronomical telescope. It could offer astrophysics significant cost and complexity savings, potentially greater pointing and thermal stability, and higher spatial resolution for an equivalent cost. RSA is also relatively mature. The U.S. Government has invested nearly a third of a billion dollars in the development of technologies relevant to large RSA systems. It is likely that these technologies and concepts can be leveraged by the astrophysics community to yield the highest performing, lowest cost, large aperture astrophysics mission. To be useful for astrophysics, aspects of RSA need to be analyzed in an astronomical context. Because of the rectangular aperture there is a trade between aspect ratio of the rectangular mirror, resolution and collecting area. This is illustrated in Table 2 were four examples of performance parameters for various aspect ratios are shown and compared to a 12-meter filled aperture similar to that discussed in the AURA report1. The RSA Integration Time Factor is the multiplicative factor needed to achieve the same signal-to-noise relative to the 12-meter filled aperture example. Table 2RSA Parameters and Performance Comparison
The analysis of RSA for astrophysics is in its earliest stages. For the analysis we are initially assuming a launch with an existing launch vehicle and a 5-meter fairing, but later in the study we will examine launching RSA with the Space Launch System (SLS) launch vehicle and its significantly larger fairing and greater lift capacity. We have identified a few critical attributes that are being addressed in detail. Note that these are not independent studies; all are interleaved and feed the set of system requirements:
Preliminary results from the analyses indicate that achieving an astrophysically valuable collecting area and resolution combination is relatively straight forward; while starlight suppression systems (coronagraphs, visible nullers, and starshades) appear to perform as well with RSA as they do with filled circular apertures. There appears to be no show stoppers or performance reducers for exoplanet science. Image quality performance analysis for astrophysics has just been initiated and will explore a wide range of spin rates for the telescope. Pointing and vibrational analyses are yet to begin, but no show stoppers are expected and telescope optical performance and parameters and detector considerations are not expected to present issues. The early results from our analyses clearly indicate that RSA is a viable alternative telescope architecture that should be included in future trade analyses for the next large astrophysical observatory. 6.SUMMARYIn this paper we have discussed the analyses we have conducted of two alternative architectures for future large astronomical observatories. A primary goal of this effort is to demonstrate to NASA and the science community the absolute need to explore alternate architectures to mitigate costs, adapt to new technologies, and better integrate into future in-space assembly and servicing. The space observatory environment has changed substantially over the past decades: both positively, such as advanced mirror and wavefront control technologies and a growing in-space operations capability, and negatively, in very reduced budgets. The alternate architecture approaches discussed in this paper are very viable and address all the future science goals. To implement, they will require a culture change, but we believe we should not constrain ourselves to a future of rebuilding our grandfather’s space telescope. The EST concept focuses upon how to build a large space telescope in a flat science budget era by mitigating big cost peaks, and by encouraging and embracing the development of in-space assembly and servicing infrastructure. EST starts with a modest size off-axis telescope (equivalent to a ~7-meter filled aperture telescope) that is launched as a fully functional telescope with instruments (EST Stage 1) performing first rank science. After the passage of time (~5 years) an augmentation mission is sent to the observatory with additional mirror segments, instruments, and other needed hardware to grow it in space to a ~12-meter filled aperture observatory (EST Stage 2). Future augmentations would again increase its size and add new instruments and support hardware to create a ~20-meter filled aperture observatory (EST Stage 3). After EST Stage 3 additional augmentations are also possible either to maintain or upgrade the 20-meter telescope for decades or to grow it to even larger sizes with added mirror elements. The RSA concept offers a very different alternate architecture. The concept was developed for non-astronomical applications and has had a substantial amount of technical investment. Since the technology is relatively mature, our focus has been on analyzing its use for astrophysics, rather than developing a technical solution. RSA is a rectangular aperture telescope that rotates to fill the UV-plane. It offers astrophysics a lower cost, greater resolution, better stability, approach that seems to meet the starlight suppression needs for astrophysics. An assessment of the image quality expected from RSA is in progress Again, we encourage NASA and the astrophysics community not to be constrained to only traditional space telescope approaches, but rather to fully explore and evaluate alternate architectures for these space telescopes. The technologies that enable these different approaches are either mature or maturing rapidly, so the risk levels are very manageable. They do, however, require a culture change. These alternate approaches can offer cost savings and performance enhancements over traditional methods and can enable a more capable astrophysical observatory earlier in time than the traditional approach. ACKNOWLEDGEMENTSThe authors would like to acknowledge strong support and internal funding from Northrop Grumman Aerospace Systems and very helpful comments, suggestions, and criticisms from a variety of people, including Jonathan Arenberg, Suzanne Casement, Alberto Conti, Michael Triller, Amber Bauermeister, Mark Folkman, Keith Leavitt, Mitchell Haeri, Neil Malone, Susan B. Spencer, Douglas W. Wolfe, Marc Postman, Ken Sembach, Wes Traub, and Harley Thronson. REFERENCESDalcanton, J., et.al, From Cosmic Birth to Living Earths: The Future of UVOIR Astronomy, Association of Universities for Research in Astronomy (AURA),2015). Google Scholar
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