Over the past few years, we have developed a concept for an evolvable space telescope (EST) that is assembled on orbit in three stages, growing from a 4×12-m telescope in Stage 1, to a 12-m filled aperture in Stage 2, and then to a 20-m filled aperture in Stage 3. Stage 1 is launched as a fully functional telescope and begins gathering science data immediately after checkout on orbit. This observatory is then periodically augmented in space with additional mirror segments, structures, and newer instruments to evolve the telescope over the years to a 20-m space telescope. We discuss the EST architecture, the motivation for this approach, and the benefits it provides over current approaches to building and maintaining large space observatories.
In 2014 we presented a concept for an Evolvable Space Telescope (EST) that was assembled on orbit in 3 stages, growing from a 4x12 meter telescope in Stage 1, to a 12-meter filled aperture in Stage 2, and then to a 20-meter filled aperture in Stage 3. Stage 1 is launched as a fully functional telescope and begins gathering science data immediately after checkout on orbit. This observatory is then periodically augmented in space with additional mirror segments, structures, and newer instruments to evolve the telescope over the years to a 20-meter space telescope. In this 2015 update of EST we focus upon three items: 1) a restructured Stage 1 EST with three mirror segments forming an off-axis telescope (half a 12-meter filled aperture); 2) more details on the value and architecture of the prime focus instrument accommodation; and 3) a more in depth discussion of the essential in-space infrastructure, early ground testing and a concept for an International Space Station testbed called MoDEST. In addition to the EST discussions we introduce a different alternative telescope architecture: a Rotating Synthetic Aperture (RSA). This is a rectangular primary mirror that can be rotated to fill the UV-plane. The original concept was developed by Raytheon Space and Airborne Systems for non-astronomical applications. In collaboration with Raytheon we have begun to explore the RSA approach as an astronomical space telescope and have initiated studies of science and cost performance.
Astronomical flagship missions after JWST will require affordable space telescopes and science instruments. Innovative spacecraft-electro-opto-mechanical system architectures matched to the science requirements are needed for observations for exoplanet characterization, cosmology, dark energy, galactic evolution formation of stars and planets, and many other research areas. The needs and requirements to perform this science will continue to drive us toward larger and larger apertures. Recent technology developments in precision station keeping of spacecraft, interplanetary transfer orbits, wavefront/sensing and control, laser engineering, macroscopic application of nano-technology, lossless optical designs, deployed structures, thermal management, interferometry, detectors and signal processing enable innovative telescope/system architectures with break-through performance. Unfortunately, NASA’s budget for Astrophysics is unlikely to be able to support the funding required for the 8 m to 16 m telescopes that have been studied as a follow-on to JWST using similar development/assembly approaches without decimating the rest of the Astrophysics Division’s budget. Consequently, we have been examining the feasibility of developing an “Evolvable Space Telescope” that would begin as a 3 to 4 m telescope when placed on orbit and then periodically be augmented with additional mirror segments, structures, and newer instruments to evolve the telescope and achieve the performance of a 16 m or larger space telescope. This paper reviews the approach for such a mission and identifies and discusses candidate architectures.
The use of an external occulter, or starshade, has been proposed as one method for the direct detection and spectral
characterization of terrestrial planets around other stars, a key goal identified in ASTRO2010. Because of the
observational geometry, one of the concerns is stray light from the edge of the starshade that is scattered into the line of
sight of the telescope. We have developed a stray light model using physical properties of a realizable starshade edge
geometry and material to calculate the resulting stray light. The background signal due to stray light has been calculated
for the two telescope architectures adopted for study by the Exoplanet Exploration Program Analysis Group (ExoPAG),
a 4 m monolithic and an 8 m segmented mirror design. Using these results, we have developed a set of design
requirements and structure features that will result in a buildable system with stray light levels that meet the overall
system sensitivity requirements.
An ultra-compact optical true time delay device is demonstrated that can support 112 antenna elements with better than
six bits of delay in a volume 16"×5"×4" including the box and electronics. Free-space beams circulate in a White cell,
overlapping in space to minimize volume. The 18 mirrors are slow-tool diamond turned on two substrates, one at each
end, to streamline alignment. Pointing accuracy of better than 10μrad is achieved, with surface roughness ~45 nm rms. A
MEMS tip-style mirror array selects among the paths for each beam independently, requiring ~100 μs to switch the
whole array. The micromirrors have 1.4° tip angle and three stable states (east, west, and flat). The input is a fiber-and-microlens
array, whose output spots are re-imaged multiple times in the White cell, striking a different area of the single
MEMS chip in each of 10 bounces. The output is converted to RF by an integrated InP wideband optical combiner
detector array. Delays were accurate to within 4% (shortest delay) to 0.03% (longest mirror train). The fiber-to-detector
insertion loss is 7.82 dB for the shortest delay path.
Use of a deployable telescope will be essential if the full science objectives of the Terrestrial Planet Finder mission are to be achieved with a visible coronagraph, since the largest monolithic mirrors that can be launched into space do not have the spatial resolution required to search the habitable zone around more than ~40 of the nearest stars. Current launch vehicle fairings limit the size of monolithic telescope mirrors to ~4 meters in diameter, or ~3.5-m x 10-m if the mirror is launched standing upright, and the telescope is unfolded after reaching orbit. By comparison, a telescope with two 3.5 x 7 meter segments could be launched and deployed autonomously to provide a 14-m elliptical aperture, and a telescope with six 4-m flat-flat hexagonal segments could be launched and deployed autonomously to provide a near-circular 12-m aperture with a single ring of segments (or 20-m if a second ring is added). Future NASA missions such as LifeFinder and planet imager will also require segmented, deployable telescopes to achieve the necessary collecting area. This paper discusses the issues associated with the use of segmented optics for coronagraphs and potential solutions.
A Single Aperture, Far InfraRed Observatory, called SAFIR, is a proposed NASA mission to observe the universe at wavelengths from ~30 to 800 microns. To achieve the mission objectives, the telescope must be of order 10-m in diameter and cooled to ~4K to obtain background limited performance. Northrop Grumman Space Technology (NGST) has developed a conceptual design based on our James Webb Space Telescope (JWST) and Terrestrial Planet Finder (TPF) mission architectures that utilizes a deployable telescope and a large sunshade to achieve the desired mirror temperature. Our design concept includes a 12-m diameter on-axis Gregorian telescope, which provides the wide fields of view desired by the SAFIR science team. We describe the optical design, a packaging concept that allows this telescope to fit in a standard 5- launch vehicle fairing, and initial concepts for the telescope thermal control system.
NASA plans to launch a Terrestrial Planet Finder (TPF) mission in 2014 to detect and characterize Earth-like planets around nearby stars, to perform comparative planetology studies, and to obtain general astrophysics observations. As part of our recently completed TPF Mission Architecture study for NASA/JPL we developed the conceptual design for a Large Aperture IR Coronagraph that meets these mission objectives. This paper describes the optical design of the telescope and the coronagraph to detect and characterize exo-solar planets. The telescope design was optimized to provide a well-corrected image plane that is large enough to feed several instruments and control scattered light while accommodating packaging for launch and manufacturing limitations. The coronagraph was designed to provide a well corrected field of view with a radius > 5 arcsec around the star it occults in the 7-17 microns wavelength region. A design for this instrument as well as results of a system simulation model are presented. The methodology for wavefront error correction and control of scattered and diffracted light are discussed in some detail as they are critical parameters to enable detecting planets at separations of down to ~λ/D.
NASA wants to launch a Terrestrial Planet Finder (TPF) mission in 2014 to detect and characterize Earth-like planets around nearby stars, perform comparative planetology studies, and obtain general astrophysics observations. The detection of a 30th magnitude planet located within 80 milli-arcseconds of a 5th (Visual) magnitude star is an exceptionally challenging objective. Observations in the thermal infrared (7-17 mm) are somewhat easier since the planet is 'only' 15m fainter than the star at these wavelengths, but many severe challenges must still be overcome, including: Designing a spacecraft, a telescope and an IR coronagraph for star-planet separations equal to λ/D;(i) Providing a stable (~30K) thermal environment for the optics and isolating them from vibration sources; (ii)Developing a deployment scheme for a 28-m space telescope that can fit in an existing launch vehicle; (iii) Minimizing telescope mass to enable launch to L2 or a driftway orbit with a single launch vehicle; (iv) Generating a manufacturing plan that will permit TPF to be developed at a reasonable cost and schedule; (v) Identifying the key enabling technologies for TPF. This paper describes the IR Coronagraph we designed during our recent TPF Mission Architecture study in an effort to meet these challenges.
A common approach used in the design of many optical systems includes the use of "standard" spherical, conic and/or polynomial aspheric surface profiles during the optimization process. Many times, the use of these "standard" surface profiles yields optical systems which either meet or exceed the optical system performance requirements. There are other optical systems, however, which cannot be optimized to their required performance levels using these "standard" optical surfaces. This paper will examine one such case and show how a new type of optical surface profile can be derived which will yield "perfect" optical performance. The derived surface equations will be coded into a user-defined surface in the CODE V optical design program, a verification ray trace will be performed and the explicit surface profile will be displayed. This technique will suggest that there may be other geometries for which surface shapes may be derived in order to solve specific optical problems.
A passive millimeter-wave camera capable of generating real time displays of the imaged scene, similar to video cameras, has been under development at TRW over the past two years. The camera operates at 89 GHz, has a 15 degree(s) X 10 degree(s) field-of-view, an aperture of 18' diameter, and displays the acquired image at a frame rate of 17 Hz. A major enabling technology is the focal plane array of direct detection MMIC receivers which guarantees reliable and low cost manufacturing of this camera, in addition to providing it with unique operational features. This work reports on progress achieved to date in the development and manufacturing of this new sensor technology.
The performance of the Lightning Mapper Sensor is dependent on the temperature shifts of its narrowband spectral filter. To perform over a 10 degree FOV with an 0.8 nm bandwidth, the filter must be 15 cm in diameter and mounted externally to the telescope optics. The filter thermal control required a filter design optimized for minimum bandpass shift with temperature, a thermal analysis of substrate materials for maximum temperature uniformity, and a thermal radiation analysis to determine the parameter sensitivity of the radiation shield for the filter, the filter thermal recovery time after occultation, and heater power to maintain filter performance in the earth-staring geosynchronous environment.