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Register a new userSimultaneous Exoplanet Characterization and deep wide-field imaging with a diffractive pupil telescope
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Olivier Guyon
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High-precision astrometry can identify exoplanets and measure their orbits and masses, while coronagraphic imaging enables detailed characterization of their physical properties and atmospheric compositions through spectroscopy. In a previous paper, we showed that a diffractive pupil telescope (DPT) in space can enable sub-microarcsecond accuracy astrometric measurements from wide-field images by creating faint but sharp diffraction spikes around the bright target star. The DPT allows simultaneous astrometric measurement and coronagraphic imaging, and we discuss and quantify in this paper the scientific benefits of this combination for exoplanet science investigations: identification of exoplanets with increased sensitivity and robustness, and ability to measure planetary masses to high accuracy. We show how using both measurements to identify planets and measure their masses offers greater sensitivity and provides more reliable measurements than possible with separate missions, and therefore results in a large gain in mission efficiency. The combined measurements reliably identify potentially habitable planets in multiple systems with a few observations, while astrometry or imaging alone would require many measurements over a long time baseline. In addition, the combined measurement allows direct determination of stellar masses to percent-level accuracy, using planets as test particles. We also show that the DPT maintains the full sensitivity of the telescope for deep wide-field imaging, and is therefore compatible with simultaneous scientific observations unrelated to exoplanets. We conclude that astrometry, coronagraphy, and deep wide-field imaging can be performed simultaneously on a single telescope without significant negative impact on the performance of any of the three techniques.
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Astrometric detection and mass determination of Earth-mass exoplanets requires sub-microarcsec accuracy, which is theoretically possible with an imaging space telescope using field stars as an astrometric reference. The measurement must however overcome astrometric distortions which are much larger than the photon noise limit. To address this issue, we propose to generate faint stellar diffraction spikes using a two-dimensional grid of regularly spaced small dark spots added to the surface of the primary mirror (PM). Accurate astrometric motion of the host star is obtained by comparing the position of the spikes to the background field stars. The spikes do not contribute to scattered light in the central part of the field and therefore allow unperturbed coronagraphic observation of the stars immediate surrounding. Because the diffraction spikes are created on the PM and imaged on the same focal plane detector as the background stars, astrometric distortions affect equally the diffraction spikes and the background stars, and are therefore calibrated. We describe the technique, detail how the data collected by the wide-field camera are used to derive astrometric motion, and identify the main sources of astrometric error using numerical simulations and analytical derivations. We find that the 1.4 m diameter telescope, 0.3 sq.deg field we adopt as a baseline design achieves 0.2 microarcsec single measurement astrometric accuracy. The diffractive pupil concept thus enables sub-microarcsec astrometry without relying on the accurate pointing, external metrology or high stability hardware required with previously proposed high precision astrometry concepts.
The Wide-Field InfraRed Space Telescope (WFIRST) will be capable of delivering precise astrometry for faint sources over the enormous field of view of its main camera, the Wide-Field Imager (WFI). This unprecedented combination will be transformative for the many scientific questions that require precise positions, distances, and velocities of stars. We describe the expectations for the astrometric precision of the WFIRST WFI in different scenarios, illustrate how a broad range of science cases will see significant advances with such data, and identify aspects of WFIRSTs design where small adjustments could greatly improve its power as an astrometric instrument.
We present optical characterization, calibration, and performance tests of the Mesospheric Airglow/Aerosol Tomography Spectroscopy (MATS) satellite, which for the first time for a satellite applies a linear-astigmatism-free confocal off-axis reflective optical design. Mechanical tolerances of the telescope were investigated using Monte-Carlo methods and single-element perturbations. The sensitivity analysis results indicate that tilt errors of the tertiary mirror and a surface RMS error of the secondary mirror mainly degrade optical performance. From the Monte-Carlo simulation, the tolerance limits were calculated to $pm$0.5 mm, $pm$1 mm, and $pm$0.15$^circ$ for decenter, despace, and tilt, respectively. We performed characterization measurements and optical tests with the flight model of the satellite. Multi-channel relative pointing, total optical system throughput, and distortion of each channel were characterized for end-users. Optical performance was evaluated by measuring modulation transfer function (MTF) and point spread function (PSF). The final MTF performance is 0.25 MTF at 20 lp/mm for the ultraviolet channel (304.5 nm), and 0.25 - 0.54 MTF at 10 lp/mm for infrared channels. The salient fact of the PSF measurement of this system is that there is no noticeable linear astigmatism detected over wide field of view (5.67$^circ$ $times$ 0.91$^circ$). All things considered, the design method showed great advantages in wide field of view observations with satellite-level optical performance.
Searching for nearby exoplanets with direct imaging is one of the major scientific drivers for both space and ground-based programs. While the second generation of dedicated high-contrast instruments on 8-m class telescopes is about to greatly expand the sample of directly imaged planets, exploring the planetary parameter space to hitherto-unseen regions ideally down to Terrestrial planets is a major technological challenge for the forthcoming decades. This requires increasing spatial resolution and significantly improving high contrast imaging capabilities at close angular separations. Segmented telescopes offer a practical path toward dramatically enlarging telescope diameter from the ground (ELTs), or achieving optimal diameter in space. However, translating current technological advances in the domain of high-contrast imaging for monolithic apertures to the case of segmented apertures is far from trivial. SPEED (the segmented pupil experiment for exoplanet detection) is a new instrumental facility in development at the Lagrange laboratory for enabling strategies and technologies for high-contrast instrumentation with segmented telescopes. SPEED combines wavefront control including precision segment phasing architectures, wavefront shaping using two sequential high order deformable mirrors for both phase and amplitude control, and advanced coronagraphy struggled to very close angular separations (PIAACMC). SPEED represents significant investments and technology developments towards the ELT area and future spatial missions, and will offer an ideal cocoon to pave the road of technological progress in both phasing and high-contrast domains with complex/irregular apertures. In this paper, we describe the overall design and philosophy of the SPEED bench.
The solar gravitational lens (SGL) provides a factor of $10^{11}$ amplification for viewing distant point sources beyond our solar system. As such, it may be used for resolved imaging of extended sources, such as exoplanets, not possible otherwise. To use the SGL, a spacecraft carrying a modest telescope and a coronagraph must reach the SGLs focal region, that begins at $sim$550 astronomical units (AU) from the Sun and is oriented outward along the line connecting the distant object and the Sun. No spacecraft has ever reached even a half of that distance; and to do so within a reasonable mission lifetime (e.g., less than 25 years) and affordable cost requires a new type of mission design, using solar sails and microsats ($<100$~kg). The payoff is high -- using the SGL is the only practical way we can ever get a high-resolution, multi-pixel image of an Earth-like exoplanet, one that we identify as potentially habitable. This paper describes a novel mission design starting with a rideshare launch from the Earth, spiraling in toward the Sun, and then flying around it to achieve solar system exit speeds of over $20$ AU/year. A new sailcraft design is used to make possible high area to mass ratio for the sailcraft. The mission design enables other fast solar system missions, starting with a proposed very low cost technology demonstration mission (TDM) to prove the functionality and operation of the microsat-solar sail design and then, building on the TDM, missions to explore distant regions of the solar system, and those to study Kuiper Belt objects (KBOs) and the recently discovered interstellar objects (ISOs) are also possible.
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