Pin Chen

Current HWO coronagraph concepts incorporate dichroic beamsplitters to facilitate parallel observations in multiple wavelength bands. As an element in the highly corrected optical wavefront upstream of the coronagraph, its aberrations and polarization properties must be understood and thoughtfully integrated into the coronagraph design. Uniformity of optical characteristics of the dichroic across the clear aperture becomes critical for broadband coronagraph performance. We consider the characteristics of four representative dichroic designs.

NASA is embarking on an ambitious program to develop the Habitable Worlds Observatory (HWO) flagship to perform transformational astrophysics, as well as directly image ∼25 potentially Earth-like planets and spectroscopically characterize them for signs of life. This mission was recommended by the Astro2020 decadal survey, which additionally recommended a new approach for flagship formulation based on increasing the scope and depth of early, pre-phase A trades and technology maturation. A critical capability of the HWO mission is the suppression of starlight. To inform future architecture trades, it is necessary to survey a wide range of candidate technologies, from the relatively mature ones such as the ones described in the LUVOIR and HabEx reports to the relatively new and emerging ones, which may lead to breakthrough performance. We present a summary of an effort to survey potential coronagraph options for HWO. In particular, our results consist of (1) a database of different coronagraph designs sourced from the world-wide coronagraph community that are potentially compatible with HWO; (2) evaluation criteria, such as expected mission yields and feasibility of maturing to TRL 5 before phase A; (3) a unified modeling pipeline that processes the designs from (1) and outputs values for any machine-calculable criteria from (2); (4) assessments of maturity of designs, and other criteria that are not machine-calculable; and (5) findings and emerging trends and patterns (some of which overturn prior conventional wisdom). Although not charged to down-select or prioritize the different coronagraph designs, the products of this survey were designed to facilitate future HWO trade studies.

The Habitable Worlds Observatory (HWO), NASA’s next flagship science mission, follows in the tradition of the Nancy Grace Roman Space Telescope (Roman) and other preceding great observatories. HWO will directly image and characterize Earth-like exoplanets and their atmospheres, with the capability to detect biosignatures and potentially answer the question “are we alone?” HWO will also serve as a powerful general astrophysics observatory, enabling breakthroughs in galaxy evolution, stellar astrophysics, and dark matter studies. Currently in preformulation, the project has established Exploratory Analytic Cases (EACs), a series of architectural concept designs used to assess the mission’s demanding science objectives while exploring challenging engineering parameters. We describe the first three EACs, starting with observing strategies and error budget formulation and then progressing to design formulations, trade studies, and lessons learned; we also discuss the integrated modeling pipeline, a key multidisciplinary system-level analysis capability, and analysis findings as applied to the first EAC. These activities set the stage for the follow-on EACs 4 and 5, which will further explore the trade space and prepare for the baseline design that will support the Mission Concept Review (MCR).

A primary goal of the Habitable Worlds Observatory (HWO) is to detect and measure the abundance of biosignature molecules, such as water (H2O) and oxygen (O2), in the atmosphere of Earth analogs. This is expected to require deep spectroscopic observations lasting hundreds of hours per planet. In this context, it is essential to optimize the spectral resolution of the spectrograph to both maximize the number of planets that can be studied over the lifetime of the mission and also to reduce the risks of false detections. The purpose of this work is to provide a framework to explore the spectral resolution design trade-space for HWO. This framework must be valid and comparable across all spectral resolutions from low (R10,000), and account for the spectral correlation of the residual starlight (i.e., speckle noise chromaticity). Leveraging the concept of “template matching,” we develop a simulation toolkit based on the Python package EXOSIMS to compute the detection significance of planets and molecules. We then simulate observations of Earth analogs around 164 stars using representative mission parameters to explore the effects of the detector noise and the correlated speckle noise floor. Our findings suggest that a moderate- or high-resolution spectrograph (R>1000) will provide higher sensitivity to critical molecules compared with a low-resolution spectroscopy mode (e.g., R∼140). The correlated speckle noise may also entirely suppress our ability to detect biosignatures at low spectral resolutions. We conclude that a more comprehensive study, combined with detailed models of its stability and other sources of correlated noise, is necessary to fully explore the trade space of spectral resolution and detectability of key species.

A primary goal of NASA’s Habitable Worlds Observatory (HWO) concept is to explore with high completeness the habitable zones (HZs) of ∼100 stellar systems and potentially acquire spectra of ∼25 terrestrial-type planets in the HZ of Sun-like stars (implying planet/star flux ratios on the order of 10−10), which places tight constraints on the performance of observatory systems. In particular, coronagraph instrumentation needs to be matured for higher throughput, deeper contrasts, and better broadband performance while also taking into account their sensitivity and ability to mitigate the impact of telescope instability and wavefront error (WFE), which can have a profound impact on exo-Earth imaging. Due to the specific focus on the quantity of observed/characterized exo-Earths, the success of various proposed HWO mission architectures is often represented by the estimated exo-Earth candidate yield. Computation of the minimum exposure time to achieve the required signal-to-noise on a given target, using an exposure time calculator (ETC), is a key part of yield estimation. Moreover, one relies on ETCs not only for yields but also to identify key observatory parameters that have large effects on expected exposure and might warrant dedicated technology development. The impacts of coronagraph sensitivity, WFE, and wavefront sensing and control (WFS&C) have been well studied in the context of developing error budgets for missions and instruments such as the Roman Coronagraph Instrument. Nonetheless, there is currently no easily accessible and open-source way to directly incorporate the effects of these key parameters into calculating exposure times to perform trade studies for HWO. To address this gap, we developed the error budget software (EBS)—an open-source tool that synthesizes sensitivity, WFE, and WFS&C information for a variety of temporal and spatial scales, and directly interfaces with the open-source yield code EXOSIMS to produce exposure times. We also demonstrate how EBS can be used for mission error budgeting using the example of the Ultrastable Observatory Roadmap Team observatory design. This includes a series of both single and multivariate parameter explorations using EBS, where we identify trends between raw contrast and WFE, and detector noise and energy resolution. We also highlight the existence of bifurcation points—points at which mission performance dramatically changes for only a moderate variation of the observatory/instrument parameter considered.

A primary goal of NASA's Habitable Worlds Observatory (HWO) mission concept is to explore the Habitable Zones (HZ) of ~100 stellar systems and acquire spectra of ~25 terrestrial-type planets (with planet/star flux ratios on the order of 1E-10) which places tight constraints on the performance of observatory systems. In particular, coronagraph instrumentation needs to be matured for higher throughput, deeper contrasts, and better broadband performance, while also considering their sensitivity and ability to mitigate the impact of telescope instability and wavefront error (WFE), which can have a profound impact on exo-Earth imaging. The success of various proposed HWO mission architectures is often represented by the estimated exo-Earth candidate yield. Computation of the minimum exposure time to achieve the required signal-to-noise on a given target, using an exposure time calculator (ETC), is a key part of yield estimation. The impacts of coronagraph sensitivity, WFE, and wavefront sensing and control (WFS&C) have been well studied in the context of developing error budgets for missions and instruments such as the Roman Coronagraph Instrument, but there is currently no easily accessible way to incorporate the effects of these key parameters into calculating exposure times for HWO. To address this, we developed the Error Budget Software (EBS) - an open-source tool that synthesizes sensitivity, WFE, and WFS&C information for a variety of temporal and spatial scales and directly interfaces with the open-source yield code EXOSIMS to produce exposure times. We demonstrate how EBS can be used for mission error budgeting using the example of the Ultrastable Observatory Roadmap Team (USORT) observatory design. This includes both single and multi-variate parameter explorations using EBS where we identify trends between raw contrast and wavefront error, and detector noise and energy resolution.

We present here the current state of a collection of promising ultraviolet technologies in preparation for the Habitable Worlds Observatory. Working with experts representing a significant number of groups working in the ultraviolet, we summarize some of the leading science drivers, present an argument for a 100-nm blue wavelength cutoff, and gather the current state of the art of UV technologies. We present the state of the art of contamination control, a crucial piece of the UV instrument plan. We explore next steps with individual technologies, as well as present paths forward with system-level testing and development.

The near-ultraviolet (NUV) wavelength range contains a valuable ozone absorption feature for characterizing the atmospheric composition of Earth-like exoplanets. Both the LUVOIR and HabEx decadal mission concept studies baseline instrumentation for obtaining photometry of directly imaged exoplanets down to wavelengths of 200 nm. Both of their proposed implementations present challenges: in the case of the HabEx starshade occulter, a separate spacecraft is required; in the case of a NUV channel within the main coronagraph instrument, non-UV-optimized optics limit the performance. However, the science requirements allow the relaxation of some of the trades of a dedicated near-UV coronagraph. For instance, given that the inner working angle (IWA) of the coronagraph is a function of the wavelength, and one would only consider the characterization of planets detected in the visible, the IWA of the NUV coronagraph can be relaxed to meet the IWA of the visible coronagraph. In addition, given that a NUV coronagraph would be used for a follow-up observation after a visible detection, it could be designed to produce a half dark zone, allowing the simplification of the optical design. Here, we present the case for a standalone NUV coronagraph instrument for the Habitable Worlds Observatory based on the science requirements, developed as part of the Coronagraph Technology Roadmap study commissioned by NASA’s Exoplanet Exploration Office. We describe the trades and rationale behind the recommendation of having a separate NUV coronagraph instrument and show the evaluation of several coronagraph designs in the NUV. We assess the effect of polarization aberrations, discuss relevant technologies, and describe potential NUV detectors and wavefront sensing and control strategies.

The National Aeronautics and Space Administration’s (NASA) Astrophysics Division funds the development of cutting-edge technologies through multiple programs to enable its strategic missions to achieve ambitious and groundbreaking science goals. These technology development efforts are managed by the Cosmic Origins program, Exoplanet Exploration Program, and Physics of the Cosmos program. The 2020 Astronomy and Astrophysics Decadal Survey, “Pathways to Discovery in Astronomy and Astrophysics for the 2020s,” recommended a pan-chromatic set of missions, including three Great Observatories and three Probe-class missions that could collect unprecedented data over the coming decades. We show the correlation between these strategic missions and the current astrophysics technology gaps, as well as recent and current technology maturation projects funded to help close these technology gaps. We also cover how these investments advanced their technology readiness levels and where they have been infused into missions and projects.

Ground-based Precision Radial Velocity (PRV) measurements are inevitably impeded by contamination from telluric absorption features, particularly in the infrared region. Thus, it is crucial to improve modeling of the telluric absorption features down to the spectral noise level. As part of the efforts towards improved PRV measurements, we have taken an existing atmospheric trace gas retrieval algorithm (GFIT) and have successfully adapted it to fit the telluric absorption features in stellar spectra down to the spectral noise level (typically ∼1%). We have established a stellar spectral fitting processing pipeline, Stellar-GFIT, to analyze a series of stellar spectra observed by two spectrographs, PARVI (1.1–1.76 μm) commissioned at the Palomar Observatory (Palomar Mountain, CA) and iSHELL (1–5 μm) deployed on the IRTF (Mauna Kea, HI). For this, we have (1) implemented a Gaussian instrumental line shape function, (2) generated atmospheric models (consisting of temperature, pressure, and volume mixing ratios of all the known trace gases) for the particular observation sites and times, (3) employed the most up-to-date spectroscopic parameters in the target spectral regions, and finally (4) developed a series of spectral fitting intervals of ∼60 cm⁻¹ width, i.e., micro-windows, customized to the individual orders of each spectrograph. Stellar-GFIT is also capable of handling non-telluric features, such as transitions from a gas cell placed in the starlight beam and stellar features if a model spectrum template is available for the target star. We present spectrum fits from the observations of various target stars and discuss the performance and advantages of our novel approach. One of the major strengths of Stellar-GFIT is an ability to adjust the abundance of atmospheric trace gases simultaneously with determining the stellar doppler shift, mitigating any adverse impacts of short-timescale variations of water vapor.