Tyler D. Groff

The LUVOIR architecture »a» coronagraph instrument

In preparation for the Astro 2020 Decadal Survey NASA has commissioned the study four flagship missions spanning to a wide range of observable wavelengths: the Origins Space Telescope (OST, formerly the Far-Infrared Surveyor), Lynx (formerly the X-ray Surveyor), the Large UV/Optical/Infrared Surveyor (LUVOIR) and the Habitable Exoplanet Imager (HabEx). One of the key scientific objectives of the latter two is the detection and characterization of the earth-like planets around nearby stars using the direct imaging technique (along with a broad range of investigations regarding the architecture of and atmospheric composition exoplanetary systems using this technique). As a consequence dedicated exoplanet instruments are being studied for these mission concepts. This paper discusses the design of the coronagraph instrument for the architecture “A” (15 meters aperture) of LUVOIR. The material presented in this paper is aimed at providing an overview of the LUVOIR coronagraph instrument. It is the result of four months of discussions with various community stakeholders (scientists and technologists) regarding the instrument’s basic parameters followed by meticulous design work by the the GSFC Instrument Design Laboratory team. In the first section we review the main science drivers, presents the overall parameters of the instrument (general architecture and backend instrument) and delve into the details of the currently envisioned coronagraph masks along with a description of the wavefront control architecture. Throughout the manuscript we describe the trades we made during the design process. Because the vocation of this study is to provide a baseline design for the most ambitious earth-like finding instrument that could be possibly launched into the 2030’s, we have designed an complex system privileged that meets the ambitious science goals out team was chartered by the LUVOIR STDT exoplanet Working Group. However in an effort to minimize technological risk we tried to maximize the number of technologies that will be matured by the WFIRST coronagraph instruments.

Data reduction pipeline for the CHARIS integral-field spectrograph I: detector readout calibration and data cube extraction

Abstract. We present the data reduction pipeline for CHARIS, a high-contrast integral-field spectrograph for the Subaru Telescope. The pipeline constructs a ramp from the raw reads using the measured nonlinear pixel response and reconstructs the data cube using one of three extraction algorithms: aperture photometry, optimal extraction, or χ2 fitting. We measure and apply both a detector flatfield and a lenslet flatfield and reconstruct the wavelength- and position-dependent lenslet point-spread function (PSF) from images taken with a tunable laser. We use these measured PSFs to implement a χ2-based extraction of the data cube, with typical residuals of ∼5% due to imperfect models of the undersampled lenslet PSFs. The full two-dimensional residual of the χ2 extraction allows us to model and remove correlated read noise, dramatically improving CHARIS’s performance. The χ2 extraction produces a data cube that has been deconvolved with the line-spread function and never performs any interpolations of either the data or the individual lenslet spectra. The extracted data cube also includes uncertainties for each spatial and spectral measurement. CHARIS’s software is parallelized, written in Python and Cython, and freely available on github with a separate documentation page. Astrometric and spectrophotometric calibrations of the data cubes and PSF subtraction will be treated in a forthcoming paper.

The large UV/optical/infrared surveyor (LUVOIR): decadal mission study update

NASA commissioned the study of four large mission concepts, including the Large Ultraviolet / Optical / Infrared (LUVOIR) Surveyor, to be evaluated by the 2020 Decadal Survey in Astrophysics. In response, the Science and Technology Definition Team (STDT) identified a broad range of science objectives for LUVOIR that include the direct imaging and spectral characterization of habitable exoplanets around sun-like stars, the study of galaxy formation and evolution, the exchange of matter between galaxies, star and planet formation, and the remote sensing of Solar System objects. To meet these objectives, the LUVOIR Study Office, located at NASA’s Goddard Space Flight Center (GSFC), completed the first design iteration of a 15-m segmented-aperture observatory that would be launched by the Space Launch System (SLS) Block 2 configuration. The observatory includes four serviceable instruments: the Extreme Coronagraph for Living Planetary Systems (ECLIPS), an optical / near-infrared coronagraph capable of delivering 10−10 contrast at inner working angles as small as 2 λ/D; the LUVOIR UV Multi-object Spectrograph (LUMOS), which will provide low- and medium-resolution UV (100 – 400 nm) multi-object imaging spectroscopy in addition to far-UV imaging; the High Definition Imager (HDI), a high-resolution wide-field-of-view NUV-Optical-NIR imager; and Pollux, a high-resolution UV spectro-polarimeter being contributed by Centre National d’Etudes Spatiales (CNES). The study team has executed a second design iteration to further improve upon the 15-m concept, while simultaneously studying an 8-m concept. In these proceedings, we provide an update on these two architectures.

Laboratory and On-sky Validation of the Shaped Pupil Coronagraph's Sensitivity to Low-order Aberrations With Active Wavefront Control

We present early laboratory simulations and extensive on-sky tests validating of the performance of a shaped pupil coronagraph (SPC) behind an extreme-AO corrected beam of the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) system. In tests with the SCExAO internal source/wavefront error simulator, the normalized intensity profile for the SPC degrades more slowly than for the Lyot coronagraph as low-order aberrations reduce the Strehl ratio from extremely high values (S.R. ~ 0.93–0.99) to those characteristic of current ground-based extreme AO systems (S.R. ~ 0.74–0.93) and then slightly lower values down to S.R. ~ 0.57. On-sky SCExAO data taken with the SPC and other coronagraphs for brown dwarf/planet-hosting stars HD 1160 and HR 8799 provide further evidence for the SPCs robustness to low-order aberrations. From H-band Strehl ratios of 80% to 70%, the Lyot coronagraphs performance versus that of the SPC may degrade even faster on sky than is seen in our internal source simulations. The 5-σ contrast also degrades faster (by a factor of two) for the Lyot than the SPC. The SPC we use was designed as a technology demonstrator only, with a contrast floor, throughput, and outer working angle poorly matched for SCExAOs current AO performance and poorly tuned for imaging the HR 8799 planets. Nevertheless, we detect HR 8799 cde with SCExAO/CHARIS using the SPC in broadband mode, where the S/N for planet e is within 30% of that obtained using the vortex coronagraph. The shaped-pupil coronagraph is a promising design demonstrated to be robust in the presence of low-order aberrations and may be well-suited for future ground and space-based direct imaging observations, especially those focused on follow-up exoplanet characterization and technology demonstration of deep contrast within well-defined regions of the image plane.

Current science requirements and planned implementation for the WFIRST-CGI Integral Field Spectrograph (IFS)

One of the key science goals of the Coronograph Instrument (CGI) on the WFIRST mission is to spectrally characterize the atmospheres of planets around other stars at extremely high contrast levels. To achieve this goal, the CGI instrument will include a integral field spectrograph (IFS) as one of the two science cameras. We present the current science requirements that pertain to the IFS design, describe how our design implementation flows from these requirements, and outline our current instrument design.

WFIRST CGI integral field spectrograph performance and post-processing in the OS6 observing scenario

The WFIRST coronagraph instrument (CGI) will have an integral field spectrograph (IFS) backend to disperse the entire field of view at once and obtain spatially-resolved, low-resolution spectra of the speckles and science scene. The IFS will be key to understanding the spectral nature of the speckles, obtain science spectra of planets and disks, and will be used for broadband wavefront control. In order to characterize, predict, and optimize the performance of the instrument, we present a detailed model of the IFS in the context of the new OS6 observing scenario. The simulation includes spatial, spectral, and temporal variations of the speckle field on the IFS detector plane, which allows us to explore several post-processing methods and assess what gains can be expected. The simulator includes the latest models of the detector behavior when operating in photon-counting mode.

Optical design of exo-planet coronagraph, integral field spectrograph, and high resolution spectrometer for LUVOIR study (Conference Presentation)

The Large UV/Optical/IR Surveyor (LUVOIR) is a concept for a highly capable, multi-wavelength space observatory with ambitious science goals. Finding and characterizing a wide range of exoplanets, including those that might be habitable, is a major goal of the study. Driven by the ambitious science goals is the challenges of the optical design. The paper will present how the optical design meets the unique challenges for coronagraphs on large segmented telescopes to achieve high contrast for a wide wavelength range from 400 nm to 1700 nm, such as the position and size of occulter masks, deformable mirror placement and separation, diffraction from a segmented mirror, tight tolerances on the optical system and each element, etc. Two types of spectrometers are designed after the coronagraph to analyze the spectrum of detected exo-planet signals: one is an Integral Field Spectrograph (IFS), and the other one is a High Resolution Spectrometer (HRS). These two spectral instruments will provide information scientists requested in searching for habitable planets. The optical designs, unique challenges, and the solutions for all coronagraph and spectral instruments will be presented. Their specifications derived from science goal will be detailed.

Modelling exoplanet detection with the LUVOIR coronagraph (Conference Presentation)

The Coronagraph is a key instrument on the Large UV-Optical-Infrared (LUVOIR) Surveyor mission concept. The Apodized Pupil Lyot Coronagraph (APLC) is one of the baselined mask technologies to enable 1E10 contrast observations in the habitable zones of nearby stars. The LUVOIR concept uses a large, segmented primary mirror (9--15 meters in diameter) to meet its scientific objectives. For such an observatory architecture, the coronagraph performance depends on active wavefront sensing and control and metrology subsystems to compensate for errors in segment alignment (piston and tip/tilt), secondary mirror alignment, and global low-order wavefront errors. Here we present the latest results of the simulation of these effects for different working angle regions and discuss the achieved contrast for exoplanet detection and characterization under these circumstances, including simulated observations using high-fidelity spatial and spectral models of planetary systems generated with Haystacks, setting boundaries for the tolerance of such errors.

Lessons for WFIRST CGI from ground-based high-contrast systems

The Coronagraph Instrument (CGI) for NASAs Wide Field Infrared Survey Telescope (WFIRST) will constitute a dramatic step forward for high-contrast imaging, integral field spectroscopy, and polarimetry of exoplanets and circumstellar disks, aiming to improve upon the sensitivity of current ground-based direct imaging facilities by 2-3 orders of magnitude. Furthermore, CGI will serve as a pathfinder for future exo-Earth imaging and characterization missions by demonstrating wavefront control, coronagraphy, and spectral retrieval in a new contrast regime, and by validating instrument and telescope models at unprecedented levels of precision. To achieve this jump in performance, it is critical to draw on the experience of ground-based high-contrast facilities. We discuss several areas of relevant commonalities, including: wavefront control, post-processing of integral field unit data, and calibration and observing strategies.

First light of the High Contrast Integral Field Spectrograph (HCIFS)

Future space-based observatories such as WFIRST will be equipped with high contrast imaging instruments designed to study extrasolar planets and disks in the absence of atmospheric perturbations. One of the most efficient techniques to achieve this goal is the combination of wavefront control and broadband coronagraphy. Being able to achieve a high contrast over a wide spectral bandwidth allows us to characterize the chemical composition of exoplanet atmospheres using an integral field spectrograph (IFS). In this paper, we report on the development of such an IFS for the High Contrast Imaging Lab (HCIL) at Princeton University, downstream of a Shaped Pupil coronagraph. Our final lensletbased design calls for the light in an 18% band around 660 nm to be dispersed with a spectral resolution of 50. We also present our new laboratory control software written in Python, allowing the import of open-source packages such as CRISPY to ultimately reconstruct 3D datacubes from IFS spatio-spectral science images. Finally, we show and discuss our preliminary first light results, reaching a contrast of ~10-5 using in-house focal-plane wavefront control and estimation algorithms with two deformable mirrors.