The design of liquid-crystal diffractive phase plate coronagraphs for ground-based and space-based high-contrast imaging systems is limited by the trade-off between spectral bandwidth and ...polarization leakage. We demonstrate that by combining phase patterns with a polarization grating (PG) pattern directly followed by one or several separate PGs, we can suppress the polarization leakage terms by additional orders of magnitude by diffracting them out of the beam. Using two PGs composed of a single-layer liquid crystal structure in the lab, we demonstrate a leakage suppression of more than an order of magnitude over a bandwidth of 133 nm centered around 532 nm. At this center wavelength we measure a leakage suppression of three orders of magnitude. Furthermore, simulations indicate that a combination of two multi-layered liquid-crystal PGs can suppress leakage to <10−5 for 1–2.5 μm and <10−10 for 650–800 nm. We introduce multi-grating solutions with three or more gratings that can be designed to have no separation of the two circular polarization states, and offer even deeper suppression of polarization leakage. We present simulations of a triple-grating solution that has <10−10 leakage on the first Airy ring from 450 to 800 nm. We apply the double-grating concept to the Vector-vortex coronagraph of charge 4, and demonstrate in the lab that polarization leakage no longer limits the on-axis suppression for ground-based contrast levels. Lastly, we report on the successful installation and first-light results of a double-grating vector Apodizing Phase Plate pupil-plane coronagraph installed at the Large Binocular Telescope. We discuss the implications of these new coronagraph architectures for high-contrast imaging systems on the ground and in space.
ABSTRACT
The time variability and spectra of directly imaged companions provide insight into their physical properties and atmospheric dynamics. We present follow-up R ∼ 40 spectrophotometric ...monitoring of red companion HD 1160 B at 2.8–4.2 μm using the double-grating 360° vector Apodizing Phase Plate (dgvAPP360) coronagraph and ALES integral field spectrograph on the Large Binocular Telescope Interferometer. We use the recently developed technique of gvAPP-enabled differential spectrophotometry to produce differential light curves for HD 1160 B. We reproduce the previously reported ∼3.2 h periodic variability in archival data, but detect no periodic variability in new observations taken the following night with a similar 3.5 per cent level precision, suggesting rapid evolution in the variability of HD 1160 B. We also extract complementary spectra of HD 1160 B for each night. The two are mostly consistent, but the companion appears fainter on the second night between 3.0–3.2 μm. Fitting models to these spectra produces different values for physical properties depending on the night considered. We find an effective temperature Teff = $2794^{+115}_{-133}$ K on the first night, consistent with the literature, but a cooler Teff = $2279^{+79}_{-157}$ K on the next. We estimate the mass of HD 1160 B to be 16–81 MJup, depending on its age. We also present R = 50 000 high-resolution optical spectroscopy of host star HD 1160 A obtained simultaneously with the PEPSI spectrograph. We reclassify its spectral type to A1 IV-V and measure its projected rotational velocity $\upsilon \sin i$ = $96^{+6}_{-4}$ km s−1. We thus highlight that gvAPP-enabled differential spectrophotometry can achieve repeatable few per cent level precision and does not yet reach a systematic noise floor, suggesting greater precision is achievable with additional data or advanced detrending techniques.
Abstract
Understanding the physical processes sculpting the appearance of young gas-giant planets is complicated by degeneracies confounding effective temperature, surface gravity, cloudiness, and ...chemistry. To enable more detailed studies, spectroscopic observations covering a wide range of wavelengths are required. Here we present the first L-band spectroscopic observations of HR 8799 d and e and the first low-resolution wide-bandwidth L-band spectroscopic measurements of HR 8799 c. These measurements were facilitated by an upgraded LMIRCam/ALES instrument at the Large Binocular Telescope, together with a new apodizing phase plate coronagraph. Our data are generally consistent with previous photometric observations covering similar wavelengths, yet there exists some tension with narrowband photometry for HR 8799 c. With the addition of our spectra, each of the three innermost observed planets in the HR 8799 system has had its spectral energy distribution measured with integral field spectroscopy covering ∼0.9–4.1
μ
m. We combine these spectra with measurements from the literature and fit synthetic model atmospheres. We demonstrate that the bolometric luminosity of the planets is not sensitive to the choice of model atmosphere used to interpolate between measurements and extrapolate beyond them. Combining luminosity with age and mass constraints, we show that the predictions of evolutionary models are narrowly peaked for effective temperature, surface gravity, and planetary radius. By holding these parameters at their predicted values, we show that more flexible cloud models can provide good fits to the data while being consistent with the expectations of evolutionary models.
ABSTRACT
The vector Apodizing Phase Plate (vAPP) is a class of pupil plane coronagraph that enables high-contrast imaging by modifying the Point Spread Function (PSF) to create a dark hole of deep ...flux suppression adjacent to the PSF core. Here, we recover the known brown dwarf HR 2562 B using a vAPP coronagraph, in conjunction with the Magellan Adaptive Optics (MagAO) system, at a signal-to-noise of S/N = 3.04 in the lesser studied L-band regime. The data contained a mix of field and pupil-stabilized observations, hence we explored three different processing techniques to extract the companion, including Flipped Differential Imaging (FDI), a newly devised Principal Component Analysis (PCA)-based method for vAPP data. Despite the partial field-stabilization, the companion is recovered sufficiently to measure a 3.94 $\mu\mathrm{ m}$ narrow-band contrast of (3.05 ± 1.00) × 10−4 ($\Delta \, {\rm m}_{3.94 \mu {\rm m}}$ = 8.79 ± 0.36 mag). Combined with archival GPI and SPHERE observations, our atmospheric modelling indicates a spectral type at the L/T transition with mass M = 29 ± 15 MJup, consistent with literature results. However, effective temperature and surface gravity vary significantly depending on the wavebands considered (1200 ≤ Teff(K) ≤ 1700 and 4.0 ≤ log(g)(dex) ≤ 5.0), reflecting the challenges of modelling objects at the L/T transition. Observations between 2.4 and 3.2 $\mu\mathrm{ m}$ will be more effective in distinguishing cooler brown dwarfs due to the onset of absorption bands in this region. We explain that instrumental scattered light and wind-driven halo can be detrimental to FDI+PCA and thus must be sufficiently mitigated to use this processing technique. We thus demonstrate the potential of vAPP coronagraphs in the characterization of high-contrast substellar companions, even in sub-optimal conditions, and provide new complementary photometry of HR 2562 B.
ABSTRACT
Clouds and other features in exoplanet and brown dwarf atmospheres cause variations in brightness as they rotate in and out of view. Ground-based instruments reach the high contrasts and ...small inner working angles needed to monitor these faint companions, but their small fields of view lack simultaneous photometric references to correct for non-astrophysical variations. We present a novel approach for making ground-based light curves of directly imaged companions using high-cadence differential spectrophotometric monitoring, where the simultaneous reference is provided by a double-grating 360○ vector Apodizing Phase Plate (dgvAPP360) coronagraph. The dgvAPP360 enables high-contrast companion detections without blocking the host star, allowing it to be used as a simultaneous reference. To further reduce systematic noise, we emulate exoplanet transmission spectroscopy, where the light is spectrally dispersed and then recombined into white-light flux. We do this by combining the dgvAPP360 with the infrared Arizona Lenslets for Exoplanet Spectroscopy integral field spectrograph on the Large Binocular Telescope Interferometer. To demonstrate, we observed the red companion HD 1160 B (separation ∼780 mas) for one night, and detect $8.8{{\ \rm per\ cent}}$ semi-amplitude sinusoidal variability with an ∼3.24 h period in its detrended white-light curve. We achieve the greatest precision in ground-based high-contrast imaging light curves of sub-arcsecond companions to date, reaching $3.7{{\ \rm per\ cent}}$ precision per 18-min bin. Individual wavelength channels spanning 3.59–3.99 μm further show tentative evidence of increasing variability with wavelength. We find no evidence yet of a systematic noise floor; hence, additional observations can further improve the precision. This is therefore a promising avenue for future work aiming to map storms or find transiting exomoons around giant exoplanets.
Context.
As an interferometric technique, sparse aperture masking (SAM) is capable of imaging beyond the diffraction limit of single telescopes. This makes SAM an important technique for studying ...processes such as planet formation at Solar System scales. However, it comes at the cost of a reduction in throughput, typically by 80–90%.
Aims.
We report on the design, construction, and commissioning of a prototype aperture masking technology implemented at the Keck OH-Suppressing Infrared Integral Field Spectrograph (OSIRIS) Imager: the holographic aperture mask. Holographic aperture masking (HAM) aims at (i) increasing the throughput of SAM by selectively combining all subapertures across a telescope pupil in multiple interferograms using a phase mask, and (ii) adding low-resolution spectroscopic capabilities.
Methods.
Using liquid-crystal geometric phase patterns, we manufacture a HAM mask that uses an 11-hole SAM design as the central component and a holographic component comprising 19 different subapertures. Thanks to a multilayer liquid-crystal implementation, the mask has a diffraction efficiency higher than 96% from 1.1 to 2.5 micron. We create a pipeline that extracts monochromatic closure phases from the central component as well as multiwavelength closure phases from the holographic component. We test the performance of the HAM mask in the laboratory and on-sky.
Results.
The holographic component yields 26 closure phases with spectral resolutions between
R
∼ 6.5 and
R
∼ 15, depending on the interferogram positions. On April 19, 2019, we observed the binary star HDS 1507 in the Hbb filter (
λ
0
= 1638 nm and Δ
λ
= 330 nm) and retrieved a constant separation of 120.9 ± 0.5 mas for the independent wavelength bins, which is in excellent agreement with literature values. For both the laboratory measurements and the observations of unresolved reference stars, we recorded nonzero closure phases – a potential source of systematic error that we traced to polarization leakage of the HAM optic. We propose a future upgrade that improves the performance, reducing this effect to an acceptable level.
Conclusions.
Holographic aperture masking is a simple upgrade of SAM with increased throughput and a new capability of simultaneous low-resolution spectroscopy that provides new differential observables (e.g., differential phases with wavelength).
ABSTRACT
NASA is engaged in planning for a Habitable Worlds Observatory (HabWorlds ), a coronagraphic space mission to detect rocky planets in habitable zones and establish their habitability. ...Surface liquid water is central to the definition of planetary habitability. Photometric and polarimetric phase curves of starlight reflected by an exoplanet can reveal ocean glint, rainbows, and other phenomena caused by scattering by clouds or atmospheric gas. Direct imaging missions are optimized for planets near quadrature, but HabWorlds ’ coronagraph may obscure the phase angles where such optical features are strongest. The range of accessible phase angles for a given exoplanet will depend on the planet’s orbital inclination and/or the coronagraph’s inner working angle (IWA). We use a recently created catalog relevant to HabWorlds of 164 stars to estimate the number of exo-Earths that could be searched for ocean glint, rainbows, and polarization effects due to Rayleigh scattering. We find that the polarimetric Rayleigh scattering peak is accessible in most of the exo-Earth planetary systems. The rainbow due to water clouds at phase angles of ∼20○ − 60○ would be accessible with HabWorlds for a planet with an Earth equivalent instellation in ∼46 systems, while the ocean glint signature at phase angles of ∼130○ − 170○ would be accessible in ∼16 systems, assuming an IWA = 62 mas (3λ/D). Improving the IWA = 41 mas (2λ/D) increases accessibility to rainbows and glints by factors of approximately 2 and 3, respectively. By observing these scattering features, HabWorlds could detect a surface ocean and water cycle, key indicators of habitability.
Computer-generated geometric phase holograms (GPHs) can be manufactured with high efficiency and high fidelity using photo-aligned liquidvcrystals. GPHs are diffractive elements, which therefore have ...a wavelength-dependent output and can generally not be used for the production of color imagery. We implement a two-stage approach that first uses the wavelength-dependent diffraction to separate colors, and second, directs these colors through separate holographic patterns. Moreover, by utilizing the geometric phase, we obtain diffraction efficiencies close to 100% for all wavelengths. We successfully create a white light hologram from RGB input in the lab. We demonstrate that this schematic allows for full control over individual (RGB) channels and can be used for wide-gamut holography by selecting any combination of wavelengths. In addition, we show with simulations how this two-stage element could be used for of true-color holograms.
Context.
The vector Apodizing Phase Plate (vAPP) is a pupil plane coronagraph that suppresses starlight by forming a dark hole in its point spread function (PSF). The unconventional and ...non-axisymmetrical PSF arising from the phase modification applied by this coronagraph presents a special challenge to post-processing techniques.
Aims.
We aim to implement a recently developed post-processing algorithm, temporal reference analysis of planets (TRAP) on vAPP coronagraphic data. The property of TRAP that uses non-local training pixels, combined with the unconventional PSF of vAPP, allows for more flexibility than previous spatial algorithms in selecting reference pixels to model systematic noise.
Methods.
Datasets from two types of vAPPs are analysed: a double grating-vAPP (dgvAPP360) that produces a single symmetric PSF and a grating-vAPP (gvAPP180) that produces two D-shaped PSFs. We explore how to choose reference pixels to build temporal systematic noise models in TRAP for them. We then compare the performance of TRAP with previously implemented algorithms that produced the best signal-to-noise ratio (S/N) in companion detections in these datasets.
Results.
We find that the systematic noise between the two D-shaped PSFs is not as temporally associated as expected. Conversely, there is still a significant number of systematic noise sources that are shared by the dark hole and the bright side in the same PSF. We should choose reference pixels from the same PSF when reducing the dgvAPP360 dataset or the gvAPP180 dataset with TRAP. In these datasets, TRAP achieves results consistent with previous best detections, with an improved S/N for the gvAPP180 dataset.
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