Exoplanet direct imaging using adaptive optics (AO) is often limited by non-common path aberrations (NCPAs) and aberrations that are invisible to traditional pupil-plane wavefront sensors (WFSs). This can be remedied by focal-plane (FP) WFSs that characterize aberrations directly from a final science image. Photonic lanterns (PLs) can act as low-order FPWFSs with the ability to direct some light to downstream science instruments. Using a PL on the SEAL (Santa Cruz Extreme AO Laboratory) high-contrast imaging testbed, we demonstrate (1) linear ranges and (2) closed-loop control. Additionally, we simulate the use of the PL in a multi-wavefront sensor AO system, in which multiple WFSs feed back to the same common-path deformable mirror. Building on previous multi-WFS AO demonstrations on SEAL, we simulate a modulated pyramid WFS to sense aberrations of high spatial order and large amplitude, and the PL to sense low order aberrations including NCPAs. We assess adaptive optics performance in this setting using three different PL wavefront reconstruction algorithms. We also provide a new method to experimentally identify the propagation matrix of a PL, making advanced model-based algorithms practical. This work demonstrates the role of photonic technologies and multi-stage wavefront sensing in the context of extreme AO and high contrast imaging.
Starlight suppression techniques for High-Contrast Imaging (HCI) are crucial to achieving the demanding contrast ratios and inner working angles required for detecting and characterizing exoplanets with a wide range of masses and separations. The advent of photonic technologies provides new opportunities to control the amplitude and phase characteristics of light, with the potential to enhance and control starlight suppression. Here, we present a focal plane optical-fiber-based nulling interferometer working with commercially available components for amplitude and phase modulation. The instrument implements single-mode fiber-coupled elements: a MEMS variable optical attenuator (VOA) matches the on-axis and off-axis starlight amplitude, while a piezoelectric-driven fiber stretcher modifies the optical path difference between the channels to achieve the π phase shift condition for destructive interference. We show preliminary lab results using a narrowband light source working at 632nm and discuss future opportunities for testing on-sky with the Astrophotonics Advancement Platform at Lick Observatory (APALO) at the Shane 3-m Telescope.
Astrophotonics, with its potential for creating low-cost, mass-producible devices, offers a path to dramatically reduce the cost of future astronomical spectrographs. However, coupling the light from large astronomical telescopes into small, photonic chip-based instruments remains a challenge. Photonic lanterns offer a potential solution. Photonic lanterns predictably decompose the inherently multimode light from a ground-based telescope into a series of single-mode outputs, thus eliminating the need for exotic optical elements or extreme AO to achieve high efficiency. We have built a custom assembly for the AO system at Lick Observatory’s 3m Shane Telescope to test photonic lantern behavior on-sky. Here we report on multiple nights of observations over the past year using a lantern with a design wavelength of 1550 nm. Our data reveals the lantern’s basic performance over a 605–1000 nm band and its time domain response to turbulent PSFs with AO correction residuals. These measurements are important for determining the efficacy of future efforts to preferentially select or combine output modes in “real-world” scenarios across scientifically useful bandwidths.
Astrophotonic technologies, specifically mass-produced “spectrometers-on-a-chip,” offer an exciting path toward dramatically reducing the cost-per-spectrum of astronomical spectrographs. This technology could one day enable significant multiplexing upgrades to fiber-based instruments and inspire new facilities capable of collecting 100,000 simultaneous spectra in both single-fiber and IFU formats. Here, we report on a new astrophotonics platform at Lick Observatory for on-sky testing of such technologies. Our initial focus is on the problem of efficiently coupling telescope light into photonic devices, which are typically optimized to work with a single mode, i.e., with diffraction-limited light. While photonic lanterns can deliver multiple single-mode outputs given multi-modal input, here we introduce the concept of Adaptive Mode Extraction (AME), which uses a second, reference lantern to select the brightest instantaneous mode or modes for injection into photonic devices. Analogous to “speckle spectroscopy,” this technique has the potential to increase the signal-to-noise ratio for faint sources by spatially filtering out the sky background. We have deployed our testing platform behind the AO system at the Shane Telescope and demonstrate that it meets requirements for our planned on-sky tests of AME, namely the ability to couple AO-corrected light from two nearby stars (within 2′′) into two dynamically-positioned lanterns, with adequate throughput (<40%) and image quality (0.15′′).
The large distance between Earth and other planetary systems makes it so that exoplanets appear as point sources to our telescopes. This is in stark contrast to the appearance of our own solar system planets, which range in angular diameter from a few arcseconds to arcminutes. Their relatively large projected size on the sky allows for detailed analysis of planetary features such as rings, atmospheric and cloud features, and more. The Planet as Exoplanet Analog Spectrograph (PEAS) instrument at Lick Observatory is designed to simulate exoplanet observations and analysis techniques using disk integrated observations of the solar system planets. PEAS uses an integrating sphere to spatially scramble the light from the planet and take a spectrum of the entire visible surface. PEAS observations of solar system planets can then be used as benchmarks for testing and validating exoplanet observations and atmospheric models. In this work, we model the throughput of the PEAS instrument, including the telescope, integrating sphere, and spectrograph components. We are able to reproduce a PEAS spectrum to within a factor of 10. We then model the throughput with possible upgrades to the system and determine which new components would produce the best efficiency.
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