On-sky Demonstration of Subdiffraction-limited Astronomical Measurement Using a Photonic Lantern

TL;DR

This work presents the first on-sky demonstration of subdiffraction-limited, mode-based astronomical measurements using a photonic lantern–fed spectrometer on the Subaru SCExAO system. By synchronizing infrared focal-plane PSF imaging with spectrally dispersed PL outputs and employing a spectral-differential self-calibration, the authors map PL responses via frame-sorted 2D response maps and recover subdiffraction information from a single telescope. They detect spectroastrometric signals in the H$\alpha$-emitting disk around the Be star $\beta$ CMi, finding a Keplerian rotation signature and a previously unseen near–far-side brightness asymmetry with a photocenter precision of $\sim$ $50\,\mu\mathrm{as}$ over $\sim10$ minutes. This demonstrates the viability, efficiency, and practicality of photonic mode–based imaging for high angular resolution spectroscopy and opens pathways for detailed studies of compact astronomical sources with existing ground-based facilities and future space missions.

Abstract

Resolving fine details of astronomical objects provides critical insights into their underlying physical processes. This drives in part the desire to construct ever-larger telescopes and interferometer arrays and to observe at shorter wavelength to lower the diffraction limit of angular resolution. Alternatively, one can aim to overcome the diffraction limit by extracting more information from a single telescope's aperture. A promising way to do this is spatial mode-based imaging, which projects focal-plane field onto a set of spatial modes before detection, retaining focal-plane phase information crucial at small angular scales but typically lost in intensity imaging. However, the practical implementation of mode-based imaging in astronomy from the ground has been challenged by atmospheric turbulence. Here, we present the first on-sky demonstration of a subdiffraction-limited, mode-based measurement using a photonic lantern (PL)-fed spectrometer installed on the SCExAO instrument at the Subaru Telescope. We introduce a novel calibration strategy that mitigates time-varying wavefront error and misalignment effects, leveraging simultaneously recorded focal-plane images and using a spectral-differential technique that self-calibrates the data. Observing the classical Be star $β$ CMi, we detected spectral-differential spatial signals and reconstructed images of its H$α$-emitting disk. We achieved an unprecedented H$α$ photocenter precision of 50$μ$as in about 10-minute observation with a single telescope, measuring the disk's near-far side asymmetry for the first time. This work demonstrates the high precision, efficiency, and practicality of photonic mode-based imaging techniques to recover subdiffraction-limited information, opening new avenues for high angular resolution spectroscopic studies in astronomy.

Figures (9)

• Figure 1: The PL-IFU concept. a. A conceptual diagram of using a PL as a compact integral-field unit. The light collected by the telescope is coupled to the multimode end of the PL at the focal plane, which is then converted into multiple single-mode beams encoding the spatial information of the incoming wavefront. Using the spectrally dispersed single-mode outputs, both spectral and spatial information can be recovered at high angular resolution within a small field of view (few $\lambda/D$), enabling detailed characterization of compact astronomical sources. b. An illustrative diagram of spatial mode decomposition of focal-plane field. Using linearly polarized (LP) mode basis that describes spatial modes of circular step-index optical fibers, a PSF with a small shift is primarily described as a linear combination of the fundamental $\mathrm{LP}_{01}$ and first-order $\mathrm{LP}_{11}$ modes, with complex-valued mode amplitudes. The complex mode amplitudes are represented as phasors (white arrows), where the arrow's length and angle correspond to the magnitude and phase of the coefficient. c. A PL has unique complex-valued mixing coefficients (transfer matrix) that map the mode amplitudes to complex amplitudes of SMF outputs; for simplicity, we describe the case of a 3-port PL here. d. By imaging SMF outputs on a detector, one measures intensities of the SMF outputs, which have spatial information of the input.
• Figure 2: The observation setup and data processing procedure. a. A simplified diagram of the observation setup. After two-stage AO correction of the light collected by the Subaru Telescope, the visible light is directed to the FIRST-PL module recording spectrally dispersed outputs of the 19-port PL, and the infrared light is sent to a high-speed detector at the focal plane with an $H$-band filter in. b. Example $H$-band PSF and corresponding PL spectrum. The PSF peak location ($x^*, y^*$) is measured from each $H$-band focal-plane frame and is associated with the corresponding PL spectrum frame. c. Frame sorting procedure. The PL spectrum data are binned by the PSF peak locations, with a resolution of $15\times 15$ spanning 48.6 mas. d. Reconstructed PL response maps. Each port's response map exhibits a unique response, with a gradual variation across wavelengths. The model responses are constructed by masking the H$\alpha$ wavelength range (where spatial signals are expected) and fitting the measured response maps with polynomial functions, then interpolating the coefficients in the masked wavelength range. Spectral-differential spatial signals are identified as deviations of the response maps from the model. e. Model reference response maps, which correspond to response maps of a point-like source. f. Image reconstruction is achieved by solving for the intensity distribution that, when convolved with the reference response maps, best describes the measured response maps. The first and second moments can be estimated by modeling the image as a point source and 2D Gaussian, respectively. Without any simplifying models non-parametric image reconstruction can be achieved.
• Figure 3: Spectral-differential spatial signal detection. a. H$\alpha$ spectrum of $\beta$ CMi observed on 2025 February 11, summed over all the 38 traces. b. Response maps (i.e., Fig. \ref{['fig:method']}e) of selected velocity channels, of one of the ports. Red solid contours and black dashed contours denote the same contour levels for the measured response maps and the modeled reference response maps. Small shifts are observed between the two contours as marked with white arrows in the zoom-in images, indicating photocenter shifts. c. Response maps subtracted by the model reference response maps (difference maps). Deviations from zero indicate that the astronomical scene departs from an unresolved central point source. The positive and negative structures indicate the shift. d. Signal-to-noise (S/N) maps of three ports, which correspond to the difference maps divided by the error maps. The spatial signals are clearly seen in the H$\alpha$ wavelength range.
• Figure 4: Photocenter analysis of the H$\alpha$ decretion disk around $\beta$ CMi. a. Two-dimensional photocenter positions for each spectral channel. Colors indicate the Doppler velocities, and gray ellipses show the 1 $\sigma$ uncertainties. The green line denotes the derived disk position angle, 126$^{\circ}$. b. H$\alpha$ spectrum of $\beta$ CMi (sum of the all 38 ports), showing double-peaked line profile. The solid and dotted lines indicate the best-fit Keplerian velocity disk models with asymmetric and symmetric power-law intensity distributions. c, d. Photocenter positions for each spectral channel, projected along the disk's major and minor axes, respectively. Photocenter measurements from two separate observations (circular and triangular markers) show strong agreement, demonstrating the repeatability of the measurement. The photocenter shift along the major axis reveals disk rotation consistent with a Keplerian velocity field. Interestingly, at low velocities, a shift along the minor axis is observed, indicating near- and far-side asymmetry of the disk. e. Best-fit intensity distribution of the asymmetric Keplerian model. f. Best-fit velocity distribution of the asymmetric Keplerian model.
• Figure 5: Image reconstruction of the H$\alpha$ decretion disk around $\beta$ CMi. a. Measured circular 2D Gaussian FWHM, representing the spatial extent of the intensity distribution in each wavelength channel. While there is an apparent FWHM enhancement around the H$\alpha$ center, it is of marginal statistical significance compared to the 1-2 mas level of systematic fluctuations across the continuum. b. The half-maximum level of reconstructed images as a function of wavelength ($|v|<180$km s$^{-1}$), for the two epochs. The colors indicate the velocity and has the same color scale as in panel a. The photocenter of each image is marked as circular and triangular symbols. The average images of the continuum level are shown in the lower right, analogous to the "beam size", the instrumental width. c. The half-maximum level of reconstructed images overlaid on best-fit asymmetric Keplerian model images for each wavelength channel from Fig. \ref{['fig:SA']}e, f. The reconstructed images show consistency with the model, tracing the shift in emission across wavelength --- both the rotation seen in Fig. \ref{['fig:SA']}f and the brightness asymmetry in Fig. \ref{['fig:SA']}e.