Spectral Map Making with SPHEREx

We present map-making methodologies and preliminary spectral data cubes for SPHEREx, a NASA Explorer mission that launched in March 2025 and has been performing an all-sky near-infrared spectral survey. The SPHEREx instrument observes from 0.75 to 5.0 microns with a spectral resolution ranging from 35 to 130 and a pixel size of 6.15". We define a nominal set of 102 wavelength channels, each of which maps the entire sky approximately twice per year. Among the main mission goals is an investigation of the cosmic history of galaxy formation through intensity mapping of the extragalactic background light (EBL), which is a primary motivation for the map maker described in this work. The SPHEREx dataset contains a wealth of additional mapping targets, e.g., resolved galaxies and nebulae and diffuse clouds of Galactic dust and gas, which display strong spectral features such as hydrogen recombination lines, molecular-hydrogen lines and emission from polycyclic aromatic hydrocarbons (PAHs). We describe how our map maker handles these various cases, how to mitigate foregrounds such as zodiacal light and upper-atmospheric emission and how to monitor and mitigate systematics and signal loss. Our maps are produced both in tangent-plane projection and in full-sky HEALPix format. Specialized maps will be released to accompany future publications from the SPHEREx Science Team, and a public mosaic tool will be made available by the NASA/IPAC Infrared Science Archive (IRSA).

💡 Research Summary

The paper presents the map‑making pipeline developed for SPHEREx, a NASA MIDEX mission launched in March 2025 that conducts an all‑sky near‑infrared spectroscopic survey from 0.75 µm to 5.0 µm with a spectral resolution ranging from R≈35 to 130. The instrument consists of six detector arrays, each 2040 × 2040 pixels, equipped with linear variable filters (LVFs) that cause the central wavelength to vary continuously along the detector’s y‑axis. This design yields a complex entanglement of spatial and spectral coverage: every exposure records all wavelengths, but each line of sight is observed at a different wavelength depending on its position on the detector. The authors divide each array into 17 nominal spectral channels, giving a total of 102 wavelength channels that map the entire sky roughly twice per year.

Key methodological steps include: (1) characterization of the LVF transmission curves and the resulting “spectral strips” that appear in individual images (e.g., Brackett‑α at 4.052 µm, He I at 1.083 µm, PAH 3.3 µm); (2) a Sun‑synchronous polar orbit that precesses 360° per year, allowing full‑sky coverage while avoiding sunlight and Earth‑shine; (3) generation of coverage maps in HEALPix (Nside = 1024, 3.4′ pixels) that record the average number of detector‑pixel hits per sky pixel, thereby identifying deep‑field regions near the ecliptic poles and any remaining coverage gaps; (4) foreground modeling and removal, notably zodiacal light (with a minimum near 3.5 µm), helium airglow from the upper atmosphere, and other faint atmospheric emissions; (5) systematic monitoring (detector temperature, bias voltages, pointing accuracy) and a simulation‑based signal‑loss correction that accounts for varying hit counts, detector non‑linearity, and LVF‑induced spectral gradients.

The pipeline produces two primary data products: (i) tangent‑plane projections preserving the native 6.15″ pixel scale for high‑resolution studies of compact objects (e.g., Cat’s Eye Nebula, Helix Nebula), and (ii) full‑sky HEALPix maps for large‑scale analyses such as intensity mapping of the extragalactic background light (EBL), PAH distribution, and H II region mapping. Specialized maps can be generated for specific science cases, for example PAH‑only maps of the Triangulum Galaxy (M33) or intensity‑mapped EBL spectra across the full wavelength range. The authors demonstrate the pipeline with several examples: spatially resolved spectroscopy of the Cat’s Eye Nebula, a PAH map of M33, and a dust‑correlated spectrum from the full sky.

All products will be released through the NASA/IPAC Infrared Science Archive (IRSA). A public mosaic tool will allow users to select wavelength channels, projection type, and coordinate system to generate custom visualizations on‑the‑fly. Future releases will include the complete set of 102 spectral‑channel data cubes, high‑resolution image tiles, and the full suite of systematic logs.

In summary, the paper delivers a comprehensive, end‑to‑end map‑making framework that leverages SPHEREx’s LVF architecture to achieve both high spectral fidelity and full‑sky coverage. By rigorously modeling foregrounds, monitoring systematics, and providing flexible data products, the pipeline enables the mission’s primary science goal—intensity mapping of the cosmic history of galaxy formation via the EBL—while also supporting a broad range of ancillary investigations (PAHs, molecular hydrogen, nebular emission lines). The methodology sets a precedent for future all‑sky spectroscopic missions and facilitates synergistic studies with higher‑resolution facilities such as JWST.