The SPHEREx Sky Simulator: Science Data Modeling for the First All-Sky Near-Infrared Spectral Survey

TL;DR

The SPHEREx Sky Simulator addresses the need to forecast instrument performance and science outcomes for a all-sky near-infrared spectral survey. It combines Python-based, HPC-friendly sky models with a detailed instrument model to generate end-to-end simulations (Levels 0–2) and auxiliary catalogs, including a QuickCatalog mode for rapid photometry. The paper documents the architecture, input sky models (Zodiacal light, DGL, compact and extended sources, satellites, planets), survey planning, and detector effects, and presents pre-launch sensitivity predictions showing a zodiacal-light–limited 5σ depth across the 0.75–5 μm range. It demonstrates the simulator’s role in pipeline validation, mission planning, and forward-modeling of systematics, with plans to expand and publicly release an on-orbit version for community-driven science analyses.

Abstract

We describe the SPHEREx Sky Simulator, a software tool designed to model science data for NASA's SPHEREx mission that will carry out a series of all-sky spectrophotometric surveys at $\sim$6'' spatial resolution in 102 spectral channels spanning 0.75 to 5 $μ$m. The Simulator software implements models for astrophysical emission, instrument characteristics, and survey strategy to generate realistic infrared sky scenes as they will be observed by SPHEREx. The simulated data includes a variety of realistic noise and systematic effects that are estimated using up-to-date astrophysical measurements and information from pre-launch instrument characterization campaigns. Through the pre-flight mission phases the Simulator has been critical in predicting the impact of various effects on SPHEREx science performance, and has played an important role guiding the development of the SPHEREx data analysis pipeline. In this paper, we describe the \skysim\ architecture, pre-flight instrument and sky models, and summarize high-level predictions from the Simulator, including a pre-launch prediction for the 5$σ$ point source sensitivity of SPHEREx, which we estimate to be $m_{\rm AB}$ 18.5--19 from 0.75 to 3.8~$μ$m and $m_{\rm AB}$ 16.6--18 from 3.8 to 5 $μ$m, with the sensitivity limited by the zodiacal light background at all wavelengths. In the future, on-orbit data will be used to improve the Simulator, which will form the basis of a variety of forward-modeling tools that will be used to model myriad instrumental and astrophysical processes to characterize their systematic effects on our final data products and analyses.

Figures (19)

• Figure 1: SPHEREx Simulator architecture.
• Figure 2: Example simulated exposure images near the north ecliptic pole. In each image, wavelength increases in the downward direction (see Table \ref{['tab:spherex_bands']}). The short-wavelength bands 1, 2, and 3 are in the top row, and the mid-wavelength bands 4, 5, and 6 are in the bottom row. The pointings are approximately equivalent for the two focal planes. The most prominent components are stars, Zodiacal light and diffuse Galactic light (DGL). The Zodiacal light is spatially smooth with a steeply rising SED at long wavelengths (see Table \ref{['tab:spherex_bands']} for wavelength coverage of each band); this creates a strong vertical gradient in the LVF images for the longest wavelengths. The DGL manifests as nebular anisotropies that are visible in the shorter-wavelength arrays. Below the mid-point of the band-4 image, the horizontal band is due to a PAH emission feature at $3.3~\mu\mathrm{m}$. At the bottom of band 3 and the top of band 4, there is a dimming due to the minimum in the optical efficiency of the dichroic beam splitter.
• Figure 3: Observation wavelengths vs. time (in days since the start of the mission) using QuickCatalog for a primary calibration source (GD71) in one mission year. The survey plan covers this particular source in two distinct epochs in this year, and is representative of the coverage of most sources away from the ecliptic pole regions.
• Figure 4: An example of simulated source photometry using QuickCatalog for a primary calibration source (GD71). Upper panel: simulated primary photometry (at the exact wavelength observed by SPHEREx). Lower panel: the secondary catalog, containing photometry binned into 102 spectral channels (blue) and synthetic photometry (orange squares) where SPHEREx photometry data are binned into synthetic photometry for LSST, 2MASS and WISE bands.
• Figure 5: Separated simulation components for array 1 for the exposure image shown in Fig. \ref{['fig:exampleImages']}. The component images can be simulated independently and then summed linearly to produce the full image. Individually resolved galaxies can be included in simulation of the extragalactic background light, or to simulate the true sky, galaxies can be instead drawn from the reference catalog. Photon noise is calculated from all optical components. Read noise and dark current are estimated from pre-launch laboratory testing.