SOLO: wide-field asteroid light curve monitoring system for SPHEREx

TL;DR

SOLO introduces a dedicated wide-field, high-cadence optical survey designed to produce absolutely calibrated asteroid light curves in the Gaia $G$-band to support the SPHEREx SSOC. The paper details the hardware (RASA-11 + Kepler 4040 FI CMOS, WG320 filter), remote control architecture, and a Python-based data-reduction pipeline (solopy) that calibrates against Gaia DR3 and generates reliable, nightly photometry for moving targets. Commissioning data demonstrate stable field-wide photometry with residuals $\lesssim 3\%$ and a 180 s limiting magnitude of $G \sim 17$--$18$, along with sample light curves showing consistency across nights and a phase-averaged scatter of $\lesssim 0.03$ mag. The authors outline an observing program to deliver on the order of $10^{3}$ absolutely calibrated Gaia $G$-band asteroid light curves per year, enabling effective rotational corrections for SPHEREx spectra and substantially enhancing the scientific value of the SPHEREx asteroid catalog.

Abstract

We present the Solar system Objects Light curve Observatory (SOLO), a wide-field, high-cadence optical survey system designed to obtain absolutely calibrated asteroid light curves, converted to the Gaia G-band photometric system, in support of the SPHEREx Solar System Object Catalog (SSOC). SOLO was installed at the Sierra Remote Observatories (SRO) in California, USA, in July 2025 and is optimized for continuous, multi-night monitoring of asteroid brightness variations. We describe the system configuration, remote operation, and data reduction pipeline, and evaluate its optical and photometric performance using commissioning data. SOLO achieves stable photometric calibration across the 11.6 deg^2 field of view and reaches a 10-sigma limiting magnitude of G ~ 17.5 for a 180 sec exposure. Sample asteroid light curves obtained over multiple nights demonstrate consistent absolute photometry at the same rotational phase, validating the estimated performance. Finally, we outline the planned operational use of SOLO in connection with NASA's SPHEREx mission. Full science operations of SOLO are scheduled to begin in January 2026. Using these data, we aim to obtain on the order of 10^3 absolutely calibrated asteroid light curves per year in the Gaia G-band, which will be used to support the construction and scientific utilization of the SPHEREx SSOC.

Figures (13)

• Figure 1: Overview of the SOLO observation system after its installation at the Sierra Remote Observatories (SRO) in California, USA, in July 2025.
• Figure 2: Design of the custom spider assembly for SOLO (shown in red). The circular interface on the front-left side attaches to the telescope entrance aperture, while the dark-gray structure on the rear-right side represents the mounted CMOS camera. A faint outline of the spider is shown in the background to illustrate its projected side view and overall geometry.
• Figure 3: Expected system throughput of SOLO, considering detector QE (dashed), filter transmittance (dotted), and atmospheric transmission (dash-dotted). The black solid line shows their product (i.e., system throughput). For comparison, the green line shows the filter transmittance of Gaia$G$-band. We adapted the atmospheric model constructed by lowtran1988ugls.rept.....K.
• Figure 4: Schematic diagram of the SOLO system configuration and data-processing pipeline. Solid and dashed lines indicate power and data connections, respectively. The three observational subsystems (Equipments), the RST-400 mount, the Kepler 4040 FI camera, and the ZWO EAF focuser, are installed at Sierra Remote Observatories (SRO) and are controlled by an on-site PC located in the control box. The SRO PC is dedicated to device control and data acquisition and receives power supply and internet connectivity from the SRO infrastructure. The system is remotely operated from Seoul National University (SNU), South Korea, via the internet. The raw data acquired at SRO are transferred to a dedicated SOLO processing server at SNU, where the primary data reduction and photometric analysis are performed, and the resulting data products are stored in the SOLO data server.
• Figure 5: An example image from SOLO, taken on July 22, 2025, with 180 sec exposure. (a) Raw image, (b) calibrated image, and (c) mask image. In panel (a), strong vignetting is visible in the four corners, along with additional shading on the top, bottom, and left edges caused by the square filter holder. After applying the flat‐field correction, these effects are largely removed in panel (b), resulting in a much more uniform background across the entire field of view. Panel (c) shows an example of the mask image, where bright stars, satellite trails, and bad pixels are masked out to exclude them as outliers during the data calibration.