The WINTER Observatory: A One-Degree InGaAs Survey Camera to study the Transient Infrared Sky

The Wide-field Infrared Transient Explorer (WINTER) is a near-infrared time-domain survey instrument operating on a dedicated 1-meter robotic telescope at Palomar Observatory. The project takes advantage of recent technology advances in time-domain astronomy, robotic telescopes, large-format sensors, and rapid data reduction and alert software for timely follow up of events. Since June of 2023, WINTER robotically surveys the sky each night to a median depth of J_AB = 18.5 mag, balancing a variety of science programs including searching for kilonovae from gravitational-wave alerts, blind surveys to study galactic and extragalactic transients and variables, and building up reference images of the near-infrared sky. The project also serves as a technology demonstration for new large-format Indium Gallium Arsenide (InGaAs) sensors for wide-field science in the near infrared without cryogenically cooled optics or detectors. WINTER’s custom camera combines six InGaAs sensors with a novel tiled fly’s-eye optical design to cover a >1 deg^2 field of view with 1 arcsecond pixels in the Y-, J-, and shortened-H-band filters (0.9 - 1.7 micron). This paper presents the design, performance, and early on-sky science of the WINTER observatory.

💡 Research Summary

The Wide‑field Infrared Transient Explorer (WINTER) is a dedicated near‑infrared (NIR) time‑domain survey instrument mounted on a 1‑meter robotic telescope at Palomar Observatory. This paper describes the instrument’s design, performance, and early on‑sky science. WINTER’s scientific goals are threefold: rapid robotic follow‑up of gravitational‑wave alerts to search for kilonovae, wide‑field NIR surveys of dust‑obscured transients, variable stars, and young stellar objects, and the construction of a deep, multi‑year reference image of the northern sky for static‑sky science.

The heart of WINTER is a custom camera that combines six large‑format Indium Gallium Arsenide (InGaAs) AP1020 focal‑plane arrays. Each sensor has 1920 × 1080 pixels with a 15 µm pitch, covering 0.9–1.7 µm (Y, J, and shortened‑H bands). InGaAs offers dark currents 2–3 orders of magnitude lower than HgCdTe at comparable temperatures, allowing background‑limited performance with only thermoelectric cooling to –40 °C, eliminating the need for cryogenic optics. The six sensors are tiled in a “fly‑eye” layout, delivering >1 deg² field of view with a 90 % fill factor and 1″ pixel scale.

Readout electronics are built around eight high‑speed 16‑bit ADCs per sensor and an Artix‑7 FPGA, providing up to 30 Hz full‑frame rates. The CTIA‑based read‑out integrated circuit (ROIC) supports video‑rate imaging but has higher read noise than traditional source‑follower designs; this is mitigated by on‑chip correlated double sampling and non‑destructive multiple reads, achieving effective read noise suitable for sky‑limited photometry. Each sensor is mounted in a hermetically sealed vacuum housing with a two‑stage thermoelectric cooler; heat is transferred via copper heat pipes to a liquid‑cooling loop, keeping the dome environment thermally stable.

Optically, WINTER uses a compact fly‑eye system of low‑index plastic lenses and a thin filter tray, delivering Y (≈1.0 µm), J (≈1.2 µm), and shortened‑H (≈1.6 µm) bandpasses with typical 5σ depths of J_AB≈18.5 mag per exposure. The instrument operates robotically each night, scanning ~400 deg² per week. A real‑time data pipeline performs image subtraction, machine‑learning transient detection, and generates alerts within ~30 seconds, enabling rapid tiling of GW localization regions.

Early science results include: (1) Simulations for the O4 observing run predicted up to five kilonova detections, but the paucity of GW events limited actual verification; nevertheless, the system demonstrated the ability to tile large error regions quickly. (2) A dust‑obscured supernova search using Y‑J colors recovered >60 % of transients missed by optical surveys, confirming WINTER’s strength in heavily reddened environments. (3) Exoplanet transit observations at Las Campanas showed sub‑5 mmag photometric precision, validating the InGaAs detectors for high‑precision time‑series work. (4) Initial stacking of nightly images yields J≈20 mag depth in 30 min, providing a valuable static‑sky map for Galactic structure, brown‑dwarf census, and high‑redshift quasar selection.

Overall, WINTER demonstrates that large‑format InGaAs sensors, coupled with a novel fly‑eye optical design and FPGA‑based fast readout, can deliver cost‑effective, wide‑field NIR time‑domain surveys without cryogenic cooling. The instrument serves both as a scientific facility and a technology pathfinder for future NIR surveys, including synergy with the upcoming Nancy Grace Roman Space Telescope and other infrared time‑domain projects.