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

Abstract: The Wide-field Infrared Transient Explorer (WINTER) is a new near-infrared time-domain survey instrument installed on a dedicated 1-meter robotic telescope at Palomar Observatory. The project takes advantage of the 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 near-infrared photometry without cryogenic cooling. WINTER's custom camera combines six InGaAs sensors with a novel tiled fly's-eye optical design to cover a >1 degree-squared 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.