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

TL;DR

WINTER addresses the need for wide-field, near-infrared time-domain imaging on a cost-effective platform by deploying six InGaAs sensors in a novel fly's-eye optical design on a 1 m Palomar telescope. The paper details the instrument, its custom readout electronics and software, and a rigorous on-sky performance evaluation, including a comprehensive assessment of conversion gain, quantum efficiency, dark current, and nonlinearity. Despite a substantial shortfall in quantum efficiency (observed ~5–10% vs design ~80%), WINTER achieves a median depth of about $J_{AB}\approx18.5$ mag in 16 minutes and demonstrates a robust data-reduction pipeline capable of rapid transient identification and VoToO follow-up using GPU-accelerated processing. The work provides a practical demonstration of InGaAs viability for ground-based NIR time-domain astronomy, delivers early science results (including kilonova follow-up attempts and obscured transients), and establishes a foundation for future wide-field NIR surveys in the Rubin/ Roman era and beyond.

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.

Figures (20)

• Figure 1: WINTER mounted on its dedicated robotic telescope, including the enclosed optics and sensor box, electronics support boxes, liquid cooling, the filter tray and a cable wrap.
• Figure 2: One of the WINTER InGaAs sensors (left) with custom readout electronics (right). The sensor is cooled by a two-stage TEC in the vacuum-sealed housing. The housing is mounted to a copper sled with copper heat pipes drawing heat away from the electronics to a heat exchanger leading to a liquid cooling loop. The five readout boards power the sensor and provide local control of each sensor through an Artix 7 FPGA.
• Figure 3: A raw image of WINTER's focal plane with six sensors. The sensors are named by position in the physical instrument, including a port and starboard side with three sensors each. The panel presents the six sensor frames arranged to mirror their layout on the focal plane, with the color scale indicating the measured counts. Prominent artifacts include areas where sensors have partially failed due to glow-spot contamination—seen in the Port C panel and the circular imprint in the Star C—as well as variations in dark current driven by thermal shifts, evidenced by the elevated background in the Port B relative to Port C above it.
• Figure 4: Histogram of photon transfer curve (PTC) slopes measured for individual pixels within one readout channel of a WINTER detector. The PTC slope (DN/e$^{-}$) is determined from the linear relationship between variance and mean signal in flat field images at different exposure levels. The inverse of this slope yields the conversion gain for each pixel. The distribution shows 255,000 individual pixel measurements (light blue histogram) with a Gaussian fit overlaid (dark blue line). The fitted mean gain is 2.5 $\pm$ 0.9 e$^{-}$/DN (1$\sigma$ standard deviation of the pixel-to-pixel variation). The designed gain specification of 1.6 e$^{-}$/DN (green dashed line) falls within the distribution but is significantly lower than the fitted mean.
• Figure 5: Measured quantum efficiency (QE) of one WINTER sensor at visible and near-infrared wavelengths. A low response is expected outside of the near-infrared (WINTER's three filter wavebands are highlighted in blue); however, the measured near-infrared QE is lower than expected at $\sim$5% as opposed to the designed 80%. This test assumes a gain of 2.5 e-/DN.