The 7-Dimensional Telescope and the 7-Dimensional Sky Survey: status report on final commissioning and survey operation

Abstract

The 7-Dimensional Telescope (7DT), developed by Center for the Gravitational-Wave Universe at Seoul National University, is a multi-telescope array designed to identify the electromagnetic counterparts of gravitational-wave events. 7DT consists of twenty 50cm telescopes equipped with CMOS cameras, each providing a 1.25 deg2 field of view. This configuration provides IFU-like data with low spectral resolution (R=30-70) by utilizing 40 medium-band filters of 25nm width spanning from 400 to 900nm, distributed across the array. Since achieving first light in October 2023, comprehensive commissioning has been carried out to evaluate instrument functionality and verify science procedures. These activities included optomechanical alignment, focus measurement, optical assessment, the generation of master calibration frames, spectrophotometric calibration, and evaluation of astrometry and image depth. To verify the scientific capabilities of the 7-Dimensional Sky Survey (7DS), observations were conducted on a wide range of astronomical targets, including nearby galaxies (e.g., PHANGS sample), extragalactic fields (e.g., UDS field for photometric redshift tests), Galactic objects, and solar system objects. 7DS comprises three tiered surveys: the Reference Imaging Survey (RIS), which covers over 20,000 deg2 of the Southern Hemisphere sky and aims to provide reference images for difference image analysis for gravitational-wave counterparts and other transient objects with a single 5-minute exposure visit, the Wide-area Time-domain Survey (WTS), which will cover 800-1,200 deg2 with a 10-14 day cadence, prioritizing areas with ancillary near-infrared data, such as VIKINGS or the LSST Deep Drilling Fields, and the Intensive Monitoring Survey (IMS), which focuses on the SPHEREx Deep Field South, near the ecliptic south pole, with a 1-day cadence. RIS commenced in July 2024, and IMS began in April 2025. WTS will commence in 2026. This presentation summarizes the 7DT commissioning status, the early scientific results derived from the commissioning data, and the current operational status of the 7DS.