What is 7DS?

On 17 August 2017 the merger of two neutron stars, GW170817, was seen in gravitational waves and then found in light. It remains the only gravitational-wave event with a confirmed electromagnetic counterpart. The reason is not that such mergers are rare but that their counterparts are hard to catch: a kilonova peaks near absolute magnitude −15 to −17 and fades by about half a magnitude a day, and the sky region a detector network reports for it covers hundreds to thousands of square degrees. Only about a dozen gravitational-wave sources have ever been localized to better than 100 square degrees — and a patch that size is expected to contain of order a hundred unrelated transients over a week.

That is the problem 7DT was built to solve. Searching an area that large quickly requires a wide field and fast slewing; separating a kilonova from the supernovae, novae and detector artifacts inside it requires spectral information. Conventional practice is to image first and follow up spectroscopically, which costs nights on large telescopes and time the source does not have. 7DT collapses the two steps into one: every exposure already carries a low-resolution spectrum of every source in the field.

7DT is designed, built and operated by the Center for the Gravitational-wave Universe at Seoul National University, with support from the National Research Foundation of Korea, funded by the Korean government (MSIT). The Center came to the project through GECKO, the Gravitational-wave Electromagnetic Counterpart Korean Observatory — a network of existing Korean-accessible telescopes that followed up gravitational-wave alerts during the O3 observing run. Its campaign on GW190425, the first binary neutron star merger of that run, covered 621 candidate host galaxies inside a 7,460 square-degree localization and found no kilonova. The limits of working with borrowed time on telescopes not designed for the task were the direct argument for building one that was.

Since first light in October 2023 the array has grown from twelve to sixteen operational units, and from twenty to thirty-five installed medium-band filters, against a design of twenty telescopes and forty filters. Commissioning has reached closure, robotic operation runs unattended, and the science program — the 7-Dimensional Sky Survey (7DS) — is under way on two of its three tiers.