What is 7DS?

Most optical surveys measure a source in a few broad bands, which constrains its spectrum only weakly. Identifying what a source is — and, for anything that varies, what is changing about it — then requires spectroscopic follow-up on a larger telescope, which is expensive and cannot be applied to more than a small fraction of detections. The result is a large gap between the number of sources a survey finds and the number it can characterize.

7DS closes that gap by putting the spectral information into the survey itself. Imaging through a set of medium bands rather than a few broad ones gives every source a low-resolution spectrum at the moment it is detected, for the whole field at once and without follow-up. The immediate motivation was the search for optical counterparts to gravitational-wave events, where candidates must be classified quickly and in large numbers; the same capability applies to any survey question that depends on knowing what a source is rather than only how bright it is.

7DS is a spectral-mapping survey of the southern sky. Rather than measuring a few broadband colors, it images through medium-band filters of about 25 nm width, so each visit yields a spectral energy distribution at R = 30-70 for every source in the field. Applied over 23,000 square degrees, this produces a homogeneous low-resolution spectroscopic map of the southern sky, and applied repeatedly it measures how those spectra change with time.

The survey combines two capabilities that are usually separate. Spectral mapping: each visit samples the spectrum of every source in a 1.25 square-degree field at R = 30-70 between 375 and 875 nm, which is enough to locate the 4000 Angstrom break and strong emission lines, and so to estimate redshifts and stellar populations directly from imaging. Time domain: the same field can be revisited on cadences from one day to two weeks, so the spectral measurement becomes a time series rather than a single epoch.

Sampling a spectrum at this resolution costs exposures: a full medium-band set is many more frames than a broadband survey takes for the same field. The array is what makes that affordable, and the trade is set out under the telescope and the survey design.