Design
One tile grid underlies everything the array does — survey tiers, target-of-opportunity pointings and difference imaging alike — and three tiers divide the available nights between area, cadence and depth.
A single grid for the whole program
All 7DS observations follow a common tile pattern generated from a HEALPix pixelization of the celestial sphere, with adjacent pointings overlapping by about 5 arcminutes in right ascension and 4 arcminutes in declination near the celestial equator and by more toward the poles. Tiles are numbered T00000 to T28519 in order of increasing declination, covering everything accessible to the array from the south pole to +30 degrees.
The RIS tile grid is the operational reference for the entire program: WTS and IMS point at the same tile centers, and target-of-opportunity observations use them wherever the field allows. Because every survey shares the grid, data from any tier coadd homogeneously with data from the others, and difference imaging always runs against a consistent reference. A visit is three consecutive 100-second exposures, coadded to a 300-second frame - a number set by the unguided tracking capability of the mount and the read noise of the detector.
| Tile centers | HEALPix pixelization of the celestial sphere |
|---|---|
| Tile range | T00000 – T28519, increasing declination |
| Sky coverage | South pole to +30° |
| Overlap near equator | ≈ 5′ in R.A., 4′ in Dec. |
| Standard visit | 3 × 100 s, coadded to 300 s |
| Typical night | ≈ 30 tiles, ≈ 3,000 exposures |
Why three surveys and not one
Area, cadence and depth cannot all be maximized at once out of a fixed number of nights. Rather than settle on one compromise, 7DS spends its time at three separate points in that trade — and ties them together by putting all three on the same tile grid, so data from any tier coadds homogeneously with data from the others.
Reference Imaging Survey
Area over depth and cadenceGive every part of the southern sky a medium-band reference image, once.
Nothing can be identified as a transient without an earlier image of the same sky in the same bands to subtract. RIS exists to build that reference, and its scale is set by the problem it serves: a gravitational-wave localization can fall anywhere, so the reference has to cover everything the array can reach. Covering 23,000 deg² at all forces the minimum per tile — one visit of 3 × 100 s — which is why RIS is the shallowest tier. The compensation is that a reference map made of forty medium bands is itself a homogeneous spectrophotometric survey, so the same exposures that enable difference imaging also support galaxy population studies, photometric redshifts and quasar identification at no extra cost in telescope time.
Wide-area Time-domain Survey
Cadence over areaFollow what changes on timescales of weeks, and stack deep enough to reach faint galaxies.
A single epoch says what is there; it does not say what is changing. WTS gives up most of the sky to revisit a smaller area every 10 to 14 days — an interval matched to what actually evolves on that scale: reverberation in active galactic nuclei, long-period variable and eclipsing stars, and slow extragalactic transients such as superluminous supernovae and tidal disruption events. Repetition also accumulates: roughly 90 to 100 visits per tile stack to 22.0–22.5 mag across most of the medium bands, deep enough for precise photometric redshifts on galaxies far below the single-visit limit. Fields are chosen where near-infrared data already exist — the VIKING footprint, the Rubin Deep Drilling Fields — because those are wavelengths 7DT cannot reach for itself.
Intensive Monitoring Survey
Depth and cadence over areaCatch variability within a single night, and build the deepest photometry 7DS will produce.
Some sources change faster than any wide survey can follow. IMS gives up area almost entirely — about 8.5 deg², seven tiles — in order to observe every available night, which is the only way to characterize short-period variables, active galactic nuclei on short timescales, and any transient that appears inside the field. Observing the same tiles nightly for five years also coadds to the deepest single-tile photometry the survey will reach, about 23.6 mag at peak sensitivity. The field position was a deliberate choice rather than a convenient one: it overlaps the SPHEREx Deep Field South, so 7DT optical medium-band photometry and SPHEREx near-infrared spectrophotometry cover the same volume.