Reference Imaging Survey
One medium-band visit to every tile the array can reach, to give the southern sky a reference image against which anything that changes can be found.
What this survey is for
Give every part of the southern sky a medium-band reference image, once.
Difference imaging requires an earlier image of the same sky in the same bands. RIS builds that reference, and its scale follows from the problem: a gravitational-wave localization can fall anywhere, so the reference must cover everything the array can reach. Covering 23,000 deg² limits the time available per tile to a single visit of 3 × 100 s, which makes RIS the shallowest of the three surveys. A reference map built from forty medium bands is also a homogeneous spectrophotometric survey in its own right, so the same exposures support galaxy population studies, photometric redshifts and quasar identification.
Area over depth and cadence
| Area | 23,000 deg² |
|---|---|
| Region | Dec < +20° |
| Cadence | Single visit |
| Visit | 3 × 100 s, coadded to 300 s |
| Single-visit depth | 19.1 mag, 5σ in m600 |
| Tile range | T00000 – T25471, plus northern extension |
Where it has observed
The survey as designed: all 25,472 tiles of the reference grid are drawn in gray, and the ones with science exposures are colored over them, so what is left to observe is the gray. Because RIS covers everything the array can reach, this grid is also the footprint of the survey as a whole. Hover a tile for its own figures. Depth and integration time are estimates: open-shutter time is recorded for the survey rather than per tile, so integration time on a tile is its frame count times the survey mean. Depth is counted differently, against the RIS visit itself: frames in m600 at the fiducial 100 s each, measured against the 19.1 mag reached by one 3 × 100 s visit — background-limited, so the 5σ limit improves as the square root of the time. Depth counts only the frames taken in m600, since a tile visited in many filters is no deeper in any one of them. The whole survey footprint, with per-tile detail under the pointer and a search by position, is on the data access page.
What has been observed so far
15,513 of 25,472 tiles observed
Counted over the original grid, T00000–T25471. The northern extension carries the tiling to Dec +30° and is counted separately. A full cycle is anticipated by the end of 2027.