Wide Fast Deep (WFD)

The Wide Fast Deep (WFD) composes the bulk of the survey visits. The WFD sky includes low-dust extinction area, useful for extragalactic science, as well as higher stellar density areas (with higher dust extinction), useful for galactic science. The total area in the WFD is approximately 19.6k square degrees.

All of the WFD area receives approximately the same number of visits per pointing (around 800), although the balance of visits between different filters varies between the low-dust and other areas.

The low-dust WFD uses a rolling cadence, alternating high and low activity areas on the sky each year. There is no rolling in year 1, to provide uniform all-sky coverage at DR2 for better calibration and catalog creation.

Visits in the WFD are typically obtained in pairs - the first and second visit to a pointing are separated by about 33 minutes. These pairs are obtained in different filters except for y band: u+g, u+r, g+r, r+i, i+z, z+y, y+y. Using different filters for pairs allows determination of colors for variables and transients. About 3% of all pairs (4% of all visits) receive a later (3-7 hours later) third visit in the same filter as one of the earlier pair. While these triplets are a small fraction of the overall visits, they provide valuable opportunities for fast transient identification.

In general, each pointing receives a return visit between 2 to 4 days later, when the pointing is in an active rolling season. Inactive seasons have a lower cadence. These return visits will often be in a different set of filters, but this depends on the lunar phase.

The filters in use depend on the lunar phase, with u and y band being loaded into and out of the camera every two weeks. Approximately 7 days after new moon, the u band filter comes out of the camera and the y band filter is put in; this process reverses 7 days before new moon.