Survey

About RACS

The Rapid ASKAP Continuum Survey (RACS) is an observatory led project that began observations in April 2019. The first epoch observations covered the whole southern sky to +41 deg Declination with ASKAP. The survey observations for the entire region took ~230 hours of telescope time (excluding repeated fields). The median field RMS is ~0.25 mJy/beam.

Multiple passes are planned in future to cover ASKAP’s full frequency range and additional epochs of the survey are planned to search for transient sources. The ASKAP observing band is split into three bands and RACS will cover each of these bands (and are referred to as RACS-low, RACS-mid and RACS-high) in a dedicated epoch (the epoch number is appended to the band e.g. RACS-low1 is the first epoch in the low band. RACS will provide a unique opportunity to study the radio sky and radio populations, in particular AGN and their role in galaxy evolution. RACS is being used to produce a global sky model for ASKAP, and test operations from data collection and reduction to release.

Survey parameters (first data release)

Survey	RACS-low	RACS-mid	RACS-high
Frequency	887.5 MHz / 943.5 MHz	1367.5 MHz	1655.5 MHz
Bandwidth	288 MHz	144 MHz	~200 MHz
Sky Coverage	-90° < δ < +49°	-90° < δ < ~+49°	-90° < δ < ~+48°
Tiling	903 tiles / 1493 tiles	1493 tiles	1493 tiles
Integration per tile	15 minutes	15 minutes	15 minutes
Footprint	square_6x6 / closepack36	closepack36	closepack36
Beam spacing	1.05 deg / 0.9 deg	0.90 deg	0.90 deg
Surveyed area	~36,200 sq. deg	~36,194 sq. deg	~35,955 sq. deg.
Median Sensitivity	0.25 mJy/beam	~0.2 mJy/beam	~0.2 mJy/beam
Resolution	~15 arcsec	~10 arcsec	~8 arcsec

Sky coverage

ASKAP has a large field-of-view as a result of innovative Phased-Array Feed (PAF) technology developed by CSIRO. This allows up to 36 beam directions to be observed simultaneously in a so-called “footprint”. RACS-low makes tile observations using the square_6x6 footprint as shown below. RACS-mid, RACS-high and RACS-low3 make use of the closepack36 footprint.

ASKAP field-of-view

ASKAP field-of-view showing positions for all 36 beams of a square_6x6 footprint

The figure below is an orthographic view of the celestial sphere showing the arrangement of the RACS-low1 observing tiles. Ranks of tiles are centred on a series of declinations from +37.6 to -68.7 degrees, giving full sensitivity from −71.3 to +40.2 degrees. A quasi-rectangular grid of tiles is placed over the zone south of declination −71 degrees, centred on the south celestial pole. The first RACS data release (RACS-low1) covers the whole southern sky to +41 deg Declination.

Tile placement on the celestial sphere.

RACS-low1 tile placement on the celestial sphere.

Frequency coverage

The figure below shows the ASKAP sensitivity over its frequency range. The purple line traces the median value of central beams of all 36 antennas. Bands affected by radio-frequency interference are shaded grey. ASKAP’s three tuning ranges are shown and labelled. The proposed RACS bands are shown as orange bars. The first RACS data release was taken in “Band 1” (RACS-low1) with a central frequency of 887.5 MHz and 288 MHz of bandwidth (Note that RACS-low3 shifted the central frequency up to 943.5 MHz where sensitivity is slightly improved). RACS “Band 2” (RACS-mid) data has been observed and released. RACS “Band 3” (RACS-high) and a second epoch of RACS-low (RACS-low2) have also been observed and an initial pass of processing performed.