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. This survey 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-low1 (first data release)||RACS-mid||RACS-high|
|Frequency||887.5 MHz||1367.5 MHz||1655.5 MHz|
|Bandwidth||288 MHz||144 MHz||~200 MHz|
|Sky Coverage||-90° < δ < +41°||-90° < δ < ~+49°||-90° < δ < ~+48°|
|Tiling||903 tiles||1493 tiles||1493 tiles|
|Integration per tile||15 minutes||15 minutes||15 minutes|
|Beam spacing||1.05 deg||0.90 deg||0.90 deg|
|Surveyed area||34,240 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|
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 and RACS-high make use of the closepack36 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 covers the whole southern sky to +41 deg Declination.
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. RACS “Band 2” (RACS-mid) data has been observed and the first data release and publication is currently being reviewed. 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.