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As a survey-class instrument,
ASKAP is
perfectly placed to survey the sky. In fact, we love the southern
skies so much that we just can't but help surveying it over and over
again with the Rapid ASKAP
Continuum Survey (RACS)! Traditional radio telescopes can take
many years to complete an all-sky survey but with ASKAP can perform
such a task in a matter of weeks (if done in a concerted effort) or a
few months if run as a low-priority filler observation. RACS (RACS-low
1) was first observed April 2019--October 2019 (on and off), this was
followed by RACS-mid which observed December 2020 - March 2021, and
RACS-high
which was observed December 2021--February 2022. Only a few weeks
after completing RACS-high we began re-observing the entire sky in the
low band again (RACS-low 2) to see if improvements in the telescope
scheduling, telescope maintenance and data processing resulted in data
quallity improvements -- these observations wrapped-up on May 21st.
Overall, RACS-low 2 was completed in one-third of the time
of RACS-low 1 (despite running as low priority to other pilot survey
observations that were ongoing), it covered more sky area (an extra
1500 square degrees to overlap more with the Northern sky), and
improved sensitivity by about 25%! The image below shows a comparison
of the coverage and sensitivity of the two surveys. In the images,
lighter shades highlight regions of increased sensitivity, dark shades
highlight where sensitivity is limited e.g. due to "glare" from bright
sources along the Galactic Plane (within the dashed lines) or as a
result of missing antennas that were undergoing maintenance. Having
both RACS-low 1 and RACS-low 2 will allow us to search for transients
and variable sources by comparing "before" and "after"
images. Ultimately though, the main impetus behind re-observing
RACS-low is to combine it with data from RACS-mid and RACS-high to
create a Global Sky Model - this will effectively be radio colour map
of the sky that will not only be valuable for science but will also
aid to speed-up and improve processing of Science Surveys (which begin
later in the year).
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