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3rd of June 2022
Surveying the sky faster and better than before - RACS-low 2
Emil Lenc & Vanessa Moss
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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