by Emil Lenc and the RACS team

A second epoch of RACS (Rapid ASKAP Continuum Survey) in the mid-frequency band has been completed and made available to the astronomical community via CASDA (the CSIRO ASKAP Science Data Archive). This is the sixth time in as many years that RACS has observed and mapped the entire southern sky (Twice in the low band at 887.5 MHz: RACS-low1 and RACS-low2; once in the low band at 943.5 MHz: RACS-low3; twice in the mid-band at 1367.5 MHz: RACS-mid1 and RACS-mid2; and once in the high-band at 1655.5 MHz: RACS-high1). With each RACS epoch we not only learn more about the radio sky but also find new ways to improve the way we make observations and the way we process the data. These improvements are ultimately incorporated into the telescope operations to improve the data products from the main survey science projects. Observations of RACS-mid2 were used to implement and test a number of these improvements since the original RACS-mid1 epoch. These include the use of improved models of how the telescope “sees” the sky – effectively corrective glasses for the telescope to help it form more accurate images of the sky; improved calibration to reduce atmospheric effects; and improved image processing to reduce the effect of glare from bright objects just outside of the field of view of the telescope. To minimise the impact on survey science observations, RACS-mid2 was primarily observed as a “filler” observation i.e. filling in gaps in the schedule where survey science projects can’t observe or where observing conditions are not optimal. In total, RACS-mid2 required 1493 15-minute observations to complete the survey, ultimately an additional 116 re-observations were made to replace observations that were deemed to be poor (e.g. due to system disruptions, extreme solar interference, or a rogue planet photo-bombing the field). All up 403 hours of telescope time were used to complete this epoch spread over a period of 313 days between Oct 2024 and Aug 2025. While the sensitivity of RACS-mid2 is marginally lower than that of RACS-mid1 (Solar activity was particularly strong during this period and 70% of the RACS-mid2 observations were during the day), it is much more consistent across the sky (as a result of the improved calibration and processing). The RACS team will now concentrate on what we call “RACS-all” where the existing RACS epochs are combined to improve sensitivity further and provide a description of astronomical objects across the entire ASKAP frequency range. A more detailed description of RACS and the available data products can be found on the RACS survey website.