Works with amplitude information of one polarization of a single sub-band Ü It relies on thresholding, where cutoffs depend on the surrounding signal levels A LOFAR RFI pipeline A.R. Offringa 2.1 Input !" # !$ % & ' $( ! ))*)+*+)*++ & " % % + Figure 1: Overview of the RFI ﬂag- The ﬂagger is executed on the amplitude information of one polarisation of a single sub-band of a baseline. In LO- FAR’s common operation, a sub-band consists of 256 chan- nels of 0.8 kHz resolution. The full band has 248 sub-bands. LOFAR can observe in two bands: the 10-80 MHz low band and the 110-240 MHz high band, which are observed by phys- cally different antennae. If speed is essential, the algorithm can be executed once on the Stokes-I values. Otherwise, if accuracy is more impor- ant than speed, the algorithm can be executed on the individ- ual XX and YY or LL and RR polarisations, or on all polar- sations individually. We do see some RFI that manifests in only one of the polarisations, or rotates through the polarisa- ions, and some advantage is therefore seen when ﬂagging all polarisations individually. 2.2 Iterations A part of the algorithm is iterated a few times, depicted n Figure 1 by the “Continue iterating” block. This is nec- Image Credit: A. Offringa
catch all the RFI, even if some non-contaminated data gets flagged Ü AOFlagger uses time selection steps which compares RMS values, and automatically flags anything with a sigma > 3.5 in order to quickly reach convergence This may not be ideal for observations containing transients
transient signals Ü Changes include deleting time selection, decrease ‘sliding window’ resolution in time, ignore thresholding in frequency ✻ Tested on MSSS data which showed high RFI percentages flagged in processing- starting from raw data to re-flag, demix, calibrate, and image…
(including any post- BBS flagging) the RMS values increase dramatically including big spikes in amplitude Ü When you do post-BBS flagging only (eg a simple amplitude cut), you get similar RMS values to a normal image Ü How is post-BBS flagging in general affecting transient sources?
signals into MSSS data (starting with simple point sources) Ü We need to figure out how a bright transient affects calibration and image quality when it isn’t in the sky model Image credit: Dan Calvelo
transient searches appears to work with automatic flagging Ü Image quality does not appear to be compromised for faster computational speeds Ü Future work needs to focus on testing the modified flagger on a wider range of data, eg. MSSS HBA Ü See how this can apply to AARTFAAC Credit: ASTRON Daily Image