Introduction

The AOFlagger is a flagger framework that implements several methods to deal with radio-frequency interference or RFI. Some of the implemented algorithms are the SumThreshold method, background fitting techniques (smoothing, sliding window, median filters, high-pass filters) and morphological operators. These are combined into one default strategy that performs very well on almost any observation.

The software consists of the flagger library (libaoflagger) that can be integrated into observatory pipelines by using the Application Programming Interface (API), as well as several programs to execute the flagger on measurement sets, tweak it and visualize results.

The frequencies covered by low-frequency telescopes like LOFAR and the MWA are considerably affected by radio-frequency interference (RFI), both in the low and the high band (see Figure below). Efficient RFI detection is essential to obtain high quality images.

History

The AOFlagger was originally written as part of my PhD thesis for the LOFAR Epoch of Reionization key science project, which needed a flagger with better accuracy compared to existing techniques. Since then it was made more generic and tweaked to work on data from other observatories and at other frequency ranges.

Aim and features

The AOFlagger is a tool that can find and remove RFI in radio astronomical observations. The code has been highly optimized for speed and accuracy. It is used by default for the LOFAR radio telescope. The software can run in a fully automated way, but a graphical interface (rfigui) is provided to analyse results and tweak the strategy. The preferred input file format is the Casa Measurement Set (MS) format, but single dish SDFits files are also supported.

I believe that the AOFlagger is the best available flagger, both in terms of accuracy and speed. It has been succesfully used on several interferometric telescopes, including LOFAR, WSRT, VLA, GMRT, ATCA and MWA, and the single-dish telescopes Parkes and Arecibo 305m.

Besides the flagger, the software consists of tools to efficiently visualize data in different ways, such as plotting time-frequency diagrams and power spectra. It provides the programs aoflagger, rfigui, aoqplot, aoquality and some other tools.

Figure: Overview of the RFI spectrum in LOFAR observations