Versions applicable to data release

During the course of survey operations, improvements were made to the Apercal pipeline in addition to bug fixes. The table of released observations contains the specific Apercal version (including github hashtag) used for the processing of that dataset. In addition, we assign “friendly” names that encompass the main changes from a data quality point of view. These versions are described below, in chronological order:

• Apercal_300 (300 MHz): Survey observations at the start of the survey period were processed with the full 300 MHz of data. Flagging all RFI present at low frequencies is a challenge and unflagged RFI affected the calibration of the whole band. Data processed  with the full 300 MHz were not ingested. However, the raw observational data that was processed in this mode falls under the data release.
• Apercal_150 (150 MHz): Starting with observations in August 2019, only the top 150 MHz (1280-1430 MHz) of the Apertif band, which is relatively RFI free, was processed. This resulted in a strong increase of data quality for the final data products, and thus data processed in this mode of the pipeline is considered for release. Note that the frequency range 1280-1292.5 MHz is automatically flagged as part of this pipeline version as it is heavily impacted by RFI.
• Apercal_150_ACF  (Auto-correlation flagging): Internal quality assessment checks the auto-correlations of dishes. High auto-correlation values indicate a dish where the formed compound beam suffers from quality issues; these data are flagged as they contribute to direction-dependent errors in final images. The cross-calibration is then rerun.
• Apercal_150_ACF_C-D (RTC-RTD flagging): The RTC-RTD baseline has an elevated (noise) response level, related to the introduction of the synch-optics boards (see System notes). In order to avoid bias in the calibration / flux scale, this baseline is flagged.
• Apercal_150_ACF_C-D_BPF (Bandpass-phase flagging): An additional internal QA check; if the phases of the bandpass solution have too large a rms, that dish is flagged and the cross-calibration is carried out again.