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As shown in Fig. 1, LoTSS-DR2 consists of 841 pointings and it covers a total of 5634 square degrees which corresponds approximately to our contiguous coverage at the time of beginning the LoTSS-DR2 processing run. The data release is formed by two contiguous regions that are centred at approximately 12h45m00s +44◦30′00′′(RA-13 region) and 1h00m00s +28◦00′00′′(RA-1 region) and span 4178 and 1457 square degrees, or 626 and 215 pointings, respectively. The data were taken between 2014-05-23 to 2020-02-05 as part of the LoTSS projects LC2_038, LC3_008, LC4_034, LT5_007, LC6_015, LC7_024, LC8_022, LC9_030, LT10_010 and the co-observing projects LC8_014, LC8_030, DDT9_001, LC9_011, LC9_012, LC9_019,LC9_020,COM10_001,LC10_001,LC10_010, LC10_014,LT10_012,LC11_013,LC11_016,LC11_019, LC11_020, LC12_014. All the data that were processed as part of this data release are stored in the LOFAR Long Term Archive (LTA) with approximately 62% in Forschungszentrum Jülich, 32% in SURF8and the remaining 6% in Poznan. The vast majority of pointings were observed for a total of 8 hrs with 48 MHz (120-168 MHz) of bandwidth which allows for two pointings to be observed simultaneously with current LOFAR capabilities. However, primarily due to the co-observing program10through which we exploit the multi-beam capability of LOFAR and accumulate LoTSS data simultaneously with observations conducted for other projects, for 18 of the pointings in LoTSS-DR2 we have used data that has the same frequency coverage but a total integration time of∼16 hrs. The overall observing time utilised for this data release is 3451 hrs and the volume of archived data that was processed is 7.6 PB. Thus the average data size for an 8 hr pointing (two observed simultaneously) is 8.8 TB but there is
significant variation because data that have been recorded since 2018-09-11 are typically five times smaller than those before this date due to Dysco compression (Offringa 2016) being utilised by the radio observatory prior to ingesting data into the LTA in more recent observations.
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Fig. 1. The status of the LoTSS observations as of April 2021 and the approximate current sensitivity coverage (accounting for station projection and typical sensitivities achieved to date) overlaid on the Haslam et al. (1982) 408 MHz all-sky image (corresponding to yellow to deep red colours, with associated contours). The yellow and black outlines show the LoTSS-DR1 and LoTSS-DR2 areas respectively and the small black dots show the 3168 LoTSS grid positions. LoTSS-DR1 included 63 pointings, LoTSS-DR2 includes 841 pointings; we have fully observed 1623 pointings including those of DR2, a further 154 pointings are partly observed and observations still need to be conducted for 1391 pointings to complete the survey. |
To process the data they are first ‘staged’ in the LTA; staging is the procedure of copying data from tape to disk and is necessary to make the large archived datasets available for transfer to a compute cluster. The data are then processed with a direction independent (DI) calibration pipeline that is executed on compute facilities at Forschungszentrum Jülich and SURF (see Mechev et al. 2017 and Drabent et al. 2019). These compute clusters are connected to the local LTA sites with sufficiently fast connections to mitigate the difficulties that would be experienced if we were to download these large datasets to external facilities. Unfortunately data transfer issues are not yet fully mitigated as we currently do not process data on a compute cluster local to the Pozna ́n archive and instead we copy these data (6% of LoTSS-DR2) to Forschungszentrum Jülich or SURF for processing.
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The mosaic images, and catalogues derived from them, have significant overlap so when producing the final full-area catalogue we remove duplicate sources by only keeping those in a given mosaic if they are closest to the centre of that particular mosaic. Our final full-area catalogue consists of 4,396,228 radio sources made up of 5,121,366 Gaussian components. The overall sensitivity distribution is shown in Fig. 2 and some example maps from the data release are shown in Fig. 3.
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