LOTSS DR2 - Where to find the data

User Interfaces

The LOw-Frequency ARray (LOFAR) Two-metre Sky Survey (LoTSS) Data Release 2 (LoTSS DR2) is hosted on the SURF Data Repository, and exposed to the community through the standard Virtual Observatory protocols. This section describes how the LoTSS DR2 data products can be queried and accessed by using different applications.

Introduction

The science-ready data products as described in Available data products are exposed through standard Virtual Observatory (VO) protocols to facilitate their access and exploration. You can find an introduction to the usage of the VO on this page. In the following we will be refering to several VO standards (SIA, DataLink, etc.) which are all discussed in the VO documentation.

The LoTSS DR2 data itself is hosted on the SURF Data Repository (sdr) system. Also the raw observations from which the survey data has been derived are accessible through the Lofar Long-Term Archvie. The main data products (images) are all directly accessible through the VO (either through the Simple Image Access, or via DataLinks to auxiliary data products). The raw data is stored on tape, which means that they need to be put on disk for access. This process is refered to as "staging". The DataLink entries to the files on tape all point to the page of the sdr which lists all the data from a specific pointing. An eample of such a page is shown in Fig. 1. This page shows all data that is directly downloadable (with the status "online" or "external), data that is in the process of being staged (with the status "staging") or data that needs to be staged (with the status "offline").

To obtain more information about a file, you can click on the small arrow next to it. This will show a description (the same as is used in the DataLink), a persistent identifier that can be used to cite the data product in its own respect and a checksum (of the Adler32 format) that one could use to check the integrity of a downloaded product.

We recommend astronomers to use the VO interfaces described below for data discovery. Some of the data is stored on tape. This is indicated in the data description in the DataLink document that is returned by the VO query. To access files on tape, users need to obtain an account which gives them the possibility to "stage" the data from the tape to a disk from which it can be downloaded. Data that is stored on disk is directly accessible through the VO.

In particular, the protocols offered are the Tabular Access Protocol (TAP), Simple Application Messaging Protocol (SAMP) and the Simple Image Access protocol (SIA). TAP and SAMP enables queries to explore the data in a tabular form using tools such as TOPCAT. TOPCAT is an interactive graphical viewer and editor for tabular data, it enables the interactive exploration of large tables performing several types of plotting, statistics, editing and visualization of tables. SIA enables the rapid display of images and cubes through all sky atlas tools such as ALADIN. ALADIN is an interactive sky atlas allowing the user to visualize digitized astronomical images/cubes and superimpose entries from astronomical catalogues or databases.

Some data products are directly accessible, in which case the link on this table will initiate a direct download. However, the larger (and rawer) data products are stored on tape (in which case the text 'on tape' appears in the description). The link will then bring you to the entry of the corresponding pointing in the SURF Data repository (Fig. 1). If data is on tape, the status is either "online", in which case it can be downloaded by pressing the "Download" button, or "offline" in which case it can be requested to be put online by pressing the "Request" button. Please note that you need to be logged in to perform the request. If a request has been correctly performed, the status of the file will change to "staging" until it becomes "online". 

The source and Gaussian cone search forms each return a table with source positions and properties. As before, the long descriptions are available using tool tips. The columns "Mosaic_URL" links to the ancillary data product page of the mosaic from where the Gaussian or source was extracted.

Data Usage Policy

Papers making use of this data should include the following acknowledgement:

This work makes use of data from the Apertif system installed at the Westerbork Synthesis Radio Telescope owned by ASTRON. ASTRON, the Netherlands Institute for Radio Astronomy, is an institute of the Dutch Research Council (“De Nederlandse Organisatie voor Wetenschappelijk Onderzoek, NWO).

In addition, they should include the LoTSS DR2 PID:

CHECK DOI

and cite the imaging survey data release paper. This is not yet available but should be soon. Until then, it can be referenced as:

Shimwell, T.W., et al. “The LOFAR Two-metre Sky Survey (LoTSS). V. Second data release" accepted for publication in Astronomy & Astrophysic