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Selecting a data collection allows the user to perform a cone search through a webform (Fig. 40). The result is either a source or Gaussian list, or a table of data products of that given class overlapping a given pointing. The size of the continuum images as well as the cubes extend beyond the 10% primary beam level for cleaning the secondary lobes of bright offset sources. To ensure that the search is done in the area of maximum sensitivity the search is performed on a maxim radius of 0.75 degrees from the center (this represents the average value of where the sensitivity drops). This value can be modified using the Max distance from center. A different output with respect to the default can be customized using More output fields selection button.

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The result is a table in the requested output format in which every row corresponds to a data product (Fig. 41).

In each row there is a column, Product key, which is a link that allows the user to download the fits file of the image of interest. The column titles should generally be self-descriptive. However, the long human-readable description of the content of each column is a tooltip that will appear when hovering over the column name.

The selected target and the position of the individual pointings can be visualized using the Quick plot button at the top of the window of the results of the search query (Fig. 41).

In the column Related products another link connects to a page containing a list of links to additional related data that can be useful to interpret or reanalyze that given product, for which a preview is provided (Fig. 42).

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Fig. 41 Result of an image search query. Click for a bigger image.

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The columns shown in Figure 41 are the most informative for the astronomers (e.g. position, observing frequency, observing date, quality assessment, format etc), please note that more columns are available but not displayed here. The complete set of columns can be visualized via topcat as described below or using More output fields selection button in the search query. Querying the released data is also possible using e.g. TOPCAT using TAP. Via the TAP protocol, it is possible to query the registry in a more flexible way using an enriched SQL syntax called ADQL. An example is given in Fig. 43 : click the link indicated with the red ellipse on the left panel Try ADQL and place your ADQL query on the query form.

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The table names to use in the query form of Fig. 43/2, are summarized in Table 10. The URL for the query is then: https://vo.astron.nl/lotss_dr2/q/{Table name}/form (e.g. https://vo.astron.nl/lotss_dr2/q/src_cone/form}.

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Once completed, the catalogue will be visible in the left panel of TOPCAT (Table List). Click on the new entry as shown by the arrow in Fig. 44. At this point the table browser will open showing the content of the DACHS results (PLACE HOLDER use DR1 in selection). From here any TOPCAT tool can be used for further inspection and analysis of the results. Alternatively the table can be saved in various formats and used locally with other programs (e.g. python scripts etc).

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From the TOPCAT menu bar, select VO and in the drop down, select Table Access Protocol (TAP) as shown by the red arrow in Fig. 45.

This will open the Table Access Query window where the ASTRON VO TAP server is listed. Select it and click on Use Service at the bottom of the window (Fig. 46). Another tab will open showing the Apertif DR1 data collection. Select one, e.g. continuum image, and enter a query command in the bottom panel, an example of which is indicated by the red arrow in Fig. 47. Submit the query using Run Query. This will show the resulting table in the Table list view shown before in Fig. 44. From here, any TOPCAT tool can again be used. As mentioned in the previous section, the query result in vo.astron.nl will display a subset of the columns of the Apertif DR1 table (e.g. position, observing frequency, observing date, quality assessment, format etc). The complete set of columns (e.g. pipeline version, wcs references etc) belonging to each data collection of the Apertif DR1 can be explored using the option described in this section.

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Another useful way to inspect the Apertif DR1 data collection, but also other data collections exposed via the ASTRON-VO, is the ivoa-obscore table. The same selection as before can be used (Fig. 47) but instead of selecting Apertif_dr1 tables, the table ivoa.obscore is to be selected. In this way it is possible to glance over all the data collections exposed via the ASTRON-VO. This might be useful for instance to enable multi-wavelength science exploring LOTSS and Apertif DR1 data or, as mentioned in the case of the ADQL query, to visualize multiple data collections at once.

Having ALADIN open, and once you are satisfied with the resulting table, it can be sent to ALADIN following the instructions of Fig. 48.

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Fig. 48 How to transfer the TOPCAT query results to ALADIN. Click for a bigger image.

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Open ALADIN and on the left panel for SIAP: select Others > SIA2 > astron.nl > The VO @ASTRON SIAP Version 2. Alternatively for TAP select Others > TAP > astron.nl > The VO @ASTRON TAP service (Fig. 49). A pop-up window will open. Click load, and enter a query using the Server selector (Fig. 50) or TAP access with astron.nl/tap (Fig. 51) to select the target of interest for SIAP and TAP respectively.

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Downloading images or cubes in ALADIN is also possible (see Fig. 54). The user will need to click on the url-link in the column access_url of the bottom panel. Then, once the image is loaded, click on the right panel as shown in Fig. 54. From here the usual functionality of ALADIN can be used.

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The ADQL form is another option, and below we provide an example query that also provides information about the calibrators used for each beam. This query is specific to the continuum_images data product but can be adapted to other (beam-based, processed) data products by replacing the table name, e.g., for polarization cubes/images use pol_cubes (see Table 10 for a full list of the available tables).

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