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The VO data collections can easily be the Apertif DR1 data collection tables can be accessed using TOPCAT, an interactive graphical viewer and editor for tabular data. Since the ASTRON VO is registered to the so-called registry of registries, TOPCAT can find the relevant services in its query menus. In the VO menu of TOPCAT you can pick the SIA query menu item, which will bring up a form as shown in Fig. 5. When selecting one of the data collections the SIA URL will be automatically filel filled in. One can then either type coordinates, or an object name and click the "Resolve" button which uses Simbad to obtain coordinates for that object. When clicking OK, a table of the data products found will appear in the main TOPCAT window. Alternatively, the table can be saved in various formats and used locally with other programs (e.g. python scripts etc).

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In several places, ADQL was mentioned as a way to construct queries to the data and catalogues. ADQL is a query language resembling SQL, so the main concepts are familiar to those acquainted with SQL. The full  reference documentation (i.e. standard definition) contains all possible commands. Here we show some simple examples of table queries. You can perform queries using ADQL on the ADQL form (see Fig. 4), or for instance in TOPCAT (see Fig. 6). Also note that TOPCAT has some built in examples implemented.

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