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The Hierarchical Progressive Survey (HiPS) protocol defines a way to visualise sky surveys by breaking them up in different hierarchical views which represents different zoom levels. This makes it possible to scroll through a survey in a way that is comparable to using Google Maps (or other online mapping/satellite tools). ASTRONs HiPS collections are all available in Aladin and the most important ones are available through other tools, like ESASky. However users can also have direct access to all our HiPS collections through hips.astron.nl. Typically, HiPS projections are used to visually look through the data from a survey, and combine the coordinates with table-based services to obtain source information or data. The following table shows an overview of the ASTRON HiPS server contains the following data collections.

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Data access through DaCHS interface

The DaCHS system comes with a web view on the services offered. Even though the interface is somewhat rudimentary, it does offer the relevant functionality to query data in the ASTRON VO. The main page, listing all services (as listed in the table above) can be fount at https://vo.astron.nl . Specific services are linked to that (either by clicking on the name of the collection, or the Q icon next to it.

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Anchor dachs-query dachs-query Fig. 1 Query search form for continuum images. Click for a bigger image.

The result is a table in the requested output format in which every row corresponds to a data product (Fig. 2). 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 result is a table (like Fig. 2) which consists of a link to the data product, a thumbnail image  that appears when hovering it (both only if querying for a data product) and relevant metadata describing the results. The Quick Plot button on top of the results page can be used to quicikly plot the numerical results (e.g. making an RA/DEC overview of the images or sources). The Send via SAMP button can be used to send the result set to an application using the SAMP protocol. This essentially means that if you have another application open which supports the SAMP protocol (as do both TOPCAT and Aladin), clicking this button will make the table appear in those applications.

In some cases, auxiliary data is available for the primary image/cube results (e.g. raw visibilities, calibration solutions, etc). Those files are connected to the primary result using a DataLink document. The link in the table is generally shown with the name dlmeta. This is essentially a table with links and predefined descriptions. In the first column the link is shown, together with an (in some cases estimated) file size. The second column consists of a human-readable description and the last column shows the relation between the linked dataset and the primary product, using the vocabulary defined for this, making the result machine-readable. An example of a DataLink document is shown in Fig. 3.

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One special service is the ADQL query service, shown on Fig 4.. This contains a single box that can be used to provide custom queries using the Astronomical Data Query Language (ADQL), which in essence is a dialect of SQL. Some more information on, and examples of ADQL can be found further below.

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Fig. 4 ADQL query form. Click for a bigger image.

Using the DaCHS interface can be useful for users but the advantage of using a standard is that generic tooling can be used to query and access the data. Two very well-known tools are TOPCAT and Aladin. We will give an overview 

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The VO data collections can easily beThe Apertif DR1 data collection tables can be accessed using TOPCAT, an interactive graphical viewer and editor for tabular data. Since the ASTRON VO is regsitered 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 in. One can then either type coordinates, or an object name and click the "Resolve" button which uses Simbad to obtain coordinates for that obiect. When clicking OK, a table of the data products found will appeared 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).

Access to the tables (and the observant reader may realise here that the image data is also tabulated) in the ASTRON VO can be done using the TAP standard. The way to do this is very similar to SIA. First select TAP in the VO menu

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. Then you will see a list of registered TAP services. You can query those by keyword. The most obvious keyword to use to find the ASTRON VO is ASTRON. Click the Find Services and the URL will be filled in the TAP URL box. Clicking the Use Service button brings you to a window that shows a list of all the tables in the data centre and a box in which you can type ADQL. Clicking the Examples button gives you several example queries you can use to find data. In the example in Fig. 6 we are querying for the first 1000 items from tne ivoa.obscore table (which is the table that contains information on all data products in the ASTRON VO).  Running the query will, again, result in a table in the the TOPCAT main window.

TOPCAT and SAMP

If you have found data in the DACHS web overview, and you have TOPCAT open, you can press the Send via SAMP on the results page (see Fig. 2). When you do this, the table will automatically appear in TOPCAT. Click on the new entry as shown by the arrow in Fig. 7. At this point the table browser will open showing the content of the DACHS results). From here any TOPCAT tool can be used for further inspection and analysis of the results.

When making selections in TOPCAT, you can broadcast the resulting table using SAMP. For example Aladin will then read the table and load it in the main interface. To do this, start Aladin and push the button with the antenna indicated in Fig 8. You will see the table appear as one of the layers in Aladin.

Access via Aladin

One of the most popular tools to access data through the VO is Aladin Sky Atlas. There are both a browser and standalone desktop edition. When refering to Aladin in the following, this will apply to the desktop edition. Aladin is a tool that makes it possible to visualise tables and images and perform some analysis on them.

HiPS

Because the ASTRON VO has been registered with a central registry, all HiPS data collections can be visualised through Aladin. However, some of our collections have been "promoted" by the Aladin admins, and they show up at a different place in the menu structure. The promoted collections are shown in the HiPS overview at the top of this page. To bring up a HiPS, either select the Image → Radio entry in the menu (yellow directory icons) where you can see the promoted collections, or the Others → HiPS → astron.nl (purple directory icons). Alternatively you can use the select box to (partially) type the name of the HiPS you want in the background.

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Images

Catalogues

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Fig. 6: TAP query form in TOPCAT (menu on the left), finding ASTRON VO (in the middle) and querying one table (in this case the obscore) using the Full table example query (on the right).

The data can be sent from vo.astron.nl to TOPCAT using one of the two protocols: SAMP or TAP. The two subsections below provide a description on how to access the tabular data using either SAMP(link to Send via SAMP subsection) or TAP(link to VO Table Access Protocol (TAP) subsection).

Send via SAMP

With TOPCAT opened, and once you are satisfied with the output of the cone search in the Astron VO webform, click the grey button “Send via SAMP” as shown on the top of the output list of Figure 3. Authorize the connection and wait until the download is completed.

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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Image Removed Fig. 44 TOPCAT table browser view of the Apertif DR1 data collection tables. Click for a bigger image.

VO Table Acess Protocol (TAP)

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.

The position of the targets can be visualized using the option skyplot in TOPCAT, once the search query has been sent via SAMP or TAP as described in this section.

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Image Removed Fig. 45 TOPCAT menu bar for VO services. Click for a bigger image.

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Image Removed Fig. 46 TOPCAT TAP service query form. Click for a bigger image.

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Image Removed Fig. 47 TOPCAT menu bar for VO services. Click for a bigger image.

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

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Catalogues

The Apertif DR1 VO data collection can also be discovered directly via ALADIN either via simple image access protocol (SIAP) or tabular access protocol (TAP). The examples shown here require the desktop version of ALADIN.

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After loading, the data collection catalogues can be plotted on the main panel by selecting them first on the right panel (e.g. highlighted in blue in Figures 14 and 15) and then by selecting the regions of interest on the bottom panel as shown in Figures 14 and 15. From here the usual functionality of ALADIN can be used.

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Fig. 52 Example of data collection selection via SIAP in ALADIN. Click for a bigger image.

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Fig. 53 Example of data collection selection via TAP in ALADIN. Click for a bigger image.

Images

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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