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In some cases, the table describing the data will contain a so-called DataLink document that links the primary data product (often an image or cube) to related data products (e.g. raw visibilities, calibration solutions). The types of data relations that are allowed within the VO are gathered in a vocabulary. This makes it easier to make the results machine-readable.
Another relevant VO standard to mention here is the Simple Application Messaging Protocol (SAMP). This protocol makes it possible for applications to share information. This allows users to for example query the VO with one application, visuaklise the results with another and do analysis with a third one.
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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 net next to it.
Selecting a data collection allows the user to perform a cone search through a webform (Fig. 1). The result is either a source or Gaussian listcatalogue (list of sources, Gaussians or visibilities), or a table of data products (images, cubes) 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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In some cases, the number query parameters can differ, in which case the field description explains what the data is that can be submitted to them.
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 Fig. 1 Query search form for continuum images. |
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.
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Fig. 41 Result of |
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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. | dachs-query | dachs-query |
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 Fig. 42 Links to data products related to the target of interest. The top two items represent the primary data product (ie the one that can be directly downloaded from the table view) and a link to the thumbnail of that data product. The other products are the anciliary data. Click for a bigger image. Note that the number of related products is too long to fit readably in a single screen shot. |
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 anciliary data product page of the mosaic from where the Gaussian or source was extracted (like e.g. Fig 42).
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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Fig. 3 Links to data products related to the target of interest. The top two items represent the primary data product (ie the one that can be directly downloaded from the main result view) and a link to the thumbnail of that data product. The other products are the anciliary data. Click for a bigger image. Note that the number of related products is too long to fit readably in a single screen shot. |
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 belowThe 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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 Fig. 43/2 4 ADQL query form. |
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}.
It is possible to query all the available dataproducts at once by using the table ivoa.obscore and by appending to the ADQL statement “where obs_collection=” it is possible to limit the search to the apertif_dr1 only.
Access via TOPCAT
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
Access via TOPCAT
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 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 appead in the main TOPCAT window.
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Fig. 5: SIAP query form in TOPCAT, querying for data collections with LOFAR in the description, and querying for 15 arc seconds around IC 708 in LoTSS-DR1. |
Querying a table through TAP
The Apertif DR1 data collection tables can be accessed using TOPCAT, an interactive graphical viewer and editor for tabular data. 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).
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