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One of the tables in vo.astron.nl is called ivoa.obscore . This table is a special table which follows the ObsCore standard defined by the IVOA and it contains key observational information describing all the data products (images, cubes, etc) in our VO service. Currently, there is not service to directly query this table, but it can be done using ADQL.
List of vo.astron.nl services
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The data published in the VO can also be accessed using a web browser at https://vo.astron.nl. The web interface can be used to perform simple queries. The power of the VO lies in the fact that several applications exist that can interact with the standards we offer. In the architecture, there is a distinction between services and tables. A service is an entity that takes user input and provides a table as output. The tables themselves are the entities that actually hold the data. Tables can also be queried directly, for instance using ADQL. The colomns of the table are:
Service name | The (human-readable) name of the service |
| Type | The type of this service. Serrvices of type system are not coupled to specific data collections. The services that provide access to data are either of type image or cube. The image cutout services also offer images, but those are cutouts as define above. Finally there are catalogues which are either catalogues of astrophysical objects, or catalogues which tabulate the properties of the raw data files, that can be used to request them for download (in the Apertif DR1 case). |
| Table name | Name of the table in the TAP service, or N/A for services that are not directly querying a table. |
| Service description | Human-readable description of the service |
| Link to col. descs | Link to the service info page in DACHS. For services that relate to a table (i.e. anything but the system services), this will contain three tables listing columns. The first table shows the input fields, which are the fields that can be used to query the service (typically those are RA, DEC and search radius). The next table shows the default output of a query, which is the subset of the table that has a verbosity of 20 or less. The third table shows all the columns present in the data table, including the ones with verbosity levels higher than 20. In the examples we will show how to access those columns. The table columns on the service info page show the column name (i.e. the term to query on using ADQL), the table header (e,g. when querying on DACHS this ends up as the table header for a column), a human-readable description, the unit of the data in this field, and the Unified Content Descriptor (UCD) which is a machine-readable definition of the data in the column. The goal of a UCD is to make sure that clients know how to handle the data in the column. For the average user those are probably not too relevant. |
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This web interface provides a page on which all the collections present in the registry are listlisted
| Service name | Type | Table name | Service description | Service info |
|---|---|---|---|---|
| ADQL Query | system | N/A | This provides a form that can be used to perform ADQL queries and get results in different formats. | info (no columns) |
| LBCS Calibrator Search | catalogue | lbcs.main | Catalog of calibrators from the | info |
| LoLSS - Image Cutout Service | image cutout | lolss.mosaic | info | |
| LoLSS source catalog | catalogue | lolss.source_catalog | Source catalogue of the | info |
| LoTSS-DR1 Cross-Matched Source Catalogue | catalogue | hetdex.main_merged | info | |
| LoTSS-DR1 Image Archive | image | hetdex.hetdex_images | Images of the . | info |
| LoTSS-DR1 Image Cutout Service | image cutout | hetdex.hetdex_images | Cutouts from the images of the . | info |
| LoTSS-DR1 Raw Radio Catalogue Cone Search | catalogue | hetdex.main | info | |
| LoTSS-PDR Image Archive | image | lofartier1.img_main | Images of the . | info |
| LoTSS-PDR Image Cutout Service | image cutout | lofartier1.img_main | Cutouts from the images of the . | info |
| LoTSS-PDR Source Catalogue | catalogue | lofartier1.main | info | |
| M) Apertif DR1 - Continuum images | image | apertif_dr1.continuum_images | Continuum images of the Apertif First Data Release (Apertif DR1). | info |
| M) Apertif DR1 - HI spectral cubes | cube | apertif_dr1.spectral_cubes | HI spectral cubes of the Apertif First Data Release (Apertif DR1). | info |
| M) Apertif DR1 - Polarization images and cubes | image/cube | apertif_dr1.pol_cubes | Stokes V image and Stokes Q and U cubes of the Apertif First Data Release (Apertif DR1). | info |
| MSSS Verification Field Images | image | mvf.msssvf_img_main | info | |
| MSSS Verification Field Sources | catalogue | mvf.main | info | |
| S) Apertif DR1 - Field calibrated visibilities | catalogue | apertif_dr1.calibrated_visibilities | Catalogue of the properties from the calibrated visibilities of the fields from the Apertif First Data Release (Apertif DR1). | info |
| S) Apertif DR1 - Field raw visibilities | catalogue | apertif_dr1.raw_visibilities | Catalogue of the properties from the raw visibilities of the fields from the from the Apertif First Data Release (Apertif DR1). | info |
| S) Apertif DR1 - Flux calibrator raw visibilities | catalogue | apertif_dr1.flux_cal_visibilities | Catalogue of the properties from the calibrated visibilities of the flux calibrators from the Apertif First Data Release (Apertif DR1). | info |
| S) Apertif DR1 - Pol. calibrator raw visibilities | catalogue | apertif_dr1.pol_cal_visibilities | Catalogue of the properties from the calibrated visibilities of the polarisation calibrators from the Apertif First Data Release (Apertif DR1). | info |
| SAURON HI Survey Images | image | sauron.mom0 | info | |
| SAURON HI Survey Velocity Fields | image | sauron.main | info | |
| TGSSADR Image Archive | image | tgssadr.img_main | Images of the | info |
| TGSSADR Image Cutout Service | image cutout | tgssadr.img_main | Cutouts from the images of the | info |
| TGSSADR Source Catalogue | catalogue | tgssadr.main | Catalogue of the radio sources in the . | info |
| The VO @ ASTRON TAP service | system | N/A | (info only): description on how to access the tables using the TAP protocol. | info (no columns) |
| Apertif DR1 beam cubes (table only, no service) | apertif_dr1.beam_cubes | Beam cubes from the Apertif First Data Release (Apertif DR1), no connected service but accessible via TAP and linked from the other Apertif DR1 tables through DataLink. | N/A |
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Selecting a data collection allows the user to perform a cone search through a webform (Fig. 1). The result is either a catalogue (list of sources, Gaussians or visibilities), or a table of data products (images, cubes) of that given class overlapping a given pointing. 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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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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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.
| Info | ||
|---|---|---|
| ||
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. Click for a bigger image. |
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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.
| Info | ||
|---|---|---|
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 Fig. 4 ADQL query form. Click for a bigger image. |
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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).
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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. 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.
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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.
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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.
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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. Fig. 9 shows the HiPS of LoTSS DR1 image, and the list of ASTRON HiPSes and where to find them (please note that for presentational purposes only the ASTRON data is displayed in this screen shot).
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The SIA and TAP services are also available directly from Aladin. First of all, load a HiPS (any HiPS will do; but it may be useful to use the HiPS from the same data release). To find the images, you can select Others → SIA → astron.nl → <data collection name> (purple directory icons). When double-clicking the collection (or clicking t he Load button) the popup screen. On top of the HiPS image, small shapes will appear that each represent the central pointing of each of the images (see Fig. 10; left). Note that the default colour of the shapes is red but the colour in the screenshot has been changed for contrast. When clicking one of the shapes the meta data of the selected object appears in a table below the figure and clicking on the image URL will then initiate a download of the original image, superimposing it on top of the HiPS (Fig. 10; right). Note that the stack on the right of the Aladin screen shows all loaded layers and that the layer in which the image was loaded is showing the percentage of the image loaded.
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Access to catalogues through TAP is handled in a way comparable to images through SIA. Open ALADIN and on the left panel for SIAP: select Others → CS → astron.nl → <data collection name> (purple directory icons). A pop-up window will open. Click load. Note that "in view" is clicked bu default, meaning that the source in view are being loaded by default. Also note that if the Field of View is too large, the loading may fail because of a overflow on the server side (ie the query resulted in too many results). After loading, each of the catalogue entries is superimposed on top of the HiPS image as, again, a small red circle (and again this coulour has been changed in the screen shot for contrast). You can then click the "select" button and select several sources (by drawing a rectangle around them) and the table of properties will appear beneath the HiPS. See Fig. 11 for a view of those steps.
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