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This page desribes access to the data in ASTRON's data holdings using the Virtual Observatory standards. Those standards support access to data like catalogs and images. Also cutout services are offere
Data access protocols
The VO consists of several different protocols. In this section we describe the import ant protocols, and provide the acronyms that are generally used in the VO tooling. Most data in vo.astron.nl consist of tables, that can be accessed using the Tabular Access Protocol (TAP). Images are offered using the Simple Image Access Protocol (SIA) which in essence is a table with a field containing a link to an image. A few services offer cutout fuctionality (using SODA). A cutout is basically a service that returns an image (or more generally a data product) that only contains the data inside the cone that has been requested by the user.
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.
HiPS
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. Some collections are promoted in Aladin to be curated by CDS. The concrete meaning of this will be discussed in the Aladin part of this page.
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Apertif first Data Release (DR1) - Uncalibrated continuum flux
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We introduce the most important standards on this page and give some usage exaples taylored to the ASTRON data holdings. If you want more advanced information on how to use the VO standards, you can start with the IVOA website for astronomers and the IVOA wiki page on educational resources. For our data holdings we use the DAta Center Helper Suite (DACHS), which offers a web interface of its own. We will link to several places in DACHS in the documentation but in principle all information showed by DACHS can also be accessed using any VO tools.
Data access protocols
The VO consists of several different protocols. In this section we describe the import ant protocols, and provide the acronyms that are generally used in the VO tooling. Most data in vo.astron.nl consist of tables, that can be accessed using the Tabular Access Protocol (TAP). Images are offered using the Simple Image Access Protocol (SIA) which in essence is a table with a field containing a link to an image. A few services offer cutout fuctionality (using SODA). A cutout is essentially a service that returns an image (or more generally a data product) that only contains the data inside the cone that has been requested by the user.
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.
HiPS
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.
| Collection name (and link) | Description | promoted |
|---|---|---|
| TGSS ADR | Yes | |
| Apertif DR1 | Apertif first Data Release (DR1) - Uncalibrated continuum flux | Yes |
| LoTSS DR1 low | Lofar Two-Metre Sky Survey (LoTSS) first data release (DR1) low resolution (20") | No |
| LoTSS DR1 high | Lofar Two-Metre Sky Survey (LoTSS) first data release (DR1) high resolution (6") | Yes |
The ObsCore table
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
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. |
Done till here
In particular, the protocols offered are the Tabular Access Protocol (TAP), Simple Application Messaging Protocol (SAMP) and the Simple Image Access protocol (SIA). TAP and SAMP enables queries to explore the data in a tabular form using tools such as TOPCAT. TOPCAT is an interactive graphical viewer and editor for tabular data, it enables the interactive exploration of large tables performing several types of plotting, statistics, editing and visualization of tables. SIA enables the rapid display of images and cubes through all sky atlas tools such as ALADIN. ALADIN is an interactive sky atlas allowing the user to visualize digitized astronomical images/cubes and superimpose entries from astronomical catalogues or databases.
The data published in the VO can also be accessed using a web browser at https://vo.astron.nl. This web interface provides a page on which all the collections present in the registry are listed, including the published LoTSS DR2 data sets:list
| Service name | Type | Table |
|---|
| name | Service |
|---|
| description | Service info |
|---|---|
| ADQL Query |
| system | 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 | info | ||
| LoLSS - Image Cutout Service | image cutout | info | ||
| LoLSS source catalog | catalogue | info | ||
| LoTSS-DR1 Cross-Matched Source Catalogue | catalogue | info | ||
| LoTSS-DR1 Image Archive | image | info | ||
| LoTSS-DR1 Image Cutout Service | image cutout | info | ||
| LoTSS-DR1 Raw Radio Catalogue Cone Search | catalogue | info | ||
| LoTSS-PDR Image Archive | image | info | ||
| LoTSS-PDR Image Cutout Service | image cutout | info | ||
| LoTSS-PDR Source Catalogue | catalogue | info | ||
| M) Apertif DR1 - Continuum images | image | info | ||
| M) Apertif DR1 - HI spectral cubes | cube | info | ||
| M) Apertif DR1 - Polarization images and cubes | image/cube | info | ||
| MSSS Verification Field Images | image | info | ||
| MSSS Verification Field Sources | catalogue | info | ||
| S) Apertif DR1 - Field calibrated visibilities | catalogue |
| info | ||||
| S) Apertif DR1 - Field raw visibilities | catalogue | info | ||
| S) Apertif DR1 - Flux calibrator raw visibilities | catalogue | info | ||
| S) Apertif DR1 - Pol. calibrator raw visibilities | catalogue | info | ||
| SAURON HI Survey Images | image | info | ||
| SAURON HI Survey Velocity Fields | image | info | ||
| TGSSADR Image Archive | image | info | ||
| TGSSADR Image Cutout Service | image cutout | info | ||
| TGSSADR Source Catalogue | catalogue | info | ||
| The VO @ ASTRON TAP service | system | info (no columns) |
| Service name | Service Description | Type | Link to column descriptions |
|---|---|---|---|
LoTSS-DR2 Gaussian catalog cone search | catalog | info | |
| LoTSS-DR2 Source catalog cone search | |||
| LoTSS-DR2 mosaics | image |
The ObsCore table
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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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| Info | ||
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 Fig. 54 Example of image selected from the Apertif DR1 displayed in ALADIN. Click for a bigger image. |
ADQL
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Python access
The data collection and the table content can be accessed directly via python using the pyvo tool. Working directly in python the tables and the data products can be simply queried and outputs can be customized according to the user’s needs, without the involvement of TOPCAT or ALADIN.
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