• in query form say we use the default but more can be found using the service info and topcat/pyvo.

Introduction

The science-ready data products as described in Available data products are exposed through standard Virtual Observatory protocols to facilitate their access and exploration.

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.

Done till hereThe LoTSS DR2 data is hosted on the SURF Data Repository (sdr) system. We recommend astronomers to use the VO interfaces described below for data discovery. Some of the data is stored on tape. This is indicated in the data description in the DataLink document that is returned by the VO query. To access files on tape, users need to obtain an account which gives them the possibility to "stage" the data from the tape to a disk from which it can be downloaded. Data that is stored on disk is directly accessible through the VO.

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.

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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.

HiPS

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

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