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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:
| Service name | Service Description |
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LoTSS-DR2 Gaussian catalog cone search | |
| LoTSS-DR2 Source catalog cone search | |
| LoTSS-DR2 mosaics |
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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The 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 ADQL query form. |
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 Fig. 54 Example of image selected from the Apertif DR1 displayed in ALADIN. Click for a bigger image. |
Access to raw and calibrated visibilities
Raw observations and calibrated visibilities can be discovered in the data collection as shown in Fig. 39, using the ASTRON-VO query form (Fig. 40) or via TAP using TOPCAT.
In addition, the query results via ASTRON VO for continuum images, polarization images/cubes, and HI spectral cubes provides a link to ancillary data (Fig. 42), including raw and calibrated visibilities.
These data products are stored on tape and not directly accessible for download, but until further notice access can be requested via the helpdesk (Fig. 56). The request will be processed on a best effort basis. Within the same request also the calibration tables can be made available (see “Available data products”).
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 Fig. 55 Visibility data access page. |
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 Fig. 56 Astron help center webpage. |
Primary beam images
The continuum images, HI spectral cubes and V images and Q&U cubes are not primary beam corrected. Primary-beam images may be used for mosaicking or for correction of individual images. More information about the primary beam images for Apertif, including their derivation, can be found in the “Overview of primary beam shapes for Apertif”. Specifically, “Released primary beam images” describes the primary beam images initially made available as part of this data release. As for other ancillary data products, primary beam images can be found in the query results following the links in the column Related products (Fig. 57) for continuum images, HI spectral cubes or polarization cubes.
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 Fig. 57 Links to ancillary data. Click for a bigger image. |
Python access
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 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.
An example of a TAP query and image download can be found in the python script below (it has been tested for python 3.7). The result of the query can also be plotted using python.
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In addition, they should include the Apertif DR1 LoTSS DR2 PID:
http://hdl.handle.net/21.12136/B014022C-978B-40F6-96C6-1A3B1F4A3DB0CHECK DOI
and cite the imaging survey data release paper. This is not yet available but should be soon. Until then, it can be referenced as:
AdamsShimwell, ET. AW. K., et al. “First release of Apertif imaging survey data”, in prep.“The LOFAR Two-metre Sky Survey (LoTSS). V. Second data release" accepted for publication in Astronomy & Astrophysic
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