SkyView V3.4.0: Handling Binary VOTables in SIAP data

Access to the TGSS ADR1 survey has taken a bit of a hit in the past couple of weeks. The TGSS data is served remotely by SkyView: we provide a resampling proxy to the underlying archive. As discussed in our previous blog post, the archive moved from http: to https: addresses which caused one problem. A few days after we fixed that issue, the format of the Simple Image Access (SIA) response from their server changed. We use SIA to find data at remote archives. This change had also broken our ability to serve the data. SkyView V3.4.0 is now available and should be released on August 22. It should fix this problem.

Simple Image Access is the primary protocol through which we access remote services. It’s used for many of our most popular surveys including the SDSS, 2MASS and the TGSS. Basically it allows us to send a query to the remote archives indicating that we’re looking for data in a given region and having the remote archive tell us about the images it has available nearby. We then download the files that seem useful (if we haven’t already done so in some earlier request) and do whatever resampling and mosaicking that seems appropriate.

The initial response to our SIA request is in a format called VOTable. There are several different structures that VOTable can use internally. The TGSS had been using the most common of these, the TABLEDATA/DATA structure. However they apparently switched to using BINARY/STREAM structure. This is often a little more efficient in bytes, though with most data transmission compressed it doesn’t usually make much difference.

Unfortunately earlier versions of SkyView could not support this structure, so we’ve added a couple of classes to enable SkyView to do so. Now it shouldn’t matter which of these two structures is used (though there are still other VOTable structures that we’ll worry about if we find a service the uses them). This reuses code that we’d originally developed for the HEASARC’s Xamin service so adding things in wasn’t too painful. We apologize to SkyView users for these interruptions.

Since this is a full release, once it is in place SkyView-in-a-Jar users should see their tgss.xml.gz files automatically updated (and will not need to manually delete the file as mentioned in the previous post).

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