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Apparently what is sent through SAMP is the metadata table (test with HST and Herschel, several Mars images). This goes to TOPCAP and also in Aladin. When I click a line from it in Aladin, the preview picture displays in my browser, not in Aladin, and the image is send to the browser for download. Same in TOPCAT: only the postcard_url can be plotted, not the image itself. Is that because of the tar format? Apparently tar files only contain a gz image - this should plot directly. Pointing to the gz instead of the tar would allow for direct display. Plus plotting the postcard in Aladin would help (is that a configuration issue on my side?)