Can I browse the science archive from the User Portal web page?
Logging into the User Portal is not required in order to browse the archive web interfaces, which are accessible by browsing to http://archive.eso.org or clicking on the links in the "Archive Services" card on the main User Portal page (if you are already logged in to the User Portal).
Bulk data download through the archive web interfaces requires you to be logged into the User Portal. If at the time you request the data you are not already logged in you will be presented with the User Portal login page. Direct download of individual files through the ESO Archive Science Portal (http://archive.eso.org/scienceportal/home) does not require being logged in.
Browsing and downloading via programmatic access does not require authentication (or, in fact, any interaction with the User Portal). For more details see: http://archive.eso.org/cms/eso-data/programmatic-access.html.