Is it possible to download Telescopius data base(s) so it can be used offline?
Yes. As soon as you visit telescopius, the database you’re using gets cached so even if you’re offline you can still perform searches. You might not get the target thumbnails but the results should appear.
Bear in mind some things, like target lists or the simulator, are not available offline - yet. I plan to do it but I don’t have an ETA yet.