Go to all the shops in Altador with as many Neopoints as there is inflation. If the inflation is 3.49, go there with 349np. You have to look around, or at least I did, in all the different shops with as many Nps as there is inflation in each shop, and look for the item 'Altadorian Scale', click on it, and it'll give you your next constellation.
Thanks for the hint on inflation. That helped a lot and now I've succeeded in finding the collector.
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