There was a write up on the myastroshop website about these refractors but the link isn't currently working. Apparently they specified a different centre element in the Prostar version which markedly reduced chromatic aberration compared to the scopes coming direct from Northgroup.(flame shield on
)
They had photo's on the site showing images through both versions of the scope, with no obvious blue halo through the Prostar.
If you send the scope back, perhaps you could ask if you can get the upgraded center element?