I have read multiple places that starting at 10.5, Image Server can now be installed on separate servers, but when we went to install Server (as an Image Server) on the new VMs, we were not able to attach them to our existing ArcGIS Server site (we are running 10.5.1). We have yet to migrate to ArcGIS Enterprise (we do not have Portal or Data Store). Does this mean that we are unable to install Image Server on separate machines? I'm not finding a lot of materials discussing the difference between implementing a basic ArcGIS Server implementation (no Portal or Data Store) vs ArcGIS Enterprise at 10.5 and higher. We have an immediate need for Image Server, but our Portal implementation is a little ways out (primarily due to the fact that our IT department has yet to set up ADFS - we want to federate with AD). We did find this article on Migrating standalone ArcGIS Server to ArcGIS Enterprise, but again, not a lot of info on the extensions and how they work between the different implementations. Any guidance is much appreciated. Thanks in advance.