Drupal 7
Modules:
Facet_api 7.x-1.3
Search_api 7.x-1.6
Relation 7.x-1.0-rc4

My site has two nodes, node one provides a description of various astronomical objects and node two are images and their composition details. I’ve built a many to many relationship between the two nodes and there is a node index (search_api) which contains all fields from the two content types with facets enabled on the primary fields.
 
I’ve created the faceted search page for the Astronomical objects which works fine and I now want to extend the Astronomical facets to gallery search page (. e.g. Show all images containing an astronomical object of type galaxy) which is where I have become stuck as I can’t see a way to create an index which joins relationships and nodes.
 
If this approach isn’t possible the only alternative I can think of is building out the relation type with extra fields duplicating the data that I want from either node and building the index on which just feels like a hack. Any suggestions?