I may have rushed into things a bit. Their correctors seem to be designed exclusively for photographic use judging by the schematics on the Tomytec site, as other members already stated. From an ‘astro’ point of view, I imagined the 1.4 extenders would be meant for visual use, but I now believe they are more about wildlife photography after all.
There is however a CN member (Ron Scarboro) that claims to having successfully used a 1.4 teleconverter with a diagonal on a 71fl with good results towards CA. There are also at least 2 other members with fuzzier stories, that claim to have used the extender visually but don’t give any details.
I do intend to get all 3 available correctors for the 90fl and play with them. I will post my results.