Info on "roof" prisms:
https://en.wikipedia.org/wiki/Roof_prism
Note: Olivon is bringing into the US a 2" roof prism 90 degree diagonal.
At this time, it remains to be seen what quality this prism will have, but a correct-image 2" 90 degree diagonal is not a common thing. I can see this being very popular with refractor users who also want to use their scopes for daytime use. Expected to arrive in the next couple months.