I am a Sony owner and am familiar with the Minolta 500mm mirror.
I am going to post in the Canon thread and maybe the Nikon thread as there are more of you than us Sony owners and they are offering all mounts.
Does anyone have any experience of comment on the 900mm mirrors offered/
They look cheap and I would be interested in any comments on image quality
If they are sharp enough the 900mm would be useful for birding ....john