It's not really controversial or a secret IMO, a lot of historic ateliers taught students utilizing a camera obscura. Indeed basic education is still largely based around literal copying using grids, even today.
Yeah, it's the specific combination of a camera lucida, a concave mirror, and a long-focal-length lens that I just have a bit of difficulty figuring out. Like, I can put all the components together on the table, but knowing what the distances and angles need to be to get rid of parallax is not obvious to me.
