The design uses a 45 flat secondary mirror to view the image, as in a standard Newtonian telescope.
If you want to go further, then a flat mirror is just a special case of a curved mirror.
A flame wavered on the flat mirror of the water.
Above and between the grips was a flat metallic mirror, not quite the size of a man's palm.
The image in a flat mirror has these features:
They were dancing as far as the eye could see, beneath a ceiling that was a flat mirror.
You have put your head into a flat mirror and brought it out as mad as Havelock.
No mirror, flat or otherwise, can change its Image.
He and his men must have come and gone through a flat mirror.
You can imagine how altering the normal flat mirror also could produce effects you suggest.
Wörterbuch für Astronomie