And I think they really could be a good option. Unlike the lenses, there would be no chromatic aberration with a reflector, right? Plus, you can set the focal length to whatever you want (taking practicality into account of course).
If you could find two parabolic mirrors with a focal length difference of less than a couple of inches, I suspect you could take the middle of the short focus one and the outside of the longer focus one and stack them concentrically, to get a flatter dish.
Honestly for this use case, just sliced and stacked a single dish would give you mostly-collimated light, right? If you put this behind a mild diffuser you could probably still trick your brain unto thinking it's sunlight coming through the curtains.
And I think they really could be a good option. Unlike the lenses, there would be no chromatic aberration with a reflector, right? Plus, you can set the focal length to whatever you want (taking practicality into account of course).
I might have to try it out...