The Longest Focal Length for Lenses & Mirrors

[edpell]

What is the longest focal length practical for a lens and/or shaped mirror?

I ask because it would be good if we could put a mirror at GEO and shine sunlight at one spot on the Earth 24 hours per day for energy production. The problem is sunlight is dispersing 1% so from 36,000km it would spread to 360km. Whence the lens or shaped mirror.

 

[Drakkith]

This depends on the desired amount of light you want to focus. At large scales, about 1-2 meters in diameters, mirrors are much easier to produce and transport in sections. The idea is that you would have a number of smaller mirrors, each positioned to focus on the same spot as every other one. So then you would have to find out what size mirrors you want and figure out the best and cheapest way to produce your desired result. A few bigger mirrors, or more smaller mirrors.