For atmospheric radiation observations, a portable piece of equipment such as the Microtops sunphotometer (pictured below) is useful.
Image source: Solar Light
However, while I can borrow this equipment, its price is way out of my league.
The 'Sun Target' optics intrigues me, it is designed to indicate when the optics in the instrument are pointing directly at the sun (a white dot appears in the center of the sun target when this is achieved), however, I can not figure out how these optics work.
What optical principles allow the Microtops sun target to work?