Microtops sunphotometer sun target optics 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? 
 A: The basic instrument design is described in the manual: http://solarlight.com/wp-content/uploads/2014/05/MTPMAN-RevC.pdf. An optical collimator is basically just a lens in a cylindrical borehole. The alignment of the lenses and boreholes is crucial for the precision of the instrument. The precision angular alignment with the sun can be estimated with e.g. quad-photodiodes. They are basically telling you the tricks they used in the manual. It's not rocket science, just hard enough that they know that the average Joe won't succeed in building the instrument themselves.
If you are thinking about building your own, as someone who has done instrument design for a living, I can tell you that replicating this is not going to be as easy as you think it might. Unless you want to go into the business of competing against this company, which is an excruciatingly bad business idea to begin with, I would suggest that you either buy their product (and you use it to make money with it so that it amortizes) or you get the data you are interested in some other way.
A: To expand on the comment under the answer - in the paper linked "Solar current output as a function of sun elevation: students as toolmakers", the crude method successfully employed is just a simple use of geometric optics method of finding a focal length - in this, of the hand lens used, as illustrated in the simplified diagram below:

Image source
In the case of the example in the paper, the length of the translucent-end tube is made to be slightly less than the focal length, so the sun target would be a small disk rather than a bright point - presumably to prevent the plastic tube from heating up. It appears as a white spot at the end of the translucent tube (figure 2 in the linked article).
Note, this method just employs geometric optics in a simple design.
