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For atmospheric radiation observations, a portable piece of equipment such as the Microtops sunphotometer (pictured below) is useful.

enter image description here

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?

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    $\begingroup$ As a side note: The right way to find out how a product works is by buying one and by taking it apart. Everybody in the industry does that. If you want to build a competing design you first study the industry leaders' technology. The upfront cost for that is small compared to the burn that results from reinventing the wheel. In many cases the first glimpse at a product already tells you that the competition is actually smart and that the price is justified. Once you know that, any further singing in the bathtub is usually discouraged by a smart product manager. $\endgroup$ – CuriousOne Dec 29 '14 at 23:30
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    $\begingroup$ Side note continued: In those rare cases where the only competing product in the market is poorly designed (also something that you can only learn by taking it apart) there is an attack vector, but those are rather rare events... there are plenty of smart people out there! $\endgroup$ – CuriousOne Dec 29 '14 at 23:30
  • $\begingroup$ As mentioned its price is way out of my league, can only borrow one, much less pull one apart. $\endgroup$ – user60063 Dec 29 '14 at 23:32
  • $\begingroup$ Just to clarify, as the question states, I am only asking about the optics principles for that particular component. $\endgroup$ – user60063 Dec 30 '14 at 5:27
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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.

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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:

enter image description here

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.

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