An Automated Mirror Pointer

We're lucky to be living in a time when automated systems are cheaper and easier to build than ever. That gives us a wealth of options for building an automated mirror pointer. The two that I'm considering are Arduino and Lego Mindstorms, and (for now at least) I'm making Mindstorms my baseline choice. I'm sure Arduino would work, but I'm unfamiliar with both the hardware and software, so there's a learning curve. I used Lego to build a manually adjustable mounting for my 1m pinhole viewer, and it worked great, so I see no reason it can't work for the larger version. Bear in mind that while the total system is much larger (30+ m versus 1m), the actual thing being pointed is much smaller: just a little hand mirror instead of a 1m cardboard tube.

There are several designs I've considered for the mirror pointer. The simplest is to just use a remote control. Lego motor controllers can be remote controlled: I've used this mechanism to make a small home-grown New Year's ball. But I have three problems with this approach: first, I'm not sure the range of the remote control is sufficient; second, manual control will be herky-jerky, not great if we want to make a movie of the eclipse; and third, it's not very interesting! That last one trumps any possible solutions to the other two. We want to automate this sucker.

There's a dead simple way to automate that sadly has a couple of flaws, one fatal. We could set up light detectors just barely outside the image of the Sun, and program the pointer so that when a detector fires (i.e. is illuminated), move the mirror until it is no longer illuminated. One flaw in this design is that while Lego does make light detectors, it doesn't make 30m wires. So hooking the detectors up to the controller isn't trivial. But the really fatal flaw is that we want to track an eclipse, so the image of the Sun won't remain a circle. So tracking the illuminated portion of the image won't keep the center of the eclipse fixed, and (much worse) will lose tracking entirely during totality (when the sliver of Sun flips discontinuously from one edge of the image to the other). And of course there's always the chance the Sun might go behind a cloud, which could also cause this design to lose tracking if the cloud cover lasted long enough (recall that the image moves a full diameter every two minutes, hence any loss of light longer than that would confuse the tracker).

That leaves a third design, more complicated than the other two but with a number of advantages: use astronomical knowledge to track the Sun's location, and use math to calculate the proper mirror angle to keep the image centered on a desired location. The advantages of doing it this way are: first, after an initial setup (stay tuned for how to do this!) the tracker will be fully automated, no intervention required; second, it works during cloud cover or eclipses; and finally, the entire pointer is self-contained with no need for remote control or long wires.

Next week, we'll get into the details of this design...