[Al Skierkiewicz]

ebarker's solution is the easiest and most reliable aiming device for this application. If the quadrant mechanism is tall enough or if the light sensors are close enough to the inside corners of the quad, then all four will be illuminated when the mechanism and/or the device to which it is mounted is pointing directly at the sun. It will continue to track the sun all day long, find the sun each morning or stop working when there is no sun or the there is clouds. A simple circuit could be used that would sense the intensity and then calibrated for disabling the system during times when there is not enough direct sunlight to efficiently heat the collector.
The aiming systems that Don is hinting at are known as lead screws and can be obtained for any size or power needs including moving multi ton radio telescopes up to several meters across. I have just finished refurbing a 4.5M dish that uses a lead screw driven by a right angle transmission fed by a 3/4 HP three phase motor. It can move the heated dish more than 45 degrees in a little over a minute. However, as Don discussed, these dishes when mounted in a polar mount can track a geostationary orbiting satellite. The sun however, is not geostationary and changes it's declination (elevation) throughout the year, from solstice to solstice. For large collectors, two moving adjustments are needed, one for azimuth (east to west) and one for decilination (north to south). Of course, since one only needs to heat part of the year, the declination could be set to the average center of the sun's apparent declination during the winter months and would not need nearly the azimuth arc during winter that it would need to track during the summer.
By the way, the surface of the dish you are using is textured to help reduce prime focus to prevent heating of the feed horn. The texture is selected to be large enough to efficiently reflect satellite energy (C & Ku Band) but not photons or infrared. A polished surface would be needed to produce effective heating. Aluminum foil, space blankets or something equally reflective would do the trick at least in the short term. As Don has pointed out, the heat can be extremely high so your system would need to remove the heat to prevent damage to the collector used at the dish focus. There are a number of books on the subject of solar collectors that would contain equations for predicting temperatures and suggestions for collection systems.