OVSA Expansion Project -- Focus/Brightness Test

Normally the 2.1 m dishes act as diffuse reflectors of sunlight. However, when the 2.1 m antennas get wet, or a layer of early-morning frost melts, the dish can act as a specular reflector and focus several kilowatts of solar energy on the tip of the feed. This can result in burn damage to the feed, and must be avoided by placing a brightness sensor to act as a warning when the dish reflection becomes specular. The images below show the result of some tests that were done on 2012 Oct 09 to evaluate the problem and specifically to see where one might place a sensor. This test shows that the region of enhanced brightness is large when away from the point of best focus, which suggests that a brightness sensor placed near the perimeter of the mounting ring can detect the enhanced brightness and provide a warning signal. However, the feed tip itself will shadow the ring near its center. Because the focal ratio of the dish is f/D = 0.35, if the feed length is L, the radius of the spot will be L/0.35 = 2.86 L at the base of the feed, which may make it too diffuse at that location. Another test with the feed in place may be warranted. Placing the sensor on one of the feed supports may be an alternative.

After wetting the dish with a hose and pointing the dish at the Sun, the focus spot becomes small and bright, as shown in this image. The spot can be used to find the location of best focus, although this will have to be repeated at radio wavelengths when the RF systems are in place. The brightness pattern is shown here at best focus, and the bright spot appears to be about 2.5 inches in diameter. The slight distortion from circular is due to the shadow of the person holding the box. The brightness pattern here several inches behind the spot of sharpest focus. Now an (inverted) image of the shadow of the person holding the box is apparent, but otherwise the region of enhanced brightness is at least 6 inches in diameter.
New Jersey Institute of Technology