This is a large concave plastic mirror i removed from an old advertising display. With a usable diameter of 15 inches and a focal length of 9.75 inches (f 0.65), this should be my best collector for weak diffuse signals. The receiver heads are built in standard plastic 35mm film canisters (o.d. 1-1/4") which fit into the tube snugly. The image formed by the mirror isn't the best but its main use is in just collecting as much light as possible for the detectors. I'm assuming the best detectors to use would be the larger area types. Due to the short focal length of this system, the detector diode used must have a FOV of at least 75 degrees or light will be wasted from the outer parts of the mirror. With the 10 x 10mm detector, this receiver has a theoretical field of view of 2.31 degrees (2 degrees 18 minutes), about 4 full moon widths. But due to the only approximate spherical shape of the mirror, the actual FOV is wider. The theoretical visual magnitude limit for a mirror of 15" is about +14.

Basic parts in the receiver Detail of feed horn with light detector circuit inside

*** Caution - Don't point mirrors this big at the sun! Your feed horn WILL burst into flames almost immediately! Also, if you plan to leave a receiver outside pointed up at the sky, make sure the sun isn't going to drift across its field of view during the day.

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