Flat Reflector

The Big Ear radio telescope had two large reflectors: to the north was a flat reflector and to the south was a paraboloidal (curved) reflector (look for a hot spot on the photo for this reflector).

The word "mirror" is sometimes used in place of the word "reflector". In many optical telescopes, one or more mirrors reflect the light to a focal point. A concave type of curved mirror will focus the light to a point or to a focal region. A flat mirror will reflect the light to a different direction without focussing. Similarly, radio waves (energy) can be focussed by a concave type of curved metallic structure (reflector or mirror), while a flat metallic structure (reflector or mirror) will send the radio waves (energy) to a different direction without focussing.

Flat Reflector The photo at the left shows the flat reflector plus the control house and the searchlight.
Mesh Surface The photo at the left shows the wire mesh on the flat reflector, plus a portion of the ground plane (with weeds penetrating the aluminum ground plane and climbing the flat reflector).

The flat reflector of the Big Ear radio telescope was 340 feet (104 meters) wide (in the east-west direction) and was 100 feet (30.5 meters) in slant height. The surface of the reflector was wire mesh, which reflected radio waves but allowed light waves (which have a much much smaller wavelength) to go right through the mesh. This reflector tilted through an angular range of 50 degrees, which allowed a range of declinations spanning 100 degrees to be observed (from about -36 degrees to about 64 degrees). The flat reflector was actually made up of 9 sections (or bays), each of which were moved through a small angle before moving the next bay in sequence. Typically, the flat reflector was set to a different declination after about 3 to 4 days of observation (depending on the weather; thunderstorms with lightning caused the data to be unusable). The earth's rotation was used to make it appear that the celestial sky (and the radio sources) were moving past the beam of each of the two feed horns.

The path of the radio source being observed was as follows. Assuming the source was in the southern sky and the flat reflector was set correctly to observe the source when it crossed the meridian (the north-south line in the sky), the radio energy would reflect off the flat reflector, go almost horizontally for about 500 feet (152.4 meters) from north to south across the ground plane (look for a hot spot on the photo for the ground plane), be reflected and focussed by the paraboloidal (curved) reflector back across the ground plane from south to north to the feed horns (look for a hot spot on the photo for the feed horns). The feed horns acted as funnels to collect the radio energy. That energy was converted into a very small electrical current which was amplified more than a million times by a receiver located in an underground room (called the "focus room"). The signal was processed (including being converted into digital data which was analyzed by and stored in a computer).

© 2001-2005 Ohio State University Radio Observatory & North American AstroPhysical Observatory.
Last modified: June 28, 2005