an array of fresnel reflectors is presented with an adjustable focus . the focus of the array is controlled by only a few mechanical parts and simple inputs . one preferred embodiment of this device is to collect sunlight and focus it onto a photovoltaic cell or a boiler , for the purpose of collecting solar energy . this solar collector , or heliostat , is easily adjusted so that it always directs the sun &# 39 ; s rays onto the pv cell or boiler .