Usually, the cooker is used only during this period, so no automatic Sun tracking is incorporated into it. This simplicity makes the design attractive, compared with using a paraboloid. Also, being a single curve, the trough reflector is simpler to construct. However, it suffers from lower efficiency. It is possible to use two parabolic troughs, curved in perpendicular directions, to bring sunlight to a point focus as does a paraboloidal reflector.The incoming light strikes one of the troughs, which sends it toward a line focus. The second trough intercepts the converging light and focuses it to a point. Compared with a single paraboloid, using two partial troughs has important advantages. Each trough is a single curve, which can be made simply by bending a flat sheet of metal. Also, the light that reaches the targeted cooking pot is directed approximately downward, which reduces the danger of damage to the eyes of anyone nearby. On the other hand, there are disadvantages. More mirror material is needed, increasing the cost, and the light is reflected by two surfaces instead of one, which inevitably increases the amount that is lost. The two troughs are held in a fixed orientation relative to each other by being both fixed to a frame. The whole assembly of frame and troughs has to be moved to track the sun as it moves in the sky. Commercially made cookers that use this method are available. In practical applications (like in car-headlights), concave mirrors are of parabolic shape. Spherical reflectors Spherical reflectors operate much like paraboloidal reflectors, such that the axis of symmetry is pointed towards the Sun so that sunlight is concentrated to a focus. However, the focus of a spherical reflector will not be a point focus because it suffers from a phenomenon known as spherical aberration. Some concentrating dishes (such as satellite dishes) that do not require a precise focus opt for a spherical curvature over a paraboloid. If the radius of the rim of spherical reflector is small compared with the radius of curvature of its surface (the radius of the sphere of which the reflector is a part), the reflector approximates a paraboloidal one with focal length equal to half of the radius of curvature. Vacuum tube technology Evacuated tube solar cookers are essentially vacuum sealed between two layers of glass. The vacuum allows the tube to act both as a "super" greenhouse and