ground noise, resulting in a lower antenna noise temperature. Dual reflector shaping: The presence of a second reflecting surface in the signal path allows additional opportunities for tailoring the radiation pattern for maximum performance. For example, the gain of ordinary parabolic antennas is reduced because the radiation of the feed antenna falls off toward the outer parts of the dish, resulting in lower "illumination" of those parts. In "dual reflector shaping" the shape of the secondary reflector is altered to direct more signal power to outer areas of the dish, resulting in more uniform illumination of the primary, to maximize the gain. However, this results in a secondary that is no longer precisely hyperbolic (though it is still very close), so the constant phase property is lost. This phase error, however, can be compensated for by slightly tweaking the shape of the primary mirror. The result is a higher gain, or gain/spillover ratio, at the cost of surfaces that are trickier to fabricate and test. Other dish illumination patterns can also be synthesized, such as patterns with high taper at the dish edge for ultra-low spillover sidelobes, and patterns with a central "hole" to reduce feed shadowing. Another reason for using the Cassegrain design is to increase the focal length of the antenna, to reduce sidelobes, among other advantages. Parabolic reflectors used in dish antennas have a large curvature and short focal length; the focal point is located near the mouth of the dish, to reduce the length of the supports required to hold the feed structure or secondary reflector. The focal ratio (f-number, the ratio of the focal length to the dish diameter) of typical parabolic antennas is 0.25–0.8, compared to 3–8 for parabolic mirrors used in optical systems such as telescopes. In a front-fed antenna, a "flatter" parabolic dish with a long focal length would require an impractically elaborate support structure to hold the feed rigid with respect to the dish. However, the drawback of this small focal ratio is that the antenna is sensitive to small deviations from the focal point: the angular width that it can effectively focus is small. Modern parabolic antennas in radio telescopes and communications satellites often use arrays of feedhorns clustered around the focal point, to create a particular beam pattern. These require the good off-axis focusing characteristics of a large focal ratio, and because the convex secondary reflector of the Cassegrain