Improper Design Improper Design: Mismatched optics which do not allow the full light cone to reach the focal plane unimpeded. To deliver as much light as possible to the focal plane, the primary mirror, diagonal mirror, and all other components must match. Too small a diagonal will produce a very small or non-existent zone of 100% illumination. Too large a diagonal will block some of light from reaching the primary mirror and reduce contrast. A compromise must be reached between 100% light zone size and contrast loss. A telescope designed for planetary and high power use will usually use shorter focal length eyepieces. These eyepieces have a fairly small field lens and don't need a large 100% zone. Only the central area of the field needs to be fully illuminated, because that is where the object of interest is placed for viewing. The focal ratio of the telescope is also fairly high, F 10 or more, so the light cone is very narrow. Therefore a small diagonal mirror can be used which will not reduce the contrast very much. High contrast is very desirable for planetary detail. See the example file "RED.NWT" for a 6" F11 planetary telescope. A telescope designed for deep sky, low and medium power use will most often use longer focal length eyepieces. These eyepieces have a larger field lens, and need a correspondingly larger 100% illuminated zone. The entire field should be illuminated as much as possible, so objects near the edge of the field of view are not dim. This requires care to keep the 75% and 50% zones from being vignetted. A larger diagonal mirror should be used. However, image contrast is still very important for seeing faint detail in extended objects. Using too large a diagonal can produce larger zones at the cost of degrading image quality. See the example file "BLUE.NWT" for a 10" F5.6 deep-sky telescope. If at all possible, the minor axis of the diagonal should be kept under 20% of the diameter of the primary mirror. This will keep the contrast high, which is so important for both planetary detail and detail in faint extended objects. Some sizes of telescope are just hard to make work. A small diameter primary mirror and a short focal ratio make a poor telescope. The short focal ratio wants a larger diagonal and a low profile focuser, but the focuser does not shrink in proportion with the tube size. There is also a limit on how much the clearance can be reduced between the edge of the primary mirror and the tube walls. These two factors cause a large percent of the light cone to be used up reaching from the diagonal to the focal plane. This forces the use of a larger diagonal. See the example file "BLACK.NWT" for a 4-1/4 inch F4 telescope.