Apparent and Real Field

The real field (also called the true field) is the actual amount of sky we see. In a telescope it is determined by the focal length of the objective and the diameter of the eyepiece's field stop. In the example above we saw that the apparent field of a 1-inch eyepiece has to be 60° for it to show the entire Moon with a telescope of 120 inches focal length. The real field is approximately the apparent field divided by the magnification. The apparent field is fixed for any given eyepiece.

Apart from those used on "department store" refractors, astronomical eyepieces come in two common barrel sizes — 1¼ and 2 inches outside diameter. The inside diameter of the barrel limits the size of the field stop, thus limiting the maximum real field in a given telescope. If we want an apparent field of, say, 50°, then the longest practical focal length for an eyepiece is 55 mm with a 2-inch barrel and 32 mm with a 1¼-inch. Anything longer will give a tunnel-vision effect — lower power and a constricted apparent field with no change in the real field.

Why would we want a large apparent field? Basically because it's fun. Also, for a given magnification, a larger apparent field means a larger real field, and this is nice, especially if the telescope has no drive and must follow a celestial object by manual nudging. As the apparent field grows, it's like getting closer to the porthole in a spaceship. A 50° field is very nice; 65° is quite wide and spacious; and 80° or more is thrillingly realistic and almost like being there.

Eyepieces usually grow in complexity with large apparent fields. Many traditional designs are not capable of delivering a wide field without major sacrifices in image quality. Furthermore, low f/ratio telescopes put greater demands on eyepieces. A four-element eyepiece with an apparent field of 65° might work well with an f/15 telescope, but would almost certainly give poor images if used with an f/5 objective. To have good images at f/5 with an apparent field of 65° requires a well-designed eyepiece with six or more elements.