The terrestrial telescope is hundreds of years old. Galileo is said to have invented the first terrestrial telescope in 1609. Today the most elegant and high quality telescopes are made with numerous ground glass lenses usually with more than one type of glass. Of course, these are very expensive.
There is a need for an inexpensive telescope that can be sold to and used by spectators at sporting events, for example, which telescope will provide a good quality image. It is particularly desirable that the exit pupil (explained further herein) be large, say 3 millimeters or more, the eye relief long so that the telescope can be used without removing eyeglasses and that all of the optical parts be made with one optical grade plastic. The larger the exit pupil the more brilliant the image which is especially important at sporting events at night under artificial lighting. These conditions, i.e., lenses of one index of refraction, large exit pupil and long eye relief, all tend to detract from the image quality (freedom from aberrations).
It is an advantage according to this invention to provide a telescope especially useful at sporting events and the like which does not sacrifice image quality yet has the desired large exit pupil, long eye relief and inexpensive plastic lenses. Consider that a commercially available terrestrial 6X telescope with plastic lenses has a one millimeter exit pupil, a nine millimeter eye relief and the following image characteristics:
Object field of view: 200 feet at 1,000 yards
Image quality in object field: 6 arc minutes or better over the field with 10 arc minutes blue halo at all points in field.
A 6X terrestrial telescope made according to the teachings of this application having a four millimeter exit pupil, twenty-one millimeter eye relief, and made with only five lens elements all of optical quality acrylic had the following image characteristics:
Object field of view: 275 feet at 1,000 yards
Image quality in object field: 3 arc minutes or better over full field (no color halo), 1 arc minute over central half of field (the limit of visual resolution).
It is an object of this invention to provide a relatively inexpensive erecting telescope with all lens elements made, for example, by injection molding with but one type of optical grade plastic.
It is a further object to maximize the clearly focused field of view.
It is yet another object to provide long eye relief (the distance from the surface of the eye piece to user's eye for optimum use). It is still another advantage that only four configurations of lens elements are used (one configuration is used twice) thus minimizing the initial production costs.
It is yet another advantage to provide a relatively large exit pupil diameter, say three to four millimeters.
It is a still further object to provide a substantially halo-free image (minimal color aberration).
As with any optical system, it is desirable to eliminate or reduce to an acceptable minimum the various aberrations that arise in such systems including the monochromatic aberrations; namely, spherical aberration (variation in focus with distance from optical axis), coma (variation in magnification with distance from optical axis), astigmatism (different focus for object height and width, i.e., tangential and sagittal images do not coincide), and distortion (square object produces pin cushion image, for example). These aberrations are sometimes referred to as the Siedel aberrations. Additionally, two color aberrations are potentially present including longitudinal (different color light has different focus through the system) and lateral (different color light has different magnification through the system). Eliminating or reducing these various aberrations in an elegant multi-lens optical system, in which the lens elements may have varied indices of refraction, is challenging. Hence, to provide an inexpensive telescope with only four different lens configurations (five lenses total) all made from one type of lens plastic (thus having identical index of refraction), is a unique achievement.
A terrestrial telescope made with spherical glass lenses and with the same constraint (four different lens elements, five total lenses, all with the same refractive index) would at best offer slightly smaller field of view and the image quality of the outer 25% of the field of view would be substantially poorer. By providing elliptically shaped surfaces on the outer faces of the erector lens elements (at essentially no cost penalty with injection molded plastic elements but very costly with glass elements) the image quality almost magically improves so that the image is equally sharp over the full field of view with essentially no visual accommodation by the observer's eye required. Locating the elliptical (aspherical) surface near the intermediate image formed by the object lens element makes the errors in surface figure essentially negligible while the curvature of the inner surface is substantially flattened. The herein described telescope actually exceeds the performance of an analog design made with spherical glass lenses.