1. Field of the Invention
This invention relates to the general field of optical instrumentation and, in particular, to methods and mechanisms for adjusting the position of the focal point of optics in the optical path of imaging systems.
2. Description of the Prior Art
Optical telescopes and their basic components (such as the objective, the eye-piece, and the telescopic optical-tube assembly) are some of the oldest types of optical instrumentation known in the art. They are fabricated by very diverse manufacturers and are often built according to custom designs. As a result of this diversity of fabrication, some opto-mechanical standards in the telescope industry have not been established or strictly maintained.
As an illustration, a typical eye-piece 10, shown schematically in FIG. 1, consists of an eye-piece lens 12 mounted inside an eye-piece tube 14. Due to the fact that different manufacturers use eye-piece tubes of various lengths and mount the lenses in different positions inside these tubes, the distance 1 between the focal point FE of the lens 12 and the edge 16 of the tube in otherwise optically equivalent eye-pieces differs from manufacturer to manufacturer. Similarly, as shown in FIG. 2, there is no established standard for positioning the lens 18 of an objective 20 within its housing 21 (known as the objective cell), nor are there standard objective cells. As a result, the exact position of the focal point FO of the objective lens 18 with respect to its housing 21 also differs from case to case.
Therefore, when a typical telescope 30 is assembled conventionally by combining an eye-piece 10 with an objective 20, the focal points FO, FE of the objective lens 18 and of an eye-piece lens 12, respectively, are invariably separated from each other by a distance d, as illustrated in FIG. 2. For proper operation of the telescope 30, it is required that the foci FO, FE of the coaxial objective anti eye-piece lenses 18, 12 coincide in space. Thus, to compensate for the distance d separating the foci (i.e., to re-focus the telescope), a conventional focusing mechanism is used to mechanically translate the eye-piece and/or the objective lenses with respect to each other along the axis 32 of the telescope. Such a focusing mechanism 33 is typically attached externally to the optical tube assembly 34 of the telescope.
For the purposes of this invention, the optical tube assembly (OTA) of a telescope is the portion of the telescope housing connecting the objective cell to the eye-piece. Often additional optical elements are located inside the OTA along the optical train prior to the eye-piece. Conventional focusing mechanisms (not shown in FIG. 2) typically operate as mechanically driven telescope tubes (either threaded, sliding, or geared) adapted to change the dimensions of the telescope (its so-called “foot-print”) by either extending or contracting the OTA.
The optical path of the telescope between the objective lens and its focus is often appropriately folded to reduce the overall dimensions of the instrument. This is typically achieved by positioning secondary mirrors between the objective and the eye-piece at an angle designed to reflect the beam away from its initial direction of propagation. As a result, the length of the OTA is reduced at the expense of increasing its lateral dimension. The need for a focusing mechanism, however, exists regardless of whether the imaging system of the telescope is folded or linear. Therefore, when using a conventional focusing mechanism, it is not possible to have a working telescope of fixed dimensions (foot-print), which represents a problem for miniaturization purposes or when the telescope is part of a larger opto-mechanical system that is subject to dimensional constraints.
To the extent that mere mechanical adjustment of the length of any part of the telescopic housing is used for re-focusing the telescope, this limitation is unavoidable. Thus, there remains a need for a focusing mechanism that allows the manufacture of a telescope of fixed dimension, especially in miniaturized implementations.