Abstract:
A catadioptric telescope is a modified version of a conventional Maksutov-Cassegrain optical telescope. In accordance with the invention, the reflecting surfaces of the primary mirror and the secondary spot mirror are on the second surfaces of the primary mirror and correcting lens, respectively. In further accordance with the invention, two of these telescopes can be joined together to form a binocular telescope array. The array can be easily customized to suit different remote sensing/satellite applications.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to telescopes, and more particularly relates to optical telescopes that are capable of operation in the visible and near-infrared portions of the electromagnetic spectrum. In its most immediate sense, the invention relates to optical telescopes and optical telescope arrays that are suitable for use in spacecraft (such as satellites) and other remote sensing applications. 
     Universities use nanosatellites for research in astronomy, climatology, and earth science. And, use of nanosatellites for both commercial and governmental purposes has been contemplated. For example, a nanosatellite network could be used to monitor the entire length of a pipeline in order to prevent oil or gasoline thefts by detecting persons who bring unauthorized truck-sized vehicles in the pipeline&#39;s vicinity. Alternatively, nanosatellites can be used for e.g. border control (monitoring aircraft that may be transporting drugs, monitoring movements of guerrillas) or prevention of environmental disasters (such as international fires in large extensions of protected forests). 
     An optical telescope intended for use in a spacecraft such as a nanosatellite must meet demanding constraints. It must be small, light, well-balanced, and mechanically robust. It must also be easily customizable; some nanosatellite applications will require a wide field of view, while others will require high resolution images, and still others will require the ability to acquire spectroscopic data or polarimetry data. 
     Therefore, objects of the invention are to provide an optical telescope and an optical telescope array for use in spacecraft and remote sensing applications such as nanosatellites, which telescope and array are small, light, well-balanced, mechanically robust, and easily customizable. 
     Conventional catadioptric optical telescopes of the Maksutov-Cassegrain type have excellent mechanical features; they are small, light, well-balanced, and mechanically robust. However, when used at wavelengths of between 400 and 1000 nm (visible to near-infrared radiation, which are required for nanosatellite applications) they have unacceptable levels of astigmatism, coma, and color spherical aberrations. And customizing a conventional Maksutov-Cassegrain telescope to meet the requirements of different nanosatellite applications would be quite difficult. 
     The invention proceeds from two realizations. The first of these is the realization that if a conventional Maksutov-Cassegrain telescope design is modified to employ second-surface reflection for the primary mirror and the secondary spot mirror (instead of first-surface reflection, which is conventional) the optical aberrations of the original design can be brought within acceptable limits while still preserving its advantageous features insofar as size, weight, balance, and robust character are concerned. 
     The second realization is that by using a binocular array made up of two telescopes having such a modified design, customization can be accomplished easily and inexpensively. This can be done by changing the orientation of the telescopes with respect to each other, changing the coatings on the lenses, and changing the filters that are used. If for example the telescopes are parallel with each other so that their fields of view coincide to be the same at the intended distance from the satellite, a high-resolution image can be obtained. Alternatively, if an image of a large area is desired, the telescopes can be precisely disinclined so that the fields of view at the intended distance are non-overlapping. Acquisition of spectroscopic and polarimetry data can be accomplished by using suitable coatings on the lenses and suitable filters, and it is possible to acquire both image data and spectroscopic or polarimetry data by configuring one telescope to acquire an image while configuring the other to acquire the non-image data desired. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood with reference to the following illustrative and non-limiting drawings, in which: 
         FIG. 1  is a schematic representation of the operation of a conventional catadioptric Maksutov-Cassegrain optical telescope; 
         FIG. 2  is a schematic representation of the operation of a catadioptric optical telescope in accordance with the invention; 
         FIG. 3  is a schematic diagram of a telescope in accordance with a preferred embodiment of the invention; 
         FIG. 4  is a schematic diagram of a binocular telescope array in accordance with the invention; 
         FIG. 5A  is a schematic illustration of the operation of a first preferred embodiment of a binocular telescope array in accordance with the invention; and 
         FIG. 5B  is a schematic illustration of the operation of a first preferred embodiment of a binocular telescope array in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In all the Figures, each element is always identified by the same reference numeral, and corresponding elements are identified using primed reference numerals. The Figures are not to scale; dimensions have been enlarged or reduced for clarity. 
       FIG. 1  shows a schematic representation of how a conventional catadioptric Maksutov-Cassegrain optical telescope operates in the wavelength range of 400 nm to 1000 nm. Incoming rays  2 ,  4 ,  6 , and  8  enter the entrance end  200  of the telescope through its spherical meniscus corrector lens  10 , which is made of optical glass and disperses them radially outwardly. They then strike the spherical reflective surface of the primary mirror  12  (which has an aperture  16  in its center) and are reflected back toward the corrector lens  10 , where they are made incident upon a secondary “spot” mirror  14 . After reflection from the secondary spot mirror  14 , the rays  2 ,  4 ,  6 , and  8  are directed towards a circular aperture  16  that is located in the center of the primary mirror  12 . 
     Each of the mirrors  12  and  14  is formed by a layer of reflective material located on the first surface of the mirror. (The term “first surface” is used because the ray of light is reflected from the first surface it encounters.) As a result, by the time the rays  2 ,  4 ,  6 , and  8  have reflected off the secondary spot mirror  14 , the image formed by those rays suffers from aberrations, which include distortion, astigmatism, coma, and color spherical aberration. Corrector lenses  18  are used to correct for these aberrations, and the rays  2 ,  4 ,  6 , and  8  then pass through a field flattener lens  20  to become incident upon a sensor  22  (such as a CMOS sensor) at the exit end  210  of the telescope. 
       FIG. 2  is a schematic illustration of the operation of a telescope in accordance with the invention. Here, rays  2 ,  4 ,  6 , and  8  are dispersed radially outwardly by a spherical meniscus corrector lens  10 ′ at the entrance end  200 ′ of the telescope and are incident upon the primary mirror  12 ′. The primary mirror  12 ′, is of the Mangin type; it is a negative meniscus lens with a circular aperture  16 ′ in its center. Here, the reflection is from the second surface of the primary mirror  12 ′; the primary mirror  12 ′ is made of optical glass and the rays  2 ,  4 ,  6 , and  8  pass through its first surface and are reflected only when they reach its second surface. The primary mirror  12 ′ thus acts not only as a mirror, but also as a triplet lens (because the light rays are deflected twice, once when they enter the primary mirror  12 ′ and once when they leave it). 
     After reflection from the second surface of the primary mirror  12 ′, the rays  2 ,  4 ,  6 , and  8  are made incident upon a secondary spot mirror  14 ′ that is located on the second surface of the corrector lens  10 ′. As in the case of the primary mirror  12 ′, the secondary spot mirror  14 ′ also functions as a lens because the corrector lens  10 ′ is a spherical meniscus lens. 
     As can be seen by comparing  FIG. 1  and  FIG. 2 , a telescope in accordance with the invention does not require corrector lenses located between the corrector lens  10  or  10 ′ and the primary mirror  12  or  12 ′. It requires only a field flattener lens  20 ′, which is located ahead of the CMOS sensor  22  at the exit end  210 ′ of the telescope. 
       FIG. 3  is a diagram schematically illustrating the dimensions of a preferred embodiment of a telescope in accordance with the invention. In this preferred embodiment: 
     a cylindrical baffle  30  is located in front of the corrector lens  10 ′; 
     another cylindrical baffle  32  is located in front of the primary mirror  12 ′; 
     a conical baffle  34  is located behind the corrector lens  10 ′; and 
     a filter  24  is interposed between the field flattener lens  20 ′ and the detector  22 . 
     Baffles such as  30 ,  32 , and  34  are conventionally used in Maksutov-Cassegrain optical telescopes; the baffles are made of aluminum and they block stray light. As will be discussed below, the filter  24  is selected in accordance with the data to be captured by the detector  22 . 
     The glass used in the preferred embodiment shown in  FIG. 3  is N-BK7, which has a refractive index n=1.5168. The focal length of this preferred embodiment is 1500 mm and its speed is f/10. At an intended observation distance of 700 km (i.e. the distance between a microsatellite in a 700 km orbit and at the earth) the preferred embodiment has a field of view that is 20 km in diameter. 
     In accordance with the invention, a binocular array of catadioptric optical telescopes is constructed. Advantageously, each of the telescopes is the above-discussed preferred embodiment of a telescope in accordance with the invention. As will become evident below, this permits the array to be easily and inexpensively customized for particular applications. 
     An array in accordance with the preferred embodiment is made up of two telescopes as described above. The telescopes  100  and  110  are mounted in a housing  120  ( FIG. 4 ) made of a ceramic having the same thermal coefficient as the glass in the corrector lenses  10 ′ and the primary mirrors  12 ′. The housing  120  has an entrance end  120 A where the corrector lenses  10 ′ are located and an exit end  120 B where the CMOS sensors  22  are located. 
     If a particular application requires a high-definition visual image, the housing  120  can be constructed with the axes of the telescopes  100  and  110  being non-parallel, whereby the telescopes  100  and  110  have the same approximately 20 km field of view at an intended observation distance of 700 km ( FIG. 5A ). At that distance, an array in accordance with the preferred embodiment can produce an image having a resolution of approximately 3 m. Alternatively, if it is more important to have a larger field of view, the housing  120 ′ can be constructed with the axes of the telescopes  100  and  110  being parallel, whereby the array has a field of view that is approximately 40 km wide ( FIG. 5B ). 
     A telescope in accordance with the preferred embodiment can operate in the visual and near-infrared portions of the electromagnetic spectrum, between wavelengths of 400 nm and 1000 nm. To customize a telescope and a telescope array in accordance with the invention, the coatings on the various lenses and the filters  24  are chosen to correspond to optimize the performance of the telescope and array in the portion(s) of the electromagnetic spectrum that is or are of interest. Advantageously, BEAR antireflection coating is used on lens surfaces that transmit light, and protected silver is used for surfaces that reflect light. Typical filters  24  are precision band-pass filters working at different wavelength bands, such as 400 nm-700 nm and 700 nm-1000 nm. Furthermore, an array in accordance with the invention can be customized in such a manner that one of the telescopes is optimized to operate in the visual portion of the electromagnetic spectrum while the other is optimized to operate in the near-infrared so as to collect spectroscopic or polarimetry data. Alternatively, the array can be customized in such a manner that one of the telescopes is optimized to collect spectroscopic data while the other is optimized to collect polarimetric data. In such instances, the two telescopes will usually share the same field of view, so that acquired image data correlates with acquired infrared data and so that acquired data from one portion of the electromagnetic spectrum correlates with acquired data from another portion. 
     Although a preferred embodiment has been described above, the scope of the invention is limited only by the following claims: