Spectrometer with double off-axis Schmidt telescope

Abstract: The spectrometer comprises, in combination: a slit (1) for the entry of a light beam; a collimator; a dispersion system (9); focusing means and a detector (13). The collimator comprises at least a first concave spherical mirror (3) and at least a first Schmidt plate (5) in an off-axis arrangement.

Description: The present invention relates to a spectrometer for the spectral analysis of a beam of light radiation, which can be used, for example, on orbiting satellites, on aircrafts, or for other applications. More particularly, the present invention relates to a spectrometer of the catadioptric type, in other words one based on the use of reflecting and refracting elements. Catadioptric spectrometers having a variety of configurations and characterized by the use of mirrors and lenses in various configurations are currently in existence. A spectrometer generally has a slit through which an incoming light beam enters, a collimator, a dispersion system and a detector. US-A-4984888 discloses a spectrometer where a single Schmidt plate is arranged between a single spherical mirror and a dispersing grating. The arrangement is such that the same portion of said plate is crossed twice by the beam. Focuser and collimator are formed by the same spherical mirror and the same Schmidt plate, which are crossed twice in the same area by the beam. EP-A-316802 discloses a spectrometer with a Schmidt plate arranged between the dispersion system and a focusing spherical mirror. The object of the present invention is to provide a spectrometer of the aforesaid type which has a particularly simple structure. Within the ambit of this general object, the object of a particular embodiment of the invention is to provide a spectrometer in which the glass elements can be reduced to a minimum. A further object of the present invention is to provide a wide-aperture spectrometer. The reduction of the glass elements is particularly important in spectrometers for space applications, since glass does not withstand cosmic radiation (gamma rays, protons, neutrons). This radiation causes a gradual loss of transparency of up to 70% for virtually all glass, with consequent loss of efficiency of the spectrometer. Furthermore, the "radiation hardened" glass available on the market has low transparency at the short wavelengths (below 500 nm), with the exception of quartz and crown glass. The above objects are achieved with a spectrometer according to claim 1. According to the invention, the collimator and the focusing means of the spectrometer include each a concave spherical mirror and a Schmidt plate. Schmidt plates, in combination with spherical mirrors, are known elements in the construction of what are called Schmidt telescopes. In one practical embodiment, the plate is positioned between the spherical mirror and the dispersion system. However, the positioning of the Schmidt plate between the two elements forming the dispersion system is not excluded, particularly when the dispersion system comprises, for example, two prisms. In the most suitable and convenient embodiment of the spectrometer according to the invention, at least the following elements are positioned in succession along the path of the beam from the entry point represented by the slit made in one mirror: the first spherical mirror, the first Schmidt plate, the dispersion system, a second Schmidt plate, a second spherical mirror, the detector. Optical components can also be placed in suitable positions to eliminate the field curvature and/or the image curvature of the slit (called the slit curvature) as illustrated in greater detail below. In the absence of correctors of the curvature, the beam entering the device through the slit is reflected by the first spherical mirror toward the first Schmidt plate and emerges from this to pass through the dispersion system. The dispersed beam, emerging from the dispersion system through a second Schmidt plate, is reflected by the second spherical mirror and finally reaches the detector. Characteristically, and in a different way from what is found in conventional applications, the Schmidt plate, although its axis of revolution passes through the center of the spherical mirror, is struck by the beam off-axis. The dispersion system which is used can be any system suitable for this purpose, for example a system formed by one or more dispersion prisms, one or more dispersion gratings, devices known as "grisms", or combinations of these elements. The Schmidt plate can advantageously be made from quartz or radiation-resistant glass of the crown type, as can the dispersion system. Thus all components made from flint glass are eliminated from the spectrometer, providing the advantage of eliminating components subject to loss of transparency as a result of cosmic radiation. The incoming beam which passes through the slit usually originates from a telescope which typically generates a curved image (field curvature), in other words one not lying on a flat surface. This curvature can be of opposite sign to that introduced by the optical components of the spectrometer. If the two curvatures are identical and of opposite sign, they compensate each other and no further corrective arrangements need to be provided. Conversely, it is possible that the two field curvatures (that introduced by the spectrometer and that introduced by the telescope which supplies the beam entering the spectrometer) will be of different sizes and will not compensate each other. In some cases, the telescope may supply a flat image. Depending on the circumstances, it is necessary to provide corrective optical components at suitable positions along the optical path of the spectrometer. These can comprise a curve slit, optical fiber corrective components, corrective elements of the dioptric (lens), catoptric (mirror) or catadioptric (combined) types. These field curvature compensation systems can be associated with the beam entry slit, with the detector, or with both. Systems for correcting what is called the slit curvature can also be provided. Further advantageous characteristics of the spectrometer according to the invention are indicated in the attached claims. The invention will be more clearly understood from the description and the attached drawing, which shows a practical and non-restrictive embodiment of the invention. In the drawing, Fig. 1 shows a schematic view from above of the spectrometer according to the invention, limited to its fundamental components; Fig. 2 shows a view similar to the view of Fig. 1, with the indication of possible elements for compensating the field and slit curvature; Figs. 3 and 4 show a device for correcting the slit curvature and for correcting the field curvature; Figs. 5A and 5B show schematically two configurations of a system for correcting the field curvature of the image originating from the telescope associated with the spectrometer, using optical fibers. With reference initially to Fig. 1, the spectrometer comprises an entry slit 1, whose dimensions are determined according to the optical system which generates the incoming image. In particular, and by way of example, the slit 1 can have dimensions of 12 x 0.040 mm. The slit 1 is associated with a concave spherical mirror 3, whose optical axis is indicated by A. The device also comprises a first Schmidt plate 5, a second Schmidt plate 7 and a dispersion system 9 positioned between the two Schmidt plates 5 and 7. In this example of embodiment, the dispersion system 9 comprises two prisms 9A and 9B. A second concave spherical mirror 11 is placed on the side of the dispersion system 9 opposite the mirror 3, and is associated with a detector 13. As shown schematically in Fig. 1, the beam entering from the slit 1, and represented by the lines F1, strikes the mirror 3 and is reflected (beam F2) toward the Schmidt plate 5. The beam F2 is a collimated beam, while the Schmidt plate 5 corrects the spherical aberration. The collimated beam emerging from the Schmidt plate 5 enters the dispersion system 9 and emerges in the form of its chromatic components, suitably dispersed by the output face of the prism 9B (beam F3), to pass through the plate 7 and reach the reflecting surface of the second spherical mirror 11. From here, the beam is reflected and focused (beam F4) on the detector 13. In the diagram in Fig. 1, the components for the correction of the field and slit curvature have been omitted. The system described in Figs. 1 - 4 is symmetrical, in the sense that it comprises two spherical mirrors 3 and 11, two Schmidt plates 5 and 7 and the dispersion system 9 in an intermediate position between the two plates. The Schmidt plates 5 and 7 are positioned outside the optical axis A common to the two spherical mirrors 3 and 11. However, this is not the only possible configuration of the spectrometer according to the invention. This is because, in the case described, the field of vision (in other words the long part of the slit 1) is in the orthogonal direction with respect to Fig. 1, but the instrument can also operate with the field of vision lying in the plane of Fig. 1. In the diagram in Fig. 1, the components for correcting or compensating the field curvature, introduced by the telescope and by the components of the spectrometer, and the slit curvature have been omitted. These curvatures can be corrected in various ways. Where the correction of the field curvature is concerned, a first possibility consists, as mentioned previously, in using a telescope which supplies a curved image with a field curvature equal and of opposite sign to that introduced by the spectrometer. In this case, the two curvatures compensate each other and the slit 1 is curved with the same radius of curvature as the image produced by the telescope. Figs. 2, 3 and 4 show a different solution for the compensation of the field curvature and the slit curvature. Fig. 2 shows a diagram similar to that of Fig. 1, with the indication of the positions (arrows III and IV) of the field and slit curvature compensation elements, shown on a larger scale in Figs. 3 and 4. More particularly, Fig. 3 shows a system for correcting the slit curvature, comprising a flat mirror 21 in which is made the slit 1 through which the beam F from the telescope passes. The slit 1 extends orthogonally to the plane of the figure. The beam F is reflected by a curved mirror 23 with its convex side facing the slit 1. The radius of curvature of the mirror 23 is very large and in the diagram in Fig. 3 it has been deliberately reduced from the real size to make the curvature of the mirror visible. The beam reflected by the mirror 23 is directed toward the internal reflecting surface 21A of the flat mirror 21 and from here (beam F1) toward the spherical mirror 3. Fig. 4 shows an enlargement of a component for correcting the field curvature, associated with the detector 13. It comprises a wedge-shaped cylindrical lens (optical wedge) 25, shown very schematically and with the wedge angle exaggerated with respect to the real angle, to facilitate graphic representation. The detector 13 is positioned on an inclined plane behind the optical wedge 25. Conversely, it is possible to associate the slit 1 and/or the detector 13 with a system of optical fibers which adapt the slit to the curvature of the spectrometer. This solution is shown schematically in Figs. 5A and 5B, where the optical fibers are indicated by 27. They are interfaced with the slit 1 or with the detector 13, depending on where the corrector device is positioned. This is indicated schematically in Figs. 5A and 5B by the double numbering 1; 13 associated with the component interfaced with the optical fibers 27. The two solutions in Figs. 5A and 5B correspond to situations with field curvatures of opposite sign.

Claims: A spectrometer comprising, in combination: a slit (1) for the entry of a light beam; a collimator including at least a first concave spherical mirror (3) and at least a first Schmidt plate (5); a dispersion system (9); focusing means; a detector (13); characterized in that said focusing means comprise a second concave spherical mirror (11) and at least a second Schmidt plate (7), for focusing the dispersed beam on the detector (9). The spectrometer as claimed in claim 1, characterized in that said first Schmidt plate (5) and said first spherical mirror are positioned in an off-axis configuration. The spectrometer as claimed in claim 1 or 2, characterized in that it comprises means for correcting the field curvature. The spectrometer as claimed in claim 1 or 2 or 3, characterized in that it comprises means for correcting the slit curvature. The spectrometer as claimed in one or more of the preceding claims, characterized in that said first Schmidt plate (5) is positioned between the first spherical mirror (3) and said dispersion system (9). The spectrometer as claimed in one or more of the preceding claims, characterized in that at least the following elements are arranged in succession along the path of the beam entering from said slit (1): the first spherical mirror (3); the first Schmidt plate (5); the dispersion system (9); the second Schmidt plate (7); the second spherical mirror (11); the detector (13). The spectrometer as claimed in one or more of the preceding claims, characterized in that said slit (1) is formed in a flat mirror (21) and in that said flat mirror (21) is associated with a curved mirror (23) which reflects the beam (F) entering from said slit (1) toward said flat mirror (21), which reflects the beam (F1) toward the first spherical mirror (3), said curved mirror (23) correcting the slit curvature. The spectrometer as claimed in claim 7, characterized in that it comprises a wedge-shaped cylindrical lens (25) associated with the detector (13) for correcting the field curvature. The spectrometer as claimed in one or more of the preceding claims, characterized in that said dispersion system comprises one or more dispersion prisms. The spectrometer as claimed in one or more of the preceding claims, characterized in that said dispersion system comprises at least one dispersion grating. The spectrometer as claimed in one or more of the preceding claims, characterized in that said dispersion system comprises at least one grism. The spectrometer as claimed in one or more of claims 1 to 6, characterized in that said slit is curved with a radius of curvature which compensates the curvature of the spectrometer. The spectrometer as claimed in one or more of the preceding claims, characterized in that it comprises a system of optical fibers (27) to adapt the curvature of the image entering from said slit (1). The spectrometer as claimed in claim 13, characterized in that said system of optical fibers is associated with said slit (1). The spectrometer as claimed in claim 13, characterized in that said system of optical fibers is associated with said detector (13).

Inventors: PAOLINETTI RICCARDO, PIERI SILVANO, ROMOLI ANDREA, PAOLINETTI, RICCARDO, PIERI, SILVANO, ROMOLI, ANDREA
Applicants: GALILEO AVIONICA S P A, GALILEO AVIONICA S.P.A.
Application year: 2000
Main classification: G01J3
IPC codes: G01J3, G02B17