Optical: systems and elements – Mirror – Plural mirrors or reflecting surfaces
Patent
1995-07-27
1997-08-26
Nguyen, Thong
Optical: systems and elements
Mirror
Plural mirrors or reflecting surfaces
359364, 359366, 359731, G02B 510, G02B 1700
Patent
active
056616104
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a telescope, and in particular to a telescope which is capable of forming part of an image-recording instrument located on board a satellite to carry out the observation of the Earth in infrared and/or visible wavelength ranges, for the purposes of studies or of weather forecasts.
BACKGROUND OF THE INVENTION
Such an instrument is described in the article "SEVIRI, the new imager for Meteosat second generation", by F. Pasternak et al, which appeared in "Proceedings of 1993 IGARSS Symposium, Volume III". This instrument includes an optical detection assembly situated at the location of the focal plane of the telescope, taking images of the Earth in various infrared or visible wavelength channels. This optical assembly is associated with a cooling system intended to keep the detectors at a temperature which is as low as possible; this reduces the detected noise. The telescope is a Ritchey-Chretien telescope including a primary concave mirror receiving an incoming image which is reflected back by a plane scanning mirror, and a convex secondary mirror disposed coaxially with the primary mirror and reflecting back the image reflected by the latter towards the focal plane of the telescope through a central aperture formed in the primary mirror. A disadvantage of this telescope is the relatively great height between the base of the scanning unit and the focal plane of the telescope (at least 1500 mm). This bulkiness reduces the space available for the system for cooling the detectors, and thus limits the possibilities for cooling.
Other types of telescope have likewise been proposed, in particular Gregory telescopes. However, their bulkiness likewise causes difficulties, and they frequently require relatively complex screen structures to reduce the interferences caused by the extraneous light.
As described in the aforementioned publication, the telescope is, in general, associated with a calibration source for the infrared channels. In the calibration phase, this source covers the whole of the entrance aperture of the telescope, and must therefore be of relatively large dimensions; this presents problems of bulkiness and of temperature regulation.
SUMMARY OF THE INVENTION
The object of the present invention is to propose a novel design of telescope permitting the reduction of at least some of the difficulties set forth hereinabove.
Accordingly, the invention proposes a telescope comprising at least one primary concave mirror disposed to receive the image reflected by a plane mirror and to collect the luminous flux and relay optics for enlarging the intermediate image formed by the primary mirror, The relay optics include a secondary mirror and a tertiary mirror, each having a central aperture, and which mirrors are disposed coaxially with the primary mirror. The secondary mirror receives the image reflected by the primary mirror through the central aperture of the tertiary mirror. The tertiary mirror, which is disposed in the vicinity of the focal plane of the primary mirror, receives the image reflected by the secondary mirror and reflects this image through the central aperture of the secondary mirror. The plane mirror, exhibiting a central aperture intended for the passage of the relay optics, is placed ahead of the primary mirror in the vicinity of the focal point of the latter, and can be utilized for scanning the scene observed.
The overall focal length of the telescope is the result of the successive enlargements by the secondary and tertiary mirrors. For one and the same overall focal length, it is then possible to reduce the overall dimensions of the telescope. Moreover, the splitting of the enlargement between the two mirrors reduces the sensitivity of the telescope to defects of alignment.
A significant advantage of this telescope results from the fact that the central aperture in the tertiary mirror can be of reduced cross-section, by reason of its arrangement in the vicinity of the focal plane of the primary mirror. This small aperture of t
REFERENCES:
patent: 3927254 (1975-12-01), Lessman
patent: 4017163 (1977-04-01), Glass
patent: 4101195 (1978-07-01), Frosch
patent: 4240707 (1980-12-01), Wetherell et al.
patent: 4598981 (1986-07-01), Hallam et al.
patent: 5144496 (1992-09-01), Kashima
patent: 5386316 (1995-01-01), Cook
"Seviri, the new imager for Meteosat generation", F. Pasternak et al., Proceedings of 1993 IGARSS Symposium, vol. III.
Applied Optics, vol. 17, n.degree. 17, Sep. 1978, P.N. Robb "Three mirror telescopes design and optimization", pp. 2677-2685.
Matra Marconi Space France S.A.
Nguyen Thong
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