COMPACT STEREOSCOPIC IMAGING DEVICE The present invention relates to a compact stereoscopic imaging device, and in particular to a device adapted to observation satellites for which the mass, overall size and reliability are critical parameters, in particular in relation to performance with regard to lifetime and agility. A stereoscopic imaging system with one or two telescopes is known from document US 2007/109637. Stereoscopy is achieved with a single detector. This involves the presence of a "shutter", in order to light up the detector successively with the same scene: once via the first optical pathway, and on the second occasion via the~second optical pathway. This solution is not adapted to observation satellites. Indeed, the latter require a high degree of reliability over long lifetimes, generally from 5 to 10 years. Therefore, the use of a shutter-type mechanism is generally prohibited. Document US 7 031 059 proposes a telescope with a single head having two optical branches. Stereoscopy is achieved either with two separate focal planes or with a single focal plane of large dimensions. Indeed, for the latter case, the optical axis of the two optical pathways providing stereoscopy is identical, and therefore the spacing between the two detection lines is in the ratio of the stereoscopic angle. Document US 7 119 954 relates to stereoscopy with two telescopes having different heads. The stereoscopy angle can be large, but it requires a device for recombining/registering the telescopes. Figure 1 shows a diagram of a conventional imaging optical instrument 1. This instrument includes a focusing optic 2 placed between an object plane 3 and an image plane 4 on which it forms the image of the plane 3. The object plane is, in the example shown, a strip of land which is scanned, in the object plane 3, in the direction of the arrow 5, and in the image plane according to arrow 6. The optic 2 is at a distance H from the object plane and the optical axis thereof has the reference 7. To achieve a stereoscopic shot, two detection lines or retinas 8, 9 are placed in the image plane 4, perpendicular to the scanning direction 6. The distance between the lines 8 and 9 is marked "e" (Figure 2), and it corresponds, in the object plane, to a breadth of land (breadth measured parallel to the direction 5) equal to B. The length of each of lines 8 and 9 (that are identical to each other) is marked L. In Figure 2, the running directions of the images over the detection lines 8 and 9 respectively are marked by the arrows 6A and 6B1 (or 6B2). It will be noted that, according to the chosen optical architecture, the running direction of the image over the detection line B can be optionally reversed (arrow 6B2 or 6B1) with respect to the running direction of the image over the detection line A. A conventional imaging optical instrument accepts a field that is extended to a greater or lesser extent depending on the required performance. In theory, this allows the integration of a nominal pathway and a stereoscopic pathway with a ratio B/H which is a function of the acceptable field. The ratio B/H is linked with the half-angle of separation of the pathways 6 by the relation: Tan 9 = B/(2H). To offer an advantage for the reconstitution of three-dimensional images, the ratio B/H must be greater than approximately 0.01, namely a stereoscopic angle 0 greater than 0.25° approximately (9 being the half-angle at which the portion of land of breadth B is seen). The deviation e between the detection lines is a function of the instrument focal distance f and it is given by the relation: e = 2.f.tan 9 = f. B/H. For high resolution imaging devices, the focal distances are extremely great, and the same applies to the deviation e between the detection lines. For example, for a focal length of 20 000 mm and a ratio B/H of 0.02, this deviation is 400 mm. More generally, for telescopes, since the field is rotational, the maximum spacing between the detection lines is close to the length thereof, i.e. e~L, which makes the imaging instrument extremely bulky. Current solutions use an architecture with several focal planes or with homothetic retina spacings of the ratio B/H, and the spacing between the two detection lines reaches dimensions close to the lengths of retinas: 200-1000 mm according to the use. The subject matter of the present invention is a stereoscopic imaging device that is compact, but whose compactness does not diminish its stereoscopic quality, and which has a high degree of reliability over long lifetimes. The stereoscopic imaging device in accordance with the invention is a device comprising an image-forming optical device and an optoelectronic detection device placed in the image plane of the optical device, and it is characterized in that the optical device comprises a telescope having two optical pathways with three aspheric mirrors, two of which are common to the beams received and the third is dissociated and belongs to two parts each dedicated to one of the two optical pathways of the telescope, each of these two parts comprising at least one plane mirror placed such that the detection lines of said two pathways are spaced apart at a distance of between 0 and the nominal value Dn such as Dn = f.B/H, f being the nominal focal lengtfroflhe telescope, B the breadth of the object plane area as seen in the image plane and H the distance between the object plane and the imaging device. Thus, the device of the invention is made up of a telescope with single head having two optical pathways ensuring stereoscopy, these two pathways having then-optical axes dissociated so as to end up at two coplanar image planes. The deviation between the detection lines is minimized so as to limit the overall size of the focal plane and the mass thereof. It will be noted that the second detection line can also be redundant to the first for monoscopic shots. The present invention will be better understood upon reading the detailed description of an embodiment, given by way of nonlimiting example and illustrated by the appended drawings wherein: Figures 1 and 2, that have already been described above, are simplified diagrams showing the relations established between the various parameters of a conventional imaging instrument, Figure 3 is a diagram of the image plane of an imaging instrument, showing the respective positions of the two detection lines of an imaging instrument in accordance with the invention, Figures 4-7 are diagrams, of a section along the optical axis, of several embodiments of an imaging instrument in accordance with the invention, Figure 8 is a diagram relating to the multifocal stereoscopy in the image and object planes of an imaging instrument in accordance with the invention, and Figures 9-12 are diagrams, of a section along the optical axis, of other embodiments of an imaging instrument in accordance with the invention in the multifocal version thereof. In the diagram of Figure 3, which is similar to that of Figure 2, shown in broken line are the respective positions of the detection lines 8 and 9 of a prior art imaging device, and in thick continuous line the respective positions of the two detection lines 8 (in the same position as in the prior art) and 10 (brought closer to the position 8 than in the prior art). The distance between the positions 8 and 10 is marked-d and is such that 0