Variable-curvature-mirror-based optical zooming used for space-borne targets monitoring: review and progress

At present, the space-borne optical camera installed on satellite platform used for space targets monitoring usually adopts optical system having fixed length and therefore the corresponding field of view cannot be changed either. However, searching for targets of interest needs large field of view but powerful details resolving capability depends on long focal length instead. In order to realize searching within large field of view and high-resolution imaging within small field of view by using one optical system, zooming is an ideal choice. Nowadays, optical zooming is the most popular zooming technique and by introducing zooming group and compensating group at the same time, not only the focal length could be changed but also the focal plane could be stabilized. However, mechanical moving elements based traditional optical zooming has obvious drawbacks, for example relatively low zoom speed, possible disturbance to platform stability and reliability decreasing. Therefore, optical zooming without macroscopic moving elements has been paid much attention and the key lies in the use of variable curvature mirror (VCM). By combining variable curvature mirror and optical leveraging effect, the slight variation of curvature radius of VCM can generate large optical zooming. On the one hand, the fewer the number of VCM used is, the bigger the saggitus variation of each variable curvature mirror will be. On the other hand, large zoom factor needs large saggitus variation but aberrations turn up correspondingly as well. Therefore, how to balance the number of variable curvature mirrors, the reasonable saggitus variation and better aberration compensation within limited volume are the key to system design. In this manuscript, first of all, the current development status of variable curvature mirror based optical zooming are systematically reviewed and our research progress on this technique is also introduced. After that, the design method is described and one typical design example is presented. At the same time the effectiveness of digital restoration in improving the imaging quality is demonstrated as well. Finally, the development tendency of variable curvature mirror based optical zooming is simply discussed.