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I.IntroductionIn the field of Earth observations, off-axis Three Mirror Anastigmatic (TMA) system provides a good solution to meet the requirements of high resolution and wide swath simultaneously. But compared with co-axis TMA systems, it faces more challenges in design, measuring and alignment. Studies of the correlative technologies have been developed deeply by BISME. A complete Optical Demonstration Model (ODM) (comprising mirrors, structure, baffling and a focal plane simulator) of the off-axis TMA telescope with both wide field of view (FOV) and high resolution has been built and tested at BISME. Its main objectives are:
As the representative of a flight model with regard to the technologies used, the ODM has been developed to deal with launch and in-orbit constraints. The achievements can easily apply to project products as soon as possible. II.Technical RequirementsTechnology requirements (such as application goal, orbit, ground sampling distance, pixel size, focal length, FOV, relative aperture) have been discussed time and again. Aiming at demonstration, extremely high performances and huge size will not be sought. The whole time budget, risk management and market demand have been analyzed which leads to the following technical requirements of ODM:
It gives a 51km swath with 2m ground sampling distance (GSD) from an altitude of 500km. The specified performances impose requirements on the optical and mechanical subassembly. The requirements will insure good stability of the telescope under the different environments (harsh mechanical environment, 0-gravity, vacuum and thermal variations) during its life (on the ground, during launch and in orbit). III.Technical SolutionsA.Optical DesignThe optical system has chosen the type of off-axis TMA system which evolves from co-axis TMA system. Compared with co-axis TMA system, it has the merits of wide field, unobscured and high quality. Based on the initial structural parameters from co-axis TMA system, the anticipant off-axis TMA system is obtained by means of off-axis and optimization. Technical difficulties and goals:
The optical schematic of the telescope is shown in Fig. 1. The solutions of the optical system design include:
Main mirror characteristics are given in Tab. 1. And results of optical design are shown in Tab. 2. Table I.MAIN MIRROR CHARACTERISTICS
Table II.RESULTS OF OPTICAL DESIGN
The design result of the optical system is shown MTF is 0.456 in the field of view of 6.2°×1.0°, nearly diffraction limited performance. The relative distortion is 0.01%. The tolerances are not sensitive so that MTF is only reduce 0.01 when mirrors tilt 50μrad or decenter 40μm, the change of the space between mirrors results in little changes on focal length, focal plane position and MTF. Appropriate tolerances reduce the difficulty of mechanical design, fabrication and testing. B.Structural DesignThe difficulties of structural design:
The main solutions of structure system design include:
With light-weight materials and a new structure design, the whole telescope (including mirrors, structure, baffles and a focal plane) weighs less than 60kg. The first frequency of the telescope is higher than 120Hz. Working within 205°C temperature range, the focal plane location remains stable and the maximum reduction of the telescope MTF is less than 0.02 without any focal plane adjustment. C.Mirror MeasuringThe difficulties of off-axis aspheric mirror measuring:
Solutions:
D.Optical AlignmentThe difficulties of optical system alignment:
The whole optical alignment consists of three phases: initial alignment, fine alignment, fastened and fixed, alignment results measurement and check. Initial assembly phase is a primary phase before computer-aided alignment. Obtaining the initial assembly precision depends on optic-mechanical manufacturing precision. The key problem of this phase is how to give appropriate initial assembly precision not only to provide condition for computer-aided alignment but also to reduce manufacture difficulty. As shown in Fig.4, a self-collimation measurement system for off-axis TMA telescopes has been developed. The given computer assistant alignment method included:
Figure 4.Schematic of self-collimation Interference Measure 1)Theodolite,2)Planar Mirror,3)Flat table,4)Camera,6)Interferometer,7)Multi-degrees Adjustable Structure,8)Two-degrees Adjustable Table.  When the optical alignment was almost finished, it was necessary to separate the adjusting mechanisms from the mirror assemblies, and to fasten the joints. At this time, the system state will be easy to vary. We have studied the fasten processes and have found out the know-how to ensure the system stable after separating and fastening. Measurements of specifications of the telescope included modulation transfer function (MTF), focal length, field of view (FOV) and transmittance. The static MTF in the whole FOV of the telescope was better than 0.41. E.FabricationWe have solved two problems in fabrication of ODM:
IV.Environmental TestA.Vibration testTo demonstrate stiffness and stability of telescope structure, sinusoidal vibration test and random vibration test on the ODM prototype have been respectively conducted. Before and after tests, the telescope MTF remained stable. B.Thermal testIn vacuum condition, the MTF of the ODM prototype was consistent and focal plane position was stable when the environmental temperature changed between -172°Cand 232°C. C.Simulation image testSimulation image test on ODM has been conducted. A taken picture is shown in Fig. 6. The imaging results show good sharpness and rich gradation. The test results showed that the ODM prototype possessed high stiffness and thermal stability and could meet the mission requirements. V.Performances Verified in OrbitThe optical system and key technologies of the ODM have been applied in the multispectral camera of ZY-3 Satellite (the first high resolution stereo mapping satellite in China), which was successfully launched on January 9th, 2012. From January 11th, 2012, ZY-3 satellite began to provide multi-spectral image data. A composite color picture is shown in Fig. 7. It is composite of blue, green and red band. Performances of the multispectral camera have been measured in orbit. The technologies of off-axis TMA camera has been verified and will have extensive prospects and greatly promote the development of follow up projects. ReferencesScience Report,
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