Xuewu Fan

All-reflective optical bifocal zooming system without moving elements based on deformable mirror for space camera application

The space camera with variable focal length is capable of capturing images with variable resolution and variable field of view. This is useful for space-borne reconnaissance because the camera can switch between coarse and fine reconnaissance flexibly. However, the traditional optical zooming relies on moving elements which might influence the momentum balance of the satellite platform. Therefore, we present a prototype design using the piezo deformable mirror (PDM) to realize an all-reflective optical bifocal zooming system. By changing the curvature radius of the PDM, the focal length can be switched between 48 and 192 mm without moving elements involved. With the focal length experiencing 4× magnification, the system performance is still approaching diffraction-limited performance, and the maximum stroke of the PDM is also within its physical limits. Experiments demonstrate that the principle is correct and the design is successful.

Prototype design of an all-reflective non-coaxial optical zooming system for space camera application without moving elements based on deformable mirror

Based on optical zooming used to capture images with variable resolution and field of view (FOV), an all-reflective non-coaxial optical zooming system without moving elements is designed for space camera application. In this prototype design, a deformable mirror (DM) whose curvature radius can be changed is introduced. By carefully selecting the optical power of conventional reflective mirrors surrounding the DM, the overall focal length of the imaging system can be greatly changed with slight variation of curvature radius of DM. The focal length of the system can be changed from 48mm to 192mm and the system performance is approaching diffraction-limited with diverse criteria and the maximum stoke of DM is still within its physical limits at the same time. The experimental results prove the effectiveness of DM based optical zooming and will provide a new routine for new type of space camera design in the future.

The development of high precision carbon fiber composite mirror

Due to low density, high stiffness, low thermal expansion coefficient, duplicate molding, etc., carbon fiber reinforced polymer (CFRP) is one of the potential materials of the optical mirror. The process developed for Φ300mm high precision CFRP mirror described in this paper. A placement tool used to improve laying accuracy up to ± 0.1°.A special reinforced cell structure designed to increase rigidity and thermal stability. Optical replication process adopted for surface modification of the carbon fiber composite mirror blank. Finally, surface accuracy RMS of Φ300mm CFRP mirror is 0.22μm, surface roughness Ra is about 2nm, and the thermal stability can achieve 13nm /°C from the test result. The research content is of some reference value in the infrared as well as visible light applications.

A precision carbon fiber hexapod for the installation of an optical telescope

A carbon fiber hexapod was used to install an optical telescope on the payload instrument module of a satellite. In order to reduce distortion stresses, titanium alloy brackets with flexure sections were employed to connect the end fittings of the carbon fiber struts. The design, fabrication and assembling of the carbon fiber hexapod were discussed. The developed hexapod passed the qualified level vibration test. The test results show that the hexapod could reduce vibration response in high frequency.

Carbon-fiber-reinforced polymer variable-curvature mirror used for optical zoom imaging: prototype design and experimental demonstration

Abstract. In recent years, optical zoom imaging without moving elements has received much attention. The key to realizing this technique lies in the design of the variable-curvature mirror (VCM). To obtain enough optical magnification, the VCM should be able to change its radius of curvature over a wide range. In other words, the VCM must be able to provide a large sagittal variation, which requires the mirror material to be robust during curvature variation, require little force to deform, and have high ultimate strength. Carbon-fiber-reinforced polymer (CFRP) satisfies all these requirements and is suitable for fabricating such a VCM. Therefore, in this research, a CFRP prototype VCM has been designed, fabricated, and tested. With a diameter of 100 mm, a thickness of 2 mm, and an initial radius of curvature of 1740 mm, this VCM can provide a maximum 23-μm sagittal variation and a minimum and maximum radius of curvature of 1705 and 1760 mm.

Strain measurement aided assembly for a CFRP hexapod

In order to mount a space optical telescope with long focal length on a spacecraft for an astronomy observation mission, a carbon fiber reinforced plastic (CFRP) hexapod with titanium alloy brackets was designed and fabricated. Each bracket has a pair of heads and each head has two orthogonal flexures as virtual pivots without clearance to provide flexure mounts. Because of no adjustment parts, slight differences among components and roughly assembly would result in misalignment and asymmetrical stress in the hexapod. The stresses and strains of the CFRP hexapod structure under 1G gravity load were analyzed with finite element method. In order to monitor the assembly stress and provide regulating guidance, strain gauges were stuck centrally on the bottom flexures of each bracket. Comparing the measured strains with the computed values, the low stress assembly of the CFRP hexapod has been accomplished successfully.

Optical design of a distributed zoom concentric multiscale meteorological instrument

A meteorological moderate resolution sensor requires large field of view (FOV) and low distortion imaging. At present, a fixed-focus camera combined with a whiskbroom scanning mechanism or a fixed-focus multi-camera combined with pushbroom scanning mechanism is being used. Owing to the fixed focal length of the camera, a large FOV causes the difference of imaging distance and ground imaging angle between the nadir point and the edge of the FOV to be significantly large, resulting in a large difference in the resolution between the nadir point and the edge of the FOV. The study proposes to simultaneously adopt a distributed zoom concentric multiscale system to realize a large FOV, low distortion, and high quality imaging to coordinate with different compensation lenses to achieve a different FOV corresponding to different focal lengths, where the resolution drop between the nadir point and the edge of the FOV is reduced. To ensure the same illumination of the entire FOV, the entire system possesses the same F# with different FOVs exhibiting different entrance pupil diameters. The study analyzes the principle of aberration compensation of a concentric multiscale system when both the FOV and entrance pupil diameter are changed and completes three groups of optical design of different focal lengths with uniform F#. The results indicate that the system has advantages of low distortion and high imaging quality in the entire FOV. Moreover, the resolution drop in the entire FOV is reduced to approximately 50% of the traditional design scheme. To verify the implementability of the system, a set of prototype manufacturing and imaging experiments are conducted to prove that the system has satisfactory implementability, and the imaging quality is also satisfactory.

The cooling control system for focal plane assembly of astronomical satellite camera based on TEC

The dark current noise existing in the CCD of the astronomical observation camera has a serious influence on its working performance, reducing the working temperature of CCD can suppress the influence of dark current effectively. By analyzing the relationship between the CCD chip and the dark current noise, the optimum working temperature of the red band CCD focal plane is identified as -75℃. According to the refrigeration temperature, a cooling control system for focal plane based on a thermoelectric cooler (TEC) was designed. It is required that the system can achieve high precision temperature control for the target. In the cooling control system, the 80C32 microcontroller was used as its systematic core processor. The advanced PID control algorithm is adopted to control the temperature of the top end of TEC. The bottom end of the TEC setting a constant value according to the target temperature used to assist the upper TEC to control the temperature. The experimental results show that the cooling system satisfies the requirements of the focal plane for the astronomical observation camera, it can reach the working temperature of -75℃ and the accuracy of ±2℃.

Design and fabrication of CGH for 600mm diameter SiC primary mirror surface figure testing

Computer-generated hologram (CGH) is an effective way to compensate wavefront aberration in null test of aspheric surfaces and freeform surfaces. Our strategies of CGH design for 600mm diameter SiC primary mirror surface figure testing are presented, and an experiment demonstrating the compensation test results of CGH is reported. We design a CGH including two sections on the same substrate in order to align the CGH to the incident wavefront: main section for compensating wavefront in null test, alignment section for adjusting the relative position between CGH and interferometer. In order to isolate different orders of diffraction, we used power carrier to make different orders of diffraction come to focus at different position along the axis to avoid ghost reflections. We measured the 600mm diameter SiC primary mirror using this CGH, and the surface test result is 0.033λ rms.

Misalignment induced aberration off-axis optical system

Through introducing transformed pupil vector and shifted center of aberration fields vector into the nodal aberration expansions of an axially symmetric optical system, the aberration expression in third order of an off-axis optical system and misaligned off-axis optical system are detailed. Nodal aberration characteristics of misaligned off-axis optical system are revealed only by analyzing the pupil decentration vector, aberration fields shifted vector and the aberration coefficients of the axially symmetric optical system. Actually, it is well demonstrated that the 3rd spherical aberration, 3rd coma, 3rd astigmatism in a misalignment off-axis system are comparable to the aberrations in a misalignment axially symmetric system. Otherwise it will not only induced constant 3rd spherical aberration but also constant 3rd coma and 3rd astigmatism over the field of view, when aligned an off-axis optical system elements with error axial spacing.