Guorui Ren

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.

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.

Development and alignment for SiC mirror subsystem of a space-borne telescope

A Φ600mm SiC primary mirror subsystem of a space-borne Ritchey-Chretien telescope was designed. The open-back primary mirror was made of pressure-less sintering silicon carbide (SiC), light-weighted at a ratio of approximately 70%. Minimizing the optical surface astigmatism was critical for the mirror, the astigmatism is caused mainly by gravity effects, temperature variation and the bonding process. Three invar flexure bipods were fixed on the baseplate of the telescope at first, and the posture of the primary mirror was adjusted precisely for 0.2mm gap to the bipods. 3M 2216 B/A grey adhesive was then injected into the gap between the mirror and bipod flexure, the curing process was last 72 hours in the room temperature. So the mirror was affected only by curing stress of the adhesive during the assembly process. Structural strength and dynamic stiffness of the mirror subsystem in the thermal- structural coupling state were analyzed with finite element method. Analyzed results show that the optical surface distortion is less than 1/50λ at 632.8nm RMS with three points support and less than 1/200λ RMS with 2°C temperature variation because of the flexure support and compatible support and mirror material, The optical performance test with interferometer show that the optical surface distortion caused by the curing stress of the adhesive is less than 1/50λRMS, the overall optical surface of the primary mirror is less than 1/30λ rms, which met the critical requirements for the primary mirror of the telescope.

Research on lightweight passive deployment mechanism for the secondary mirror in the deployable space telescope

In this paper, a new type of lightweight passive deployment mechanism based on the tape spring and the shape memory alloy is presented for the secondary mirror of a deployable space telescope. In this passive deployment mechanism for the secondary mirror, the high elastic potential energy of the folded tape springs is used as driving force when the support structure is extended, and the high stiffness characteristics of the circular arc cross section of the tape spring can be used to achieve structure self-locking after deployment. Then a deployable space telescope combined with lightweight passive deployable mechanism for the secondary mirror is designed for applying to nanosatellite imaging. Furthermore, a lock-release device is designed to achieve the function of locking the folded structure and releasing on orbit by taking advantage of the phase transformation characteristics of shape memory alloy with temperature changing. Finally, the correction method for the deployment error of secondary mirror is discussed. The temperature of the tape springs is controlled respectively to make a required length change. This can achieve the purpose of adjusting the position of the secondary mirror and improve the deployment accuracy.

Annular-force-based variable curvature mirror combined with multi-point actuation array to improve the surface figure accuracy: a prototype design

In recent years, a novel optical zooming technique has been paid much attention. With the help of optical leveraging effect, it is possible to alter the system focal length dramatically without moving elements involved in by only changing the curvature radius of VCM (variable curvature mirror) slightly. With no doubt, VCM is the key to realize non-moving element optical zooming and it has to provide large enough saggitus variation while still maintaining the high surface figure accuracy to ensure high quality imaging. In our previously published paper, an annular force based VCM has been designed, fabricated and tested. Experiments demonstrate that with the aperture of 100mm and thickness of 2mm, the VCM could generate a large saggitus variation exceeding 30λ (λ=632.8nm). However, the optical quality degrades very fast and this makes such a VCM unsuitable for optical imaging in visible band. Therefore in this manuscript, a multipoint actuation array, which is composed of totally 49 piezoelectric actuators, is embedded into the annular structure to aim to correct the surface figure distortion caused by large saggitus variation. The new structure model has been designed and numerical simulation indicates that the surface figure distortion could be well corrected as long as the degraded surface figure accuracy is better than 1.8λ (λ=632.8nm) (RMS). Based on this, a new prototype VCM is being fabricated and intermediate results are reported here.

Annular force based variable curvature mirror aiming to realize non-moving element optical zooming

Recently, a new kind of optical zooming technique in which no moving elements are involved has been paid much attention. The elimination of moving elements makes optical zooming suitable for applications which has exacting requirements in space, power cost and system stability. The mobile phone and the space-borne camera are two typical examples. The key to realize non-moving elements optical zooming lies in the introduction of variable curvature mirror (VCM) whose radius of curvature could be changed dynamically. When VCM is about to be used to implement optical zoom imaging, two characteristics should be ensured. First, VCM has to provide large enough saggitus variation in order to obtain a big magnification ratio. Second, after the radius of curvature has been changed, the corresponding surface figure accuracy should still be maintained superior to a threshold level to make the high quality imaging possible. In this manuscript, based on the elasticity theory, the physical model of the annular force based variable curvature mirror is established and numerically analyzed. The results demonstrate that when the annular force is applied at the half-the-aperture position, the actuation force is reduced and a smaller actuation force is required to generate the saggitus variation and thus the maintenance of surface figure accuracy becomes easier during the variation of radius of curvature. Besides that, a prototype VCM, whose diameter and thickness are 100mm and 3mm respectively, have been fabricated and the maximum saggitus variation that could be obtained approaches more than 30 wavelengths. At the same time, the degradation of surface figure accuracy is weakly correlated to the curvature radius variation. Keywords: optical zooming; variable curvature mirror; surface figure accuracy; saggitus;

Design and verification of focal plane assembly thermal control system of one space-based astronomy telescope

One space-based astronomy telescope will observe astronomy objects whose brightness should be lower than 23th magnitude. To ensure the telescope performance, very low system noise requirements need extreme low CCD operating temperature (lower than -65°C). Because the satellite will be launched in a low earth orbit, inevitable space external heat fluxes will result in a high radiator sink temperature (higher than -65°C). Only passive measures can’t meet the focal plane cooling specification and active cooling technologies must be utilized. Based on detailed analysis on thermal environment of the telescope and thermal characteristics of focal plane assembly (FPA), active cooling system which is based on thermo-electric cooler (TEC) and heat rejection system (HRS) which is based on flexible heat pipe and radiator have been designed. Power consumption of TECs is dependent on the heat pumped requirements and its hot side temperature. Heat rejection capability of HRS is mainly dependent on the radiator size and temperature. To compromise TEC power consumption and the radiator size requirement, thermal design of FPA must be optimized. Parasitic heat loads on the detector is minimized to reduce the heat pumped demands of TECs and its power consumption. Thermal resistance of heat rejection system is minimized to reject the heat dissipation of TECs from the hot side to the radiator efficiently. The size and surface coating of radiator are optimized to compromise heat reject ion requirements and system constraints. Based on above work, transient thermal analysis of FPA is performed. FPA prototype model has been developed and thermal vacuum/balance test has been accomplished. From the test, temperature of key parts and working parameters of TECs in extreme cases have been acquired. Test results show that CCD can be controlled below -65°C and all parts worked well during the test. All of these verified the thermal design of FPA and some lessons will be presented in this paper.