The FORMOSAT-8 Program aims to develop high-resolution optical remote sensing satellites through collaboration with industry, academia, and research teams in Taiwan. In order to enhance the signal-to-noise ratio (SNR) of the images, it is crucial to understand the surface scattering characteristics and implement effective measures to suppress stray light in the optical system. In this study, a combination of bidirectional reflectance distribution function (BRDF) and total integrated scattering (TIS) measurements is utilized to identify materials that exhibit ultra diffusive-matt characteristics. All the selected materials underwent environmental tests to verify their durability for space environment usage. The databases of BRDF and TIS also facilitate the construction of mathematical models representing surface scattering characteristics. All the findings of this study were utilized to arrange the surface blackening methods for various components within the optical system and proved highly advantageous for FORMOSAT-8 satellite program.
This paper describes superior candidate substrate material- aluminum nitride (AlN), for the cost-effective and high-performance
requirements for aerospace mirrors. In fact, high specific stiffness and thermal stability are not only two major
considerations to select ideal material but also machinability, dimensional stability, and cost. Silicon carbide shows the
best figure of merits 12627, but it has extremely low remove rate and expensive raw material properties. Extremely low
expansion ceramics such as Zerodur may be difficult to obtain in large quantities and its figure of merit is 1151. On the
contrary, AlN has outstanding thermal conductivity (~170 W/m°C) and maintain high figure of merit 2222 without
compromise. In our fabrication processes, the AlN substrate can be easily polished to 53±1nm Ra surface texture and
66.4±2nm RMS surface form. Its unique thermal stability, specific stiffness, and good figure of merits, associated with its
easy machinability and low-cost raw materials, makes AlN an ideal selection for superior mirror substrate in the future
space mirror optics.
From 2015, NSPO (National Space Organization) began to develop off-axis Korsch telescope system for next generation earth observation mission. The experimental Korsch telescope system is consist of five mirrors, including: (1) M1: 550mm diameter clear aperture concave primary mirror, (2) M2: Convex secondary mirror, (3) M3: Off-axis concave tertiary mirror with rectangular aperture, (4) FM1 & FM2: Two folding mirrors with rectangular aperture and flat surface. All the experimental mirrors are designed with lightweight structure and made of fused silica. Since early 2016, we collaborated with Taiwanese domestic company and manufactured all the mirrors for the experimental Korsch telescope. Moreover, we not only accomplished the assembly of M1 but also implemented the form error metrology technique to measure the surface error of M1 with high repeatability in 2017. Recently, in order to validate the structural and athermal design of opto-mechanical structures, several vibration tests and thermal experiments have been accomplished in 2018. The experimental data could not only help us to enhance the analysis accuracy of the finite element model but also benefit to our automated opto-mechanical design system. For next developed phase, the aspheric polishing and reflective coating will be achieved till 2021.
Large mirrors with lightweight structure, such as those used in the telescope system of astronomy or spaceborne applications, are susceptible to stress caused by fabrication process. Furthermore, both the residual stress and subsurface damage are critical for the lightweight tooling of glass materials.
In order to figure out the stress distribution on glass substrate, the photoelastic method has been applied to not only the astronomical optics but also the industrial optics of semiconductor equipment. However, there are several influence factors in photoelasticity measurement, including the structure of mirror, fabrication process, and metrology technique. The above factors may affect to the retardation result of photoelasticity measurement and cause the error of stress calculation. Therefore, it is important to clarify the retardation difference contribution to the corresponding influence factors.
In this study, we attempted to use photoelastic instrument to investigate the relationship between the photoelastic effect and stress of several kinds of lightweight mirrors. There are three different lightweight mirrors were adapted to the photoelasticity measurement, including: (1) GSO 12” Mirror with 12 inches diameter made by fused silica, (2) Primary Mirror (M1-B) of Formosat-5 with 450 mm diameter made by ZERODUR® , (3) Primary Mirror (M1) of Mircrosat with 380 mm diameter made by CLEARCERAM®-Z. The experimental results depict some obvious retardation differences caused by the certain influence factors and the details will be discussed below.
In 2015, NSPO (National Space Organization) began to develop the sub-meter resolution optical remote sensing instrument of the next generation optical remote sensing satellite which follow-on to FORMOSAT-5. The multi-spectral strip filter has been developed by NSPO in collaboration with MORRISON Opto-Electronics (MOE) Ltd, meeting the emerging demands of the new TDI CMOS image sensor of the Korsch type optical remote sensing instrument for next satellite mission. This paper represents the technology to deposit the multi-spectral band-pass strip filters on single synthetic silica substrate. The optical multi strip filter is installed in front of TDI CMOS image sensor to capture multi-spectral images of the earth surface. The optical multi strip filter composed of five band-pass filters on single substrate, including three bands in visible bands (400nm to 700nm) called VIS, one panchromatic band including whole visible spectrum and one band in near infrared (NIR). MORRISON Opto-Electronics (MOE) Ltd is responsible to integrate micro-structuring process base on lithography and ion beam-assisted deposition (IAD). These made multi spectral optical thin film coating in a small area with high dimension accuracy deposited possible on the substrate and achieve the robust process of patterning photoresist and removing the photoresist. By repeating the process five times, we have deposited five kinds of band-pass strip filters on single substrate.
This article presents the opto-mechanical design of a primary mirror assembly of a ground-based telescope with optimization algorithm. The prototype of ground-based telescope – GSO RC16 with 16 inches diameter blank primary mirror had been manufactured in 2016. However, a telescope with a blank primary mirror is too heavy to carry on for the stargazer. Besides, deformations caused by temperature difference and gravity will do significant effect to the large aperture mirrors with high optical performance requirements. In order to reduce the weight and maintain the stiffness simultaneously, the lightweight design and mounting interface design are critical and important. There are four types of system architectures in this project, including (1) two types of lightweight mirror designs - honeycomb type segments and sector type segments; (2) two types of mounting interface designs - retainer type support and CFRP type support. The optimization results showed that (1) the lightweight ratio of the primary mirrors are greater than 70%; and (2) the PV value of the mirrors supported by optimal mounting interfaces with gravity effect as a tilt of about 45 degrees and ±20°C temperature difference effectively less than 1/4 λ.
In 2015, NSPO (National Space Organization) began to develop the sub-meter resolution optical remote sensing instrument of the next generation optical remote sensing satellite which follow-on to FORMOSAT-5. Upgraded from the Ritchey–Chrétien Cassegrain telescope optical system of FORMOSAT-5, the experimental optical system of the advanced optical remote sensing instrument was enhanced to an off-axis Korsch telescope optical system which consists of five mirrors. It contains: (1) M1: 550mm diameter aperture primary mirror, (2) M2: secondary mirror, (3) M3: off-axis tertiary mirror, (4) FM1 and FM2: two folding flat mirrors, for purpose of limiting the overall volume, reducing the mass, and providing a long focal length and excellent optical performance. By the end of 2015, we implemented several important techniques including optical system design, opto-mechanical design, FEM and multi-physics analysis and optimization system in order to do a preliminary study and begin to develop and design these large-size lightweight aspheric mirrors and flat mirrors. The lightweight mirror design and opto-mechanical interface design were completed in August 2016. We then manufactured and polished these experimental model mirrors in Taiwan; all five mirrors ware completed as spherical surfaces by the end of 2016. Aspheric figuring, assembling tests and optical alignment verification of these mirrors will be done with a Korsch telescope experimental structure model in 2018.
This paper presents the finite element and wavefront error analysis with reverse engineering of the primary mirror of a small space telescope experimental model. The experimental space telescope with 280mm diameter primary mirror has been assembled and aligned in 2011, but the measured system optical performance and wavefront error did not achieve the goal. In order to find out the root causes, static structure finite element analysis (FEA) has been applied to analyze the structure model of the primary mirror assembly. Several assuming effects which may cause deformation of the primary mirror have been proposed, such as gravity effect, flexures bonding effect, thermal expansion effect, etc. According to each assuming effect, we establish a corresponding model and boundary condition setup, and the numerical model will be analyzed by finite element method (FEM) software and opto-mechanical analysis software to obtain numerical wavefront error and Zernike polynomials. Now new assumption of the flexures bonding effect is proposed, and we adopt reverse engineering to verify this effect. Finally, the numerically synthetic system wavefront error will be compared with measured system wavefront error of the telescope. By analyzing and realizing these deformation effects of the primary mirror, the opto-mechanical design and telescope assembly workmanship will be refined, and improve the telescope optical performance.
Bar chart patterns projects by collimator was adopted to measure contrast transfer function (CTF)values at Nyquist frequency before assembly of imaging sensor with telescope for earth-observing pushbroom imager. A relay imaging probe consisting of optical objective and 2D imaging sensor was builded to image these projected pattern and estimate the image quality of optical system before alignment of linear imaging sensor. By riding on a hexapod stage and measuring at a series focus position at several field angles, this probe provides a reference map for alignment of imaging sensor and image quality assessment. Certainly, testing result can be used to anticipate result of focusing alignment.
FORMOSAT-5 consists of a spacecraft bus and an electro-optical payload. The payload is an f/8 Cassegrain type telescope with 3.6-m effective focal length. The spacecraft has a ground sampling distance of 2-m for panchromatic and 4-m for multispectral bands, with a 24-km swath width. FORMOSAT-5 is the first space program that National Space Organization (NSPO) takes full responsibility for the complete satellite and payload system engineering. The optical system assembly (OSA) has been successfully aligned and is now undergoing final performance verification tests at system level. To help create this unique instrument, NSPO has developed the computer aided alignment method assisted with the mechanical ground support equipment to carry out the assembly, alignment, and verification of the complex systems. This method offers an integrated capability for interferometric alignment and characterization of the large instrument. A detail OSA integration and verification steps, including primary mirror, secondary mirror, corrector lens and baffles alignment are presented. This paper describes the overall capability of this method and uses decomposed Zernike polynomials from the alignment and characterization of the OSA to verify the reduction of the wavefront errors and misalignments. It further demonstrates the successful completion of the instrument and satisfaction with the main system requirements.
The paper is aimed at obtaining the deformation results and optical aberration configurations of a spaceborne mirror made of ZERODUR® glass on a main plate with three isostatic mounts for a space Cassegrain telescope. On the rear side of the main plate four screws will be locked to fix the focal plane assembly. The locking modes for the four screws will be simulated as push and pull motions in the Z axis for simplification. The finite element analysis and Zernike polynomial fitting are applied to the whole integrated optomechanical analysis process. Under the analysis, three isostatic mounts are bonded to the neutral plane of the mirror. The deformation results and optical aberration configurations under six types of push and pull motions as well as self-weight loading have been obtained. In addition, the comparison between the results under push and pull motions with 0.01 mm and 0.1 mm displacements in Z axis will be attained.
For a currently developing multispectral space Cassegrain telescope, the primary mirror with 450 mm clear aperture is
made of Zerodur and lightweighted at a ratio about 50 % to meet both thermal and mass requirement. For this mirror, it
is critical to reduce the astigmatism caused from the gravity effect, bonding process and the deformation from the
mounting to the main structure of the telescope (main plate). In this article, the primary mirror alignment, MGSE,
assembly process and the optical performance test for the primary mirror assembly are presented. The mechanical shim
is the interface between the iso-static mount and main plate. It is used to compensate the manufacture errors of
components and differences of local co-planarity errors to prevent the stress while iso-static mount (ISM) is screwed to
main plate.
After primary mirror assembly, an optical performance test method called bench test with novel algorithm is used to
analyze the astigmatism caused from the gravity effect and the deformation from the mounting or supporter. In an effort
to achieve the requirement for the tolerance in primary mirror assembly, the astigmatism caused from the gravity and
deformation by the mounting force could be less than P-V 0.02λ at 633 nm. The consequence of these demonstrations indicates that the designed mechanical ground supported equipment (MGSE) for the alignment and assembly processes meet the critical requirements for primary mirror assembly of the telescope.
The speed of light is an important physical parameter. Currently it is a common belief of the constance of the speed of light regardless of the relative velocity between the source and the observer. Because the speed of light is very fast, if the relative velocity is small compared with the speed of light, it is difficult to detect the effect of the relative velocity on the measurement of the speed of light. In this paper we present a method of comparing the speeds of starlight and the light emitting from a terrestrial source. We use a telescope to collect the light from the star having significant relative velocity with respect to the earth, e.g. Capella. Then we modulate the starlight and the light emitted from the local source into pulses i.e. these pulses leave the modulator simultaneously. After travelling 4.2 km, these pulses are detected by a receiver. If the starlight and the terrestrial light have the same speed, then these pulses must arrive at the receiver at the same time. Our results show that the arrival times of the pulses of starlight are different from that of the local light. For example, the Capella is leaving away from the earth. The Capella pulses arrive later than the local light pulses. It indicates that the speed of Capella starlight is slower than the common believed value, c. The presented method uses one clock and one stick, so the clock synchronization problem and any physical unit transformation can be avoided.
This paper is investigated to construct the self-developed fundamental capability which can apply to the opto-mechanical design and analysis in space telescope. The mounting mechanics is found to be a key issue to dramatically affect the optical performance in optical systems. Design and experiments are conducted to study the relationship between stresses and wavefront errors of the opto-mechanical systems by use of the interferometric measurement. Zernike polynomials and wavefront fitting is performed to study the mirror mounting mechanism. It is reported that mirror mounting stresses will severely affect the wavefront errors of the optical systems. Moreover, this investigation indicates that the external stresses will increase the wavefront errors, especially in the terms of astigmatism and trefoil. In addition, the transverse shear stress is more sensitive to degrade the optical performance in this opto-mechanical system design.
Optical parameters such as radius of curvature (RoC), direction of the optical axis, offset of the apex relative to the outer
diameter, et al. of the primary mirror of a Cassegrain telescope by a coordinate measuring machine (CMM) is presented.
These parameters are measured by a novel technique developed by the authors. RoC, tilt, and wedge of a lens can also be
measured by the technique. Geometric parameters, such as diameters, central obscuration diameter, and perpendicularity
of mirror edge, the mirror, et al. can be measured taking the advantage of the geometric measurement function. The
optical and geometric parameters are measured by this method on a set of primary and secondary mirrors, and four
corrector lenses of a Cassegrain telescope.
This paper presents the analysis of static structure and wavefront error (WFE) of a telescope experimental model’s primary mirror. The experimental telescope with 280mm diameter primary mirror had been assembled and aligned in 2011. The WFE result was not perfect. In order to find out the root cause of the WFE, the static structure analysis had been applied to the structure model of the telescope assembly. Some assumed effects which may cause deformation of the primary mirror had been proposed, such as gravity effect, iso-static mount (ISM) bonding effect, thermal expansion effect, etc. According to each assumed effect, we established a corresponding model and boundary setup. The numerical model was also analyzed by static structure analysis software and opto-mechanical analysis software to obtain numerical WFE and Zernike polynomials. The numerically synthetic optical system WFE was compared with measured system WFE of the telescope. According to the results of these deformation effects of the primary mirror, we could get the root cause of the system WFE. In the next generation of telescope, the opto-mechanical design and alignment process would be refined, and applied to the new optical system.
This work presents a developing design of adjustable supporting mechanism of secondary mirror for airborne Cassegrain
optical remote sensing instruments. Several datum surfaces and shims have been designed in this mechanism, shims and
one of structural parts can be released and are properly grinded to the desired thickness. The proposed mechanism has
been verified by an experimental model. The reassembled accuracy is 5 arcsecond and ±3 μm for orientation and position
respectively. Furthermore, their adjustable accuracy can achieve 5 arcsecond and ±3 μm respectively, depending on
grinding and measurement precision. The accuracy can drive the optical system performance to design specification.
After verification, we confirm the proposed design of adjustable supporting mechanism can be applied to airborne
Cassegrain optical systems, and has good potential in spaceborne applications.
Chip sorter is one of packaging facilities in chip manufactory. Defects will occur for a few of chips during
manufacturing processes. If the size of chip defects is larger than a criterion of impacting chip quality, these flawed
chips have to be detected and removed. Defects inspection system is usually developed with frame CCD imagers.
There're some drawbacks for this system, such as mechanism of pause type for image acquisition, complicated
acquisition control, easy damage for moving components, etc. And acquired images per chip have to be processed in
radiometry and geometry and then pieced together before inspection. These processes impact the accuracy and
efficiency of defects inspection. So approaches of image acquisition system and its opto-mechanical module will be
critical for inspection system.
In this article, design and characterization of a new image acquisition system and its opto-mechanical module are
presented. Defects with size of greater than 15μm have to be inspected. Inspection performance shall be greater than
0.6 m/sec. Thus image acquisition system shall have the characteristics of having (1) the resolution of 5μm and 10μm
for optical lens and linear CCD imager respectively; (2) the lens magnification of 2; (3) the line rate of greater than 120
kHz for imager output. The design of structure and outlines for new system and module are also described in this work.
Proposed system has advantages of such as transporting chips in constant speed to acquire images, using one image only
per chip for inspection, no image-mosaic process, simplifying the control of image acquisition. And the inspection
efficiency and accuracy will be substantially improved.
In order to give consideration to both resolution and swath of airborne remote sensing instrument, pan-sharpening
method was used to get high resolution pan-sharpened image. This research provides a description of the optomechanical
system design and assembly of airborne imager, VCDi-660 (Vegetation and Change Detection imager).
Opto-mechanical components of VCDi-660 consist of six optical lenses, focal plane adjustment, bore sight alignment
adjustment, filter-exchanging mechanical device and circuit protected housing. An improved adjustment device for bore
sight alignment of the multiple-band camera in this Taiwanese airborne imaging sensor, VCDi-660, has been presented.
Target of this mechanical device is to provide a translational and rotational movement in the focal plane of VCDi-660.
Sensor can be aligned into the best focal position, thus the central point deviations for individual camera module can be
less than 3 pixels in both axes in image after alignment.
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