The interferometry is often adopted to check the optical quality and support the alignment in assembly for high precision demand optical system. The interferometer could acquire the system wavefront error (WFE) and obtain the delicate variation of aberration terms. For those optical systems applied to the space mission, the transition from atmosphere to vacuum environment would lead to the dimensional changes of mechanical structure within the scale of micrometer and eventually diminish optical performance. Consequently, as stated above, it is essential to validate the whole optical system in vacuum circumstance. Before the system actually being launched into the space, they are usually placed in the thermal vacuum chamber during ground testing in order to validate if the design could withstand the harsh environments such as high vacuum level and large temperature difference. Nevertheless, it is a big challenge to build up an in-situ optical measurement architecture for large aperture optical system in the thermal vacuum chamber due to the finite internal space of chamber, limited aperture size of transmission view port of chamber door and thermal dissipation problem of measuring instruments. In this paper, we demonstrate an innovative way of interferometry for monitoring the optical performance variation of FORMOSAT-8 (FS-8) optical system assembly (OSA) in our current vacuum chamber that the test telescope and the diverger lens were located in the vacuum environment, while the interferometer stayed in the ambient circumstance. The interferogram was successfully obtained thanks to the rigorous optical alignment process and the speical designed reference tools.
The method for aligning the focal plane assembly (FPA) with the optical structure assembly (OSA) involves finding a position with optimal focus. The experimental setup includes a collimator with an effective focal length of 10,500 mm and a free aperture of 700 mm. An electronic ground support equipment (EGSE) is responsible for capturing the sensor signals. Processing these data allows the derivation of the contrast transfer function (CTF) value using a line-pair pattern. At the beginning of the FPA alignment process, the attitude and position of the FPA are controlled with the assistance of a hexapod positioning system (a six-axis robot). After determining the attitude and the best focus position of the FPA, shimming is performed to adjust the gap between the FPA and the OSA. Based on the shim thickness required at three orientations, shim rings with the relevant thickness are installed. Due to the uncertainties from shimming, shim ring thickness manufacturing tolerance, and the residual of the tip/tilt angle between the FPA and the OSA during alignment, it is crucial to confirm and fine-tune the shim ring thickness. Once the FPA is fixed on the OSA, moving the FPA for the through-focus measurement is not feasible. However, the distance between the primary mirror and the secondary mirror of the collimator is tunable. By adjusting this distance, the collimation beam can be modified to a converging or diverging beam, thereby changing the focal plane position. Consequently, through-focus measurements can still be conducted by moving the secondary mirror of the collimator.
In the paper, we proposed a compact Korsch-type CubeSat telescope for optical remote sensing validation. The optical payload will be mounted on a 6U CubeSat named ONGLISAT. This telescope has an aperture of 92 mm and an effective focal length of 725 mm, providing a field of view of 2.2 degrees. The optical payload is equipped with a linear TDI CMOS image sensor, capable of capturing high-resolution panchromatic images with a ground sampling distance (GSD) of 2.8 meters in a low-Earth orbit at an altitude of 406 kilometers. The Korsch telescope consists of three aspheric mirrors (primary, secondary, and tertiary mirrors) as well as a roof mirror. This paper presents details of the optical design, assembly, integration, and performance test for the compact Korsch telescope. The image quality of the Korsch telescope is evaluated by the contrast transfer function (CTF). Finally, after thermal vacuum cycling testing and vibration testing, the telescope still maintains a CTF of 0.12 at 50 lp/mm line pairs. The Korsch telescope has the characteristics of a low obstruction ratio and high stray light blocking. The design of the telescope has higher resolution, a smaller size, and lower launch costs, which are beneficial for future applications in science, agriculture, commerce, and disaster relief. Furthermore, its optical quality can be quickly validated, serving as a preliminary study for larger telescopes.
In this paper, we propose a method for rapid calibration of long effective focal length collimators by the Ronchi test. Long effective focal length collimators were often used to calibrate remote sensing optical payloads or telescopes. In order to accurately verify optical system’s collimation, the effective focal length of the collimator must be at least three times longer than the optical system under test. Traditionally, people usually used interferometer to calibrate collimator. However, this method could accurately detect the collimation of the optical system, but the inspection process was quite complex and required a high-precision 6-axis moving platform to recode the interferometer position. Therefore, we propose a method for rapid calibration of long effective focal length collimators by the Ronchi test. The Ronchi test module was composed of a light source, a Ronchi ruling and a camera. We would use the module to verify the alignment of an optical system with a focal length of 10,500mm and a f-number of approximately 14.3. If the distance between the primary mirror and the secondary mirror would be the same as the ideal optical path, the position of the focal plane didn't change. By measuring changes in focal plane position, collimation could be quickly confirmed and adjusted. According to experiment results, the measurement error of the focal plane of this module is approximately ± 55 um. The collimator of emission angle error less than 1.75E-4 mrad. Form the above experiment results, the module could perform collimation verification at lower cost and faster speed. In the future, we could further reduce the measurement error by adding multi-wavelength light sources, gratings of different period frequencies and analyzing various interference fringe characteristics.
The iridescence green band and cyan tail of the wing on Papilio blumei butterfly were investigated. The bi-color phenomenon on the scales of butterfly wings was found and analyzed. The spectral change with thickness of chitin-air layers, width of air hole, total layer numbers and incident angle of light were simulated by FDTD method. 2D photonic-crystal model was applied to explain the change of reflectance spectra and color with angle. The replica of structural color and nanostructured thin films for Papilio blumei butterflies was fabricated successfully by three main techniques, PS spheres bedding, electron-beam gun evaporation and ICP etching.
KEYWORDS: Calibration, Optical filters, Sensors, Colorimetry, Error analysis, CIE 1931 color space, Principal component analysis, Photonics, Standards development, Human vision and color perception
Currently, the most popular instrument for chromaticity measurement consists of a power detector with several
color filters of transmission spectra approaching to the color matching functions from the CIE 1931 standard
observers. Unavoidably, there exist mismatches between the transmission spectrum of color filters and color
matching functions. Therefore, the calibration process is obviously very important to the accuracy and the precision
for the chromatic tri-stimulus values measurement in the colorimeters. In spite of several methods having
been developed for the calibration procedures, the correction condition is only set to one certain illumination
condition. Two most adapted standard illuminating conditions are the CIE standard illuminant D65 under correlated
color temperature (CCT) 6504K, and the CIE standard illuminant A under CCT 2856K. However, in
the general applications, the practical illuminating conditions for measurements are very difficult to be the same
as the calibration one. In this work, the impulse response analysis has been applied for the error analysis. From
the spectral mismatches of the color filters in colorimeters. A quantitative formula has thus been developed for
the evaluation of the correction for the chromatic tri-stimulus values due the mismatch of the color filters under
various CCT illumination condition. Based on this formula, the previously unknown deviation in the chromatic
measurement under different CCT condition now can be further reduced down. In summary, the filter-type colorimeters
with the calibration procedure according to our correction formula will perform much higher precision
but still keep its simple and convenient use.
This research intends to explore with a uniform color space based on the CIE 1931 x-y chromatic coordinate
system. The goal is to improve the non-uniformity of the CIE 1931 x-y chromaticity diagram such as to approach
the human color sensation as possible; however, its simple methodology still can be kept. In spite of the existence
of various kinds of the uniform color coordinate systems built up early (CIE u'-v', CIE Lab, CIE LUV, etc.), the
establishment of a genuine uniform color space is actually still an important work both for the basic research in
color science and the practical applications of colorimetry, especially for recent growing request in illumination
engineering and in display technology. In this study, the MacAdam ellipses and the Munsell color chips are
utilized for the comparison with the human color sensation. One specific linear transformation matrix is found
for the CIE 1931 color matching functions (see manuscript) to become the novel uniform ones. With the aid
of the optimization method, the transformation matrix can be easily discovered and makes the 25 MacAdam
ellipses are similar to each other in the novel uniform color space. On the other hand, the perfectiveness of the
equal-hue curves and the equal-chroma contours from the Mnusell color chips evaluates for the best optimization
conditions among several different definitions for the similarity of all the MacAdam ellipses. Finally, the color
difference between any two colors can be simply measured by the Euclidean distance in the novel uniform color
space and is still fitted to the human color sensation.
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