Flatness Measurement of a Mosaic Focal Plane by using a Coaxial Multispectral Laser
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Flatness Measurement of a Mosaic Focal Plane by using a Coaxial Multispectral Laser Chang-hua Liu∗ and Ning-Xin Guo Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences , Changchun 130033, China and University of Chinese Academy of Sciences, Beijing 100049, China
Jian-Li Wang, Tao Chen and Zhi-Yong Wu Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130033, China
Xue Cheng Suzhou Institute of Biomedical Engineering and Technology, Chinese Academy of Sciences, Suzhou 215163, China (Received 16 March 2020; revised 7 May 2020; accepted 3 June 2020) The wide-field telescope will be an important tool in the discovery and birth of new theories of astronomy in future. The detectors in this kind of telescope mostly adopt a mosaic focal plane array. The focal depth of a large-F-number optical system is very small, and the flatness of the mosaic detector should be less than half the focal depth. In this paper, a new flatness measurement and data processing method is developed. A flatness measurement platform composed of a high-precision gantry platform and a coaxial multispectral displacement meter was built. The flatness of a single detector was measured under uncooled and cooled working conditions, and data processing was conducted, in which the root mean square (RMS) and the peak–valley (PV) values of the cooled detector were 0.0017 mm and 0.0112 mm, respectively. Next, a 2 × 6 mosaic model of the focal plane with a dimension of 148.5 mm × 168.5 mm was built using a metal detector model. The measurement platform was used to measure the flatness of the mosaic focal plane. According to the measurement results, the preliminary installation and mosaic adjustment were completed. The final RMS and PV values of the mosaic focal plane are 0.009 mm and 0.0808 mm, respectively. The experimental results show that the measurement and the data processing method can accurately reflect surface information on the detectors. This suggests that the method has great potential for use in ensuring the accuracy of wide-field telescope equipment in all applicable research areas. Keywords: Focal plane, Flatness, Detector array, Laser gauge DOI: 10.3938/jkps.77.653
I. INTRODUCTION In the application of the modern digital sky survey with wide-field telescopes, the flatness of the detector is an important characteristic, especially for mosaic detectors. The mosaic focal plane is composed of several detectors, which are coplanar after precise alignment. Shown in the Fig. 1 is the hyper suprime-cam (HSC) of Japan’s Subaru Telescope. In the development process, the flatness of the detector needs to be measured many times at different stages and under different working conditions to ensure that the flatness of the detector meets the requirement of half the focal depth of the system. Generally, the ∗ E-mail:
surface of the detector is coated with an antireflection film and the reflectivity is only 1% to 4%. The detectors are sensitive optical elements, which can only be measured using a
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