Korean Journal of Optics and Photonics (한국광학회지)

Optical Society of Korea (한국광학회)
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Measurement of the Axial Displacement Error of a Segmented Mirror Using a Fizeau Interferometer

피조 간섭계를 이용한 단일 조각거울 광축방향 변위 오차 측정

  • Ha-Lim, Jang (Department of Science of Measurement, University of Science and Technology) ;
  • Jae-Hyuck, Choi (Space Optics Team, Advanced Instrumentation Institute, Korea Research Institute of Standards and Science) ;
  • Jae-Bong, Song (Space Optics Team, Advanced Instrumentation Institute, Korea Research Institute of Standards and Science) ;
  • Hagyong, Kihm (Department of Science of Measurement, University of Science and Technology)
  • 장하림 (과학기술연합대학원대학교 측정과학과 ) ;
  • 최재혁 (한국표준과학연구원 첨단측정장비연구소 우주광학팀) ;
  • 송재봉 (한국표준과학연구원 첨단측정장비연구소 우주광학팀) ;
  • 김학용 (과학기술연합대학원대학교 측정과학과)
  • Received : 2023.01.13
  • Accepted : 2023.02.03
  • Published : 2023.02.25
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Abstract

The use of segmented mirrors is one of the ways to make the primary mirror of a spaceborne satellite larger, where several small mirrors are combined into a large monolithic mirror. To align multiple segmented mirrors as one large mirror, there must be no discontinuity in the x, y-axis (tilt) and axial alignment error (piston) between adjacent mirrors. When the tilt and piston are removed, we can collect the light in one direction and get an expected clear image. Therefore, we need a precise wavefront sensor that can measure the alignment error of the segmented mirrors in nm scale. The tilt error can be easily detected by the point spread image of the segmented mirrors, while the piston error is hard to detect because of the absence of apparent features, but makes a downgraded image. In this paper we used an optical testing interferometer such as a Fizeau interferometer, which has various advantages when aligning the segmented mirror on the ground, and focused on measuring the axial displacement error of a segmented mirror as the basic research of measuring the piston errors between adjacent mirrors. First, we calculated the relationship between the axial displacement error of the segmented mirror and the surface defocus error of the interferometer and verified the calculated formula through experiments. Using the experimental results, we analyzed the measurement uncertainty and obtained the limitation of the Fizeau interferometer in detecting axial displacement errors.

조각거울은 우주용 관측위성의 주반사경을 크게 제작하기 위한 방법 중 하나로서, 여러 개의 작은 거울들을 이어 하나의 큰 거울로 이용하는 방법이다. 여러 개의 거울들을 하나의 거울로 정렬하기 위해서는 인접한 거울들 간에 기울기 오차(tilt)와 광축방향 정렬오차(piston)가 없어야 한다. 기울기 오차와 광축방향 정렬오차를 해결해야 여러 개의 거울이 한 방향으로 빛을 모으고, 이를 통해 뚜렷한 이미지를 얻을 수 있기 때문에 조각거울의 정렬오차를 나노미터 수준으로 측정할 수 있는 파면 센서가 필요하다. 기울기 오차는 조각거울을 통해 얻은 이미지를 통해 어떤 거울의 기울기가 틀어졌는지 쉽게 확인할 수 있는 반면, 광축방향 정렬오차는 이미지의 질은 떨어뜨리지만 드러나는 뚜렷한 특징이 없기 때문에 같은 방법으로 감지하기 어려워 세밀한 측정이 매우 중요하다. 이를 위해 본 논문에서는 지상용 초기 정렬시 많은 이점을 갖는 광학계 평가용 간섭계 중 하나인 피조 간섭계를 이용한다. 피조 간섭계를 사용한 복수 조각거울의 광축방향 정렬오차 측정을 위한 기초 연구로서 단일 조각거울의 광축방향 변위 오차를 측정하고, 측정불확도를 계산해서 피조 간섭계의 광축방향 변위 오차 측정 한계를 규명한다. 또한 수식을 통해 조각거울 광축방향 변위 오차와 간섭계로 측정한 표면 초점오차(defocus)의 관계를 계산했고, 도출한 수식의 타당성을 실험으로 검증했다.

Keywords

Acknowledgement

이 논문은 2022년도 정부(과학기술정보통신부)의 신소재 전개형 조각거울 및 위상측정기술 개발 사업으로 한국연구재단의 지원을 받아 연구되었음(NRF-2021M1A3B8077122).

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