Korean Journal of Optics and Photonics (한국광학회지)

Optical Society of Korea (한국광학회)
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Surface-error Measurement for a Convex Aspheric Mirror Using a Double-stitching Method

이중 정합법을 이용한 볼록비구면 반사경의 형상 오차 측정

  • Kim, Goeun (Space Optics Team, Advanced Instrumentation Institute, Korea Research Institute of Standards and Science) ;
  • Lee, Yun-Woo (Space Optics Team, Advanced Instrumentation Institute, Korea Research Institute of Standards and Science) ;
  • Yang, Ho-Soon (Space Optics Team, Advanced Instrumentation Institute, Korea Research Institute of Standards and Science)
  • 김고은 (한국표준과학연구원 첨단측정장비연구소 우주광학팀) ;
  • 이윤우 (한국표준과학연구원 첨단측정장비연구소 우주광학팀) ;
  • 양호순 (한국표준과학연구원 첨단측정장비연구소 우주광학팀)
  • Received : 2021.11.04
  • Accepted : 2021.11.15
  • Published : 2021.12.25
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Abstract

A reflecting telescope consists of a concave primary mirror and a convex secondary mirror. The primary mirror is easy to measure, because it converges the beam from an interferometer, while the secondary mirror diverges the beam and so is not easy to measure, even though it is smaller than the primary mirror. In addition, the Korsch-type telescope uses the central area of the secondary mirror, so that the entire area of the secondary mirror needs to be measured, which the classical Hindle test cannot do. In this paper, we propose a double-stitching method that combines two separate area measurements: the annular area, measured using the Hindle stitching method, and the central area, measured using a spherical wave from the interferometer. We test the surface error of a convex asphere that is 202 mm in diameter, with 499 mm for its radius of curvature and -4.613 for its conic constant. The surface error is calculated to be 19.5±1.3 nm rms, which is only 0.7 nm rms different from the commercial stitching interferometer, ASI. Also, the two results show a similar 45° astigmatism aberration. Therefore, our proposed method is found to be valuable for testing the whole area of a convex asphere.

일반적인 반사식 망원경은 오목 형태의 주경과 볼록 형태의 부경으로 구성되어 있다. 주경은 크기가 크지만 빛을 모아주는 특성으로 반사경의 파면오차 측정이 비교적 용이하다. 반면에 부경은 빛을 퍼트리기 때문에 주경보다 크기는 작지만 측정의 난이도는 더 높다고 할 수 있다. 특히 코시(Korsch) 형태의 망원경에서는 부경의 가운데 영역도 사용하는데 기존의 힌들(Hindle) 측정법으로는 가운데 부분을 측정할 수 없다. 본 논문에서는 크기가 큰 볼록비구면을 측정하기 위해 힌들 정합법(힌들 측정법과 정합법의 결합)을 적용하여 측정하고 이 측정에서 빠지는 가운데 영역은 일반간섭계의 구면파를 이용해 측정하여 두 결과를 합쳐 전체 형상 오차를 획득하는 방법을 제안하고자 한다. 제안한 방법으로 직경 202 mm, 곡률 반경 499 mm, 비구면상수 -4.613의 볼록 비구면의 형상 오차를 측정한 결과 19.5±1.3 nm rms로 측정되었다. 이 결과는 상용 정합 측정 장비로 측정한 결과와 0.7 nm rms의 근소한 차이를 보였고 반사경의 형상 오차가 45도 방향 비점수차를 가지는 것것도 일치하였다. 따라서 본 논문에서 제안한 방법이 볼록 비구면의 전체 영역을 정밀하게 측정하는데 유용함을 알 수 있었다.

Keywords

Acknowledgement

본 과제는 한국표준과학연구원의 기본사업(21011099)의 지원으로 수행되었습니다.

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