Journal of The Korean Astronomical Society (천문학회지)

The Korean Astronomical Society (한국천문학회)
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TILT CORRECTION FOR A WIDE-FIELD ON-AXIS TELESCOPE USING THE SYMMETRICITY OF OPTICAL ABERRATIONS

  • Received : 2020.12.18
  • Accepted : 2021.06.21
  • Published : 2021.08.31
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Abstract

It is difficult for observers to conduct an optical alignment at an observatory without the assistance of an optical engineer if optomechanical parts are to be replaced at night. We present a practical tilt correction method to obtain the optimal optical alignment condition using the symmetricity of optical aberrations of a wide-field on-axis telescope at night. We conducted coarse tilt correction by visually examining the symmetry of two representative star shapes obtained at two guide chips facing each other, such as east-west or north-south pairs. After coarse correction, we observed four sets of small stamp images using four guide cameras located at each cardinal position by changing the focus positions in 10-㎛ increments and passing through the optimum focus position in the range of ±200 ㎛. The standard deviation of each image, as a function of the focus position, was fitted with a second-order polynomial function to derive the optimal focus position at each cardinal edge. We derived the tilt angles from the slopes converted by the distance and the focus position difference between two paired guide chip combinations such as east-west and north-south. We used this method to collimate the on-axis wide-field telescope KMTNet in Chile after replacing two old focus actuators. The total optical alignment time was less than 30 min. Our method is practical and straightforward for maintaining the optical performance of wide-field telescopes such as KMTNet.

Keywords

Acknowledgement

This research used the KMTNet system operated by KASI, and the data were obtained at the host site of CTIO in Chile. We are grateful for comments from anonymous reviewers and the editor of this journal.

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