It is dicult 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 prac- tical tilt correction method to obtain the optimal optical alignment condition using the symmetricity of optical aberrations of a wide-eld 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-m increments and passing through the optimum focus position in the range of 200 m. The standard deviation of each image, as a function of the focus position, was tted 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 dierence between two paired guide chip combinations such as east{west and north{south. We used this method to collimate the on-axis wide-eld 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-eld telescopes such as KMTNet.

Abstract
1. INTRODUCTION
2. FOCUS MONITORING USING GUIDE CAMERA
    2.1. KMTNet Focusing System
    2.2. Software for Focus and Tilt
3. SENSITIVITY ANALYSIS OF KMTNET OPTICALSYSTEM
    3.1. KMTNet Optical System
    3.2. Sensitivity Analysis
4. TILT CORRECTION EXPERIMENT
    4.1. Optimal Focus Position
    4.2. Calculation of Tilt Angle
    4.3. Improvement in Photometry
    4.4. General Procedure to Find Tilt Angle
5. SUMMARY
REFERENCES

• Chung-Uk Lee(Korea Astronomy and Space Science Institute) Corresponding author
• Yunjong Kim(Korea Astronomy and Space Science Institute)
• Seung-Lee Kim(Korea Astronomy and Space Science Institute)
• Dong-Joo Lee(Korea Astronomy and Space Science Institute/Department of Astronomy and Space Science, Chungbuk National University)
• Sang-Mok Cha(Korea Astronomy and Space Science Institute/School of Space Research, Kyung Hee University)
• Yongseok Lee(Korea Astronomy and Space Science Institute/School of Space Research, Kyung Hee University)
• Dong-Jin Kim(Korea Astronomy and Space Science Institute)