Pointing of HAGAR telescope mirrors
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An array of seven atmospheric Cherenkov telescopes was commissioned at a high altitude site in Hanle in the Ladakh region of the Himalayas. The array called HAGAR has been designed to observe celestial γ-rays of energy >100 GeV. Each telescope is altitude-azimuth mounted and carries seven parabolic mirrors whose optic axes are co-aligned with the telescope axis. The telescopes point and track a celestial source using a PC-based drive control system. Two important issues in positioning of each HAGAR telescope are pointing accuracy of telescope axis and co-alignment of mirrors’ optic axes with the telescope axis. We have adopted a three pronged strategy to address these issues, namely use of pointing models to improve pointing accuracy of the telescopes, RA-DEC scan technique to measure the pointing offsets of the mirrors and mechanical fine-tuning of off-axis mirrors by sighting a distant stationary light source. This paper discusses our efforts in this regard as well as the current status of pointing and monitoring of HAGAR telescopes.
KeywordsTelescope pointing γ-ray astronomy Pointing model Atmospheric Cherenkov telescopes
We thank Prof. B. V. Sreekantan for his keen interest and encouragement. We also thank Dr. A. Krishnan for his guidance and helpful tips during the early stage of the telescope control system design. We thank Profs. P. N. Bhat, H. S. Mani, R. Koul and P. Bhattacharjee for their suggestions that helped reducing the pointing error. We thank the staff at Hanle for help with the operation of telescopes. Our thanks are due to IIA and TIFR staff members who have contributed at various stages of this project.
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