Development of a 0.25 m f/10 reflecting telescope consisting of a thin primary mirror and a friction drive

Abstract

Modern astronomical telescopes are comprised of high-quality optics and superior mounting designs to meet the standards and challenges of observational astronomy. Cost of commercial telescopes is a constraint for developing countries like Sri Lanka to be involved in astronomical related research. The objective of this research was to study the technological methods used in fabricating telescopes and to implement those methods to make economical telescopes locally. Therefore, a f/10 focal ratio Newtonian telescope consisting of a computerized friction drive alt-azimuth mount was fabricated with a locally ground thin telescope primary mirror made from Soda-lime glass plate having 0.25 m diameter and 0.012 m thickness. Deformation characteristics, profile formation and optical performance of the telescope primary mirror were investigated using finite element analysis, Polarization and Ronchi pattern analyses, respectively. Resolution, slew rate, torque and slippage of the fabricated friction drive were studied using a Computer Aided Design (CAD) model and an empirical approach. It was found that the profile of the mirror is a perfect parabola. The mirror was supported by a mirror cell with eighteen floating points and resultant root mean square wave-front error of the mirror due to the supporting points is 3.735x10-7m. The friction drive with the computer controller can position the telescope to a given angle with accuracies of 0.421 degrees on altitude axis and 0.03 degrees on azimuth axis. The total cost of production of the prototype telescope is about US$ 750.00. It is shown that there is a significant cost reduction and increased optical and mechanical performances of the telescope due to using the thin mirror and the friction drive mount. In this paper, the system design and the performance evaluation of the mechanical positioning and optical quality of the telescope are presented. Keywords: Newtonian telescope, mirror grinding, telescope driving system, thin telescope mirror.