Design, Fabrication and Test of Plano-Convex Lenses Using Locally Available Materials for Applications in Telescopes.

Abstract

Astronomy can be considered as a main field of science which employs sophisticated instruments with precision lenseð€€¨ to acquire data of distant objects. Therefore, the fabrication of the lenses involves'a large amount of research work and fabrication methods may vary depending on the required optical manipulation from them. Thus, in this research a fabrication method of spherical lenses was studied. Experimental lenses were grinded using two glass plates and with the help of series of abrasives. The grinded lenses were tested using standard interferometric tests such as Newton's rings and Ranchi test. Glass blanks of 7 cm diameter were cut from a 5 mm thick Soda-Lime-Silica glass plate. Silicon Carbide powder (SiC) and Aluminium Oxide (Al2O3) with different grain sizes were used as abrasives. A polishing tool was made from an optical Pitch and Cerium Oxide (CeOz) and Zirconium Oxide (ZrOz) were used as the polishing materials. The number of trial lenses were grinded to determine the time and strokes needed in each grain size of abrasives. Hence, the optimum grinding time required in each grain size was determined. Radius of curvature of the lens was calculated from the interference fringes generated in the Newton's rings test. The focal length of a thick lens was calculated using the lens maker's formula. A finished lens has a focal length of 92.401 cm. The pattern generated from the Ranchi test was compared with a standard pattern and it was evident that the lens profile is in good quality. The lens was used as an objective lens with a 20 mm eyepiece of a standard telescope. The observed images were undistorted indicating that the lens can perform well as an objective lens of a refractive telescope. This was confirmed by the results of the Star test which was done at the end of the study.