The 3.6m Devasthal optical telescope is being installed at Devasthal, Nainital, and it will provide seeing limited observational capabilities at visible and near infrared bands. The image quality of any optical telescope is very crucial to determine accurate physical properties of celestial objects and it can go bad due to variety of reasons such as poor alignment of optics and the environment of telescope.The aim of the project deals with the detection of wavefront distortion induced by misalignment of optics and other imperfections in telescope using Shack Hartmann wavefront sensor. Using modal wavefront reconstruction approach, the distorted wavefront has been reconstructed for 3.6m telescope in terms of Zernike modes and a value of RMS wavefront error of 184nm has been obtained. PSF and MTF of the image are also extracted using these Zernike coefficients. The values obtained are consistent with the image quality specification of the telescope.