A Study of Influences of Defocus Aberrations on High-Resolution Image Reconstruction for Data from the New Vacuum Solar Telescope of the YNAO

The 1m New Vacuum Solar Telescope (NVST) in the Fuxian Solar Observatory (FSO) of the Yunnan Observatories is currently the largest ground-based solar telescope in China. The primary scientific task of the NVST is high-resolution observation of the sun. Images from the ground-based NVST are inevitably blurred by atmospheric turbulences, and some high-resolution statistical reconstruction techniques (such as the speckle interferometry and speckle masking) need to be adopted to remove effects from atmospheric turbulences. However, reconstruction results with the techniques are affected by aberrations of the NVST. Due to solar radiations and environmental temperature variations there are appreciable aberration changes in a day. Changes of defocus aberrations have major contributions to overall aberration changes. In this paper we present analyses of defocus aberrations of the NVST and their temporal variations using the NVST high-resolution imaging system. We subsequently investigate influences of defocus aberrations on high-resolution statistical reconstruction of NVST data. Our preliminary estimation shows that in a typical day of observation the average and maximum variations of defocus aberrations of the NVST are about 1.8λ and 1λ, respectively. Our numerical simulations illustrate that defocus aberrations significantly influence the Module Transfer Functions (MTFs) of reconstructed images, but affect little the Phase Transfer Functions (PTFs) of the images. If defocus aberrations are above about 2λ spatial resolutions of reconstructed images for the NVST decrease rapidly as defocus aberrations increase. We conclude from these that defocus aberrations should be appropriately compensated to make reconstructed images of the NVST to have sufficient quality.