Improvement Of PSF Correction For VST Data

Cosmic shear is the weak gravitational lensing effect caused by large scale structure ofthe universe. Through measuring it, we can understand profoundly about the distributionof dark matter, dark energy and large scale structure of the universe. Thus shearmeasurement has played increasingly important role of our cosmic research.This thesis uses the original observed data supplied by the VOICE survey of VST(VLTsurvey telescope).The main procedure includes obtaining many parameters of stars bySExtractor software first, then removing lower signal-noise regions of the exposure. Theseparation of stars and galaxy are from the plot of size vs magnitude. The PSF (pointspread function) correction of stars is done by using KSB method. In this thesis5orderpolynomial fitting function is applied to correct anisotropic factor of PSF. Then the PSFcorrection of galaxies is achieved by applying the correction of stars to galaxies. Thecorrelation function of aperture mass and the decomposition of the E/B mode arecalculated to gain the weak lensing signal-E mode.The data processed in this thesis is28exposures of1square field, due to the limitationof time and data of observation. From the analysis, PSF is quite different correspond todifferent exposures. Therefore, four different methods are applied to correct PSF and thencompare the results. Firstly,28exposures is processed individually;Secondly, stack all28exposures by Swarp sotfware and then apply the same process; Thirdly, only stack theexposures with good seeing value(<0.9); Lastly,28exposures are set as different groupaccording to the median value of stars ellipticity.As the result of this thesis,the last method obtaining a relatively good lensing signals.Due to the limit of the number of exposures of our available data，thus its survey is quiteshallow. Once the observation is complete, we expcct that the method discussed in thisthesis will improve the correction of VST data.