CFHTLS Depth Surveys PSF Correction Improvement

Theories and observation demonstrate that most of matter in Universe does notemit light, which cannot be observed by optical telescope. However, we can studythe matter distribution taking weak gravitational lensing as a “observational” tool.Cosmic shear is the weak lensing effect caused by the large-scale structure of theUniverse. It is the most important methods, So far, for the research of weakgravitational lensing. The accuracy depends on the precision of ellipticitymeasurement for galaxies. PSF (Point Spread Function) is a function describing thechange of a single point such as star, to a surface source. Lights emitted by distantgalaxies are deflected by the foreground gravitational field, which has to be takeninto account in the astrophysics study. With the influence of at mospheric agitation,atmosphere turbulence, seeing and distortion from telescope, light from remoteobjects will be distorted while they are observed by CCD. How to improve theprecision of ellipticity measure for galaxies is important for cosmic shear. In thisthesis work, with the motivation of improving the accuracy of galaxy shapemeasurement, I study the improvement of PSF anisotropy correction. The date areobtained from CADC (Cannada-Franch-Hawaii Telescope Legacy Survey) center. I usethe sixth publish data for r-band and25%best seeing for analysis. In the thesis, I willtake D2(hereafter D2-r′-25) as the example to describe all the details of dataprocess and PSF correction works. In this thesis work, I use two methods to achievethe PSF correction improvement. The first method is to work on the whole onesquare degree image. Stars are selection via magnitude vs size graph, and then applypolynomial fitting function to get PSF for stars. And then I use the PSF correctionfunction to remove the PSF effects for galaxies. The second method is to cut onesquare degree image into four parts. For each part, we use the similar process as thefirst method but with lower order of fitting function. For the first method, it mayhappen that some part of the region is over-fitted, while some part is under-fitted.This problem can be easily adjusted in the second method. I check the results of PSF.ABSTRACT.