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RealGalaxy Data
The galsim.RealGalaxy class in GalSim uses images of galaxies from real astrophysical data (e.g. the Hubble Space Telescope), along with a PSF model of the optical properties of the telescope that took the
images, to simulate new galaxy images with a different (must be larger) telescope PSF. The galsim.COSMOSCatalog class takes this a step further, and can simulate either realistic images or parametric models of those images.
This modelling requires external data for the galaxy images and PSF models, which is read into a galsim.RealGalaxyCatalog or galsim.COSMOSCatalog object from FITS files. An example catalog of 100 real galaxies is in the repository itself. To get catalogs with large number of objects, the catalog to use depends on your GalSim version.
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For GalSim version 0.5.1 and later, a set of ~56 000 real galaxy images with I<23.5, or another set of ~87 000 with I<25.2, with original PSFs, can be downloaded from Zenodo:
The tar ball for the I<23.5 sample ( COSMOS_23.5_training_sample.tar.gz ) is roughly 4 GB and contains catalogs and images with a README. The tar ball for the I<25.2 sample (COSMOS_25.2_training_sample.tar.gz ) is of similar size and format. As of GalSim v1.3, GalSim comes with a script galsim_download_cosmos that downloads the I<23.5 sample. GalSim v1.4 will be updated to work with both samples, with the I<25.2 sample being the default but with keyword arguments to choose between the two. A copy of the data is also available on the GREAT3 server at https://cosmo.vera.psc.edu/Great3/public/
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The HST postage stamp data from this paper, which includes studies of the impact of blending on shear estimation in HSC, was released as part of the HSC survey's second incremental data release. The sample is larger than the above, goes to the depth of COSMOS, and does not have nearby objects masked. The links to download it and the instructions on its use are here.