This code takes data from the Hubble Space Telescope (HST) with Filter F555W (5391.43 Angstrom, central wavelength) of the spiral galaxy NGC 4258. The data being recorded and analysed is a single population 1 Cepheid Variable star (of 15 made available to me). By measuring the flux density of the Cepheid Variable (Units of Jy? Hard to tell because header doesn't specify but with some simple calculations Jy would make sense). The flux density (counts) of the cephid is then compared to the HST reference of 22.57 apparent magnitude at 1000 counts to get an apparent magnitude for the Cepheid. Once this has been collected then a Mag vs Time (or phase) plot can be used to determine the time period of the Cepheid which allows us to calculate an absolute magnitude. From this we can calculate a distance to the Cepheid (and therefore a rough estimate of the distance to NGC 4258).