Remembering from the CCD Overview lesson, each star:
• Is the same size on the focal plane – a Point Source
• Each point source has a specific shape and size at the CCD chip known as a Point Spread Function (PSF) • PSF is mostly determined by Seeing
• Seeing is the smearing of a stellar object on the CCD chip due to atmospheric effects such as temperature and turbulence
• Overall, each star has the same shape and size on the CCD chip.