The data is publicly available (as is the code), so you could just use it. The data itself is organized by region of the sky being imaged, so that calculation has already been done for us (and at the distance the stars/galaxies are at the difference in distance due to the Earth orbit doesn't matter). In the actual application, both the noise and the psf are modeled based on the actual telescope, so just adding arbitrary noise is a bit of a different problem, but probably a good start. Do note also that Celeste is a generative model (including modeling the noise), so you could just use it to generate a bunch of images for you.