The Hubble Space Telescope was able to take the Hubble Ultra-Deep Field Image thanks to the telescope being exposed to a single section of the sky for well over two months. But what I don't get is: how does keeping the telescope open for a long time help with catching distant objects? It can't be the case that it allows the light from that far away to reach us during that time, for it will have to be a big coincidence that we pointed only when it was first reaching us. I tried to search for long exposure photography, but it mostly revealed how using that we can take pictures of moving objects differently by altering the exposure time. I couldn't find much on how it helps with seeing distant objects. How does it really work? Correction: there were two sessions of observation-- one for over two months and the other for a little over a month.