10/11/2026
How is light "collected" in astrophotography? Part of the answer is long exposure, simply leaving the camera shutter open to collect a lot of light over time, but it's not the whole answer. Every camera has a limit to how much light it can collect into a single image before pixels become saturated and can't collect any more. On a sunny day this can happen in less than a second. Under dark skies it takes longer, but it's still a limit. It's also difficult to precisely track the sky for long periods. Even with excellent equipment breezes and even vibrations through the ground can slightly blur an image.
To get around these limitations, astrophotographers use what's called "stacking" to collect sometimes hours worth of light using only minutes or seconds of individual exposures. Even the Hubble and James Webb space telescopes use this technique. The famous Hubble Deep Field Image (linked below) used hundreds of roughly 7-minute exposures to collect over 39 hours worth of light from a very dim part of the sky, revealing galaxies no one knew were there.
In this example, the first frame is a single "long" exposure of one minute. The second image is a stack of roughly 60 minutes worth of light. The third features nearly 3 hours of light. This effectively increases the dynamic range (how much difference can be seen between lighter and darker areas) of the image. Darker and darker details are able to be seen and areas of light like nebulosity become more and more apparent.
Learn about how Hubble's Deep Field images used this technique and increased our understanding of the universe here:
https://science.nasa.gov/mission/hubble/science/universe-uncovered/hubble-deep-fields/
