ELi5 why do Astrophotographers need to capture hundreds of pictures of the same area in the sky and lay them on top of each-other to make a clear image? Ty in advance ❤️🙏🏼
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People stack images in astrophotography to increase the signal-to-noise (in other words, to make the things they want to see much brighter than the background).
I’m not sure how familiar you are with the basics of photography, so my apologies if this is either too basic or at too high of a level. In photography we have three tools at our disposal to change the brightness or signal to noise of our image: shutter speed, aperture, and ISO.
Shutter speed is pretty simple. The longer you leave the shutter open, the more light you will detect. If you’ve ever tried to take a picture in the dark and your camera takes a long time to take the image/things get blurry from movement, that’s because your camera tried to use a long shutter speed to account for the lack of light.
Aperture is the size of the opening through which you collect the light for your image. The wider the aperture, the more light you let in. Aperture also affects how you focus and depth of field, but that isn’t super relevant for most astrophotography applications.
The ISO controls how sensitive your detector is. A higher ISO makes your image brighter, but also makes the background brighter and causes the image to look grainy. Typically you want to use the lowest ISO possible, while still maintaining a suitable brightness.
Okay, now back to astrophotography in particular. For astrophotography, you’re trying to look at really faint stars/galaxies, meaning they have a low signal to noise and are hard to capture (which is made worse by light pollution). I already said that you want to keep your ISO relatively low, so we’ll ignore that variable for now. A bigger aperture will help a lot (with regular camera lenses this is equivalent to using a smaller f-stop). If you hook your camera up to a telescope, the diameter of the telescope is what controls the aperture.
So why not just use a really long exposure (slow shutter speed) to get adequate signal to noise? Well, the earth is spinning pretty darn fast and after a couple seconds, stars will move enough in your field of view that they start to smear out. This can lead to cool effects like [this](https://upload.wikimedia.org/wikipedia/commons/e/e5/All_In_A_Spin_Star_trail.jpg), but doesn’t give you a crisp view of the night sky, which is what most astrophotographers are after.
To get a sharp image without smearing, you can add up a bunch of images with shorter exposure times (short enough that there is no smearing). If they are overlaid and added up properly, it will be like you took a really long exposure, but didn’t have to worry about the stars moving.
There are also special devices called star trackers, which will move the camera to counteract the rotation of the earth, so you don’t have to worry about smearing. In these cases, one should in principle be able to take really long exposures, but it is often still better to stack photos.
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