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Subs is short for sub exposures - most astro photos are a combination of multiple individual exposures. As an example I typically take 5 minute sub exposures but might do 12 or more of them for each colour filter and then combine them in a stacking program like Deep Sky Stacker (I actually use Pixinsight). I've attached an example of a single 5 min sub (image 1) and a stacked set of 9x5 min subs (image 2) from the same night/target so you can see the difference in the signal to noise ratio - the stack of multiple subs has a lot less noise. Both frames are post calibration so have been corrected for flats and darks.
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