Hi Josh
Alchemy has covered some good stuff above.
Simplistically put, DSLR’s are optimised for day time “happy snap” photos, where exposures are usually sub 1 second duration.
Astrophotography often requires exposures of 30 seconds to 5 minutes duration to record sufficient details in the (dim) subject. At these long exposures, the ccd or cmos sensor begins to accumulate thermal noise, which becomes visible as hot pixels in the image.
So, your image is now made up of the following components:
- The subject matter e.g. a galaxy. (=signal)
- Hot pixels or thermal noise generated by the long exposure. (=noise)
- Optical defects in the system such as vignetting, dust spots, light gradients, etc. (=noise)
Subtracting a dark frame removes the thermal noise.
Dividing a Flat Field Frame removes the system's optical defects mentioned above.
And voila, you are left with the actual “signal” (=galaxy) and the “noise” (darks & flats) has been minimised to an acceptable level.
Cheers
Dennis