noise and signal

http://www.astro.wisc.edu/~sheinis/ast500/AY500_lect5.ppt.pdf

L14: Signal-to-Noise photon statistics and the CCD equation
http://slittlefair.staff.shef.ac.uk/teaching/phy217/lectures/instruments/L14/index.html

https://skyandtelescope.org/astronomy-blogs/imaging-foundations-richard-wright/astrophotography-gentle-introduction-noise/