End of milestone 3

docs/optics.md

@@ -529,6 +529,10 @@ times the dark current (`Detector::prnu`, `dsnu`, `hot_pixel_fraction`,
 `hot_pixel_multiple`). Because the dark factor scales the dark current, the
 dark pattern grows with the exposure, as a real one does: at 0.1 ms the
 example camera's hot pixels make 0.1 electrons, invisible; over 5 s, 5,000.
+A camera made for long exposures is calibrated: its hot pixels are found in
+dark frames and mapped out. The long-exposure example scenes (the Pleiades
+and the GEO telescopes) leave 1 value in a million hot instead
+(`hot_pixels = [1e-6, 50]` in a scene file's `[camera]`).
 
 The pattern comes from `Detector::pattern_seed`, apart from the noise's
 seed, so one camera keeps its pattern whatever the frame or noise: the same