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Ryan,
Excellent explanation !!
Can I add some further information for Peter in relation to Noise and Signal to Noise ratio (SNR) in Astrophotography which I think is important to mention and relevant to the topic of “darks”
Noise and SNR (Signal to Noise Ratio) in Astrophotography
Photons are the fundamental particles of light
Signal are photons from the celestial object or DSO together with nuisance Skyglow and Dark current ( noise )
Noise comprises of Read noise and Shot noise
Quality of images are dependent on the Signal to Noise ratio expressed as
SNR = Signal / Noise
Signal is made up of photons directly from your image target
Noise is target signal + Skyglow signal + Dark signal + Read noise square root
“Skyglow” is basically a build up of luminance in the night sky caused by natural and human sources ( light pollution, reflected light off the moon , star light etc.. )
“Dark current” ( Dark Noise ) is a thermal signal produced by the cameras electronics which subtracts the good signal ( photons ) The higher the temperature of the cameras sensor the higher the thermal signal the higher the dark current or dark noise
“Read noise” is the noise produced by your cameras sensor electronics. A lower read noise has the ability to pick up weaker signals and differences in signal levels
“Shot noise” is the fluctuations in the number of photons detected by the sensor which includes both photons from your object and skyglow. This variation in the number of good and bad photons from each exposure reduces your SNR
Ways to increase Signal
* Stack more images ( more Target Signal )
* Longer exposures
* Lower Focal Ratio or f ratio ( more photons per pixel for same time exposure ) Eg: 8” f6 newtonian telescope vs 4” f4 newtonian telescope using same camera
25% less photons are captured by the 4” scope but 3 times the photons hit each pixel on the sensor
* Quantum Efficiency ( QE ) of Camera ( higher the better ) Low is around 40% and high is around 80%
* Cool your sensor
* Use filters in urban areas
Ways to reduce Noise
* Stacking ( increases signal )
* Dark frames ( removes thermal signal eg hot pixels etc.. )
* Dithering ( removes fixed pattern noise or dark current noise )
Clear Skies
Martin
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