Many amateur astronomers who step into astrophotography understandably have many questions regarding imaging sensors. By far, one of the most frequent question is about the Noise (not sound, electronic disturbance) present in the captured frames which is a result of the heat generated in the sensor.
The CCD or CMOS sensors like the one shown on the left side are electronic devices that translate the capture of photons to electric signals, read out by the computer as an image. Now since we know that electricity, when flowing through any material which has resistance generates heat, so it does with the sensor. This heat shows up as "noise" in the image. One effective method to rid the images of this heat signature is via the process of dark subtraction. Simply speaking, the astrophotographer will stop light from entering the camera sensor by closing its cap or lid, then initiate one or multiple exposures in the camera with the exact same settings (exposure time and temperature) used to capture the image for which the dark needs to be subtracted. By closing the cap of the camera the intention is to allow no other signal to be captured in the sensor except the heat signature.
Therefore, a certain "dark" frame is the heat signature of the sensor for "that" exposure time. Also, as an example, 1 minute dark will have lesser heat shown which corresponds directly to the heat produced because of all the electricity flowing through the sensor. Similarly 5 minute will show more "heat" as the electricity flows through the sensor for a long duration, giving it "time" to heat up the sensor. Therefore, a Dark frame is also like a "Fingerprint" of that property of the sensor, which shows whether it gets heated quickly or not. This is also the reason that some CCD sensors are more expensive than others. The expensive sensors get heated less even if exposed for longer and hence display lesser noise when compared to cheaper CCD chips. The Sony ICX285AQ, used in Starlight Xpress H9, The Opticstar DS-145 ICE, and ATIK 314L series all have this sensor and are more expensive when compared to cameras featuring for example the Kodak 8300 chip.
Every exposure therefore needs to have a corresponding