Dark Spikes:

Dark frames show "salt and pepper" Dark Spikes randomly distributed in them. These spikes once they appear, remain in the sensor like a scar: they don't go away. New ones may form over time and once formed they too remain in all future images.

These dark spikes are formed in defective regions of the silicon and are contained within the pixel. The leakage rate is influenced by the electric field in the pixel and the higher the field the greater the leakage rate.

Empty wells have the highest fields and filled wells have the lowest fields. As a result dark frames will have "taller spikes" than will light frames, particularly when the light frames have high background signal level.

For this reason dark subtraction will not completely remove the dark spikes (actually bright spikes in the dark image but may leave dark spots after dark subtraction).

The curve below was made by examining light frames taken using broadband light and Halpha filtered light for two different sensors and by examining the corresponding dark frames for each that matched the exposure time and temperature. The dark frames had a minimum of 10 such darks median combined while the light frames were single frames that were analyzed.

Candidate bright spots were identified in the Light images, the coordinates and DN values of it and the four surrounding pixels noted (the four were averaged to provide a reference background level). The same pixels were then analyzed in the dark frames and again the four surrounding pixels were averaged as the background level. The delta of the spike minus the reference background was computed and recorded for the light and dark images

Finally the Light spike minus Dark spike was computed for each and was plotted versus light frame average background level (the average of the four surrounding pixels).

The data shows that for high background signal levels the spikes are taller in the dark frames than the light frames but for low background signal levels, the opposite is true. The results are consistent with the theory that teaches the strength of the electric field within the pixel regulates the dark spike current generation rate.


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