Under less extreme background conditions, significant improvement in stability can be made by referencing the signal level to the reset level, instead of electrical ground. This is done in readout method 2. The detector node voltage applicable to readout method 2 is shown schematically in Figure 26.
Note that the detector node voltage jumps when the reset FET is switched off, and this pedestal level is not removed in readout method 2. The pedestal level is different for each pixel in the array so this imprints a pedestal structure on the image, which is difficult to remove completely. The uncertainty in the value of this pedestal is known as kTC noise .
For direct imaging through broad band filters with high sky levels, the dominant noise source is still the photon shot noise of the background flux so readout method 2 gives acceptable performance.
The minimum readout time for this method is 0.3 sec.