###Exposure Time Calculator### * What does "counts" mean in the (ACS Instrument) handbook? A signal of an electron on the CCD is equivalent to one count. Therefore, the actual output signal from a CCD (sometimes referred to as DN for data number) is C/G where C is counts and G is the gain. The user needs to remember to multiply by G to compute photon events in the raw CCD images. G is 1 always for the SBC, and 1, 2, 4 or 8 for the CCDs depending on CCDGAIN.