###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.