What is the meaning of the 2x2 resolution element in the ACS ETC for an extended source ? What do I do if my source is larger than the 2x2 element ? When you are using any ACS ETC (imaging, spectroscopic or ramp) with an extended source, the ETC does all calculations for a boxsize of 2x2 pixels, or one resolution element. When a user inputs a uniform surface brightness and a signal-to-noise, the ETC calculates the exposure time necessary to reach that signal-to-noise for a 2x2 box. For sources larger than the 2x2 boxsize, scale the signal-to-noise to the appropriate boxsize (spatial extent) of your object. For a boxsize of 101x101 (infinite), you would choose a S/N 50 times smaller. (The 101x101 box is 2500 time the size of one resolution element and the square root of 2500 is 50.) The same is true whether you are using emission lines or not.