Conceptually, is three separate noise terms added in quadrature—a purely circuit-related noise term and the multiplied shot noise of the APD’s dark current and CW background photocurrent. According to Table 3, a background photon arrival rate per pixel of up to per nm of filter BW is possible in the worst case (near target scenario; 2.5 mrad DAS; 25 mm aperture). Figure 4 estimates that the best filter width we can have in this case is 9.3 nm, so the background flux in the large DAS case will be . For a , InGaAs/InAlAs APD pixel with 80% QE and 70% fill factor, operated at a mean gain of , the worst case background photocurrent is about 1.6 nA. This is about an order of magnitude larger than the APD pixel’s 0°C dark current at this gain, of about 0.2 nA. Filter width is not a problem in the far target scenario, with 0.1 mrad DAS. Table 3 gives a background photon rate of about , corresponding to about 4 pA of photocurrent, which is negligible compared to the pixel dark current. The worst case optical background combined with the APD pixel’s 0°C, dark current together contribute about RMS of multiplied shot noise at the ROIC pixel input, whereas with negligible optical background, the multiplied shot noise of the APD pixel’s dark current alone is about under these conditions. In the low-BW configuration, responding to 4-ns FWHM laser pulses, an input-referred pixel circuit noise of about can reasonably be achieved. In the high-BW configuration, responding to 1-ns FWHM laser pulses, the ROIC’s circuit noise would roughly double. Consequently, in the low-BW configuration, the difference between the worst case solar background and negligible background is RMS versus . The high-BW configuration would not be applied to the large DAS case because of its smaller format and the relaxed range precision requirement of that scenario; in the small DAS case, the optical background is negligible, and RMS for the high-BW configuration. It should also be remarked that if the APD pixel is operated at lower gain, such as , the detector shot noise is smaller. We make calculations for APD pixel gains of , , , and to find an optimal operating point.