Noise level for each of 512 stars in Q1 SC data expressed as the standard deviation in ppm over 1.0–7.5 mHz in an amplitude spectrum (square root of power). The curve passing through the lower envelope consists of simple Poisson, readout noise plus sky background terms. The boxes near magnitudes of 9.3, 11.4, and 14.9 respectively are used to delineate samples of quiet stars as discussed in the text and Figure 2. A minor spread in values is expected from differences in sensitivity and readout noise over the 84 amplifiers as well as errors in the Kepler Input Catalog magnitudes.