Boosting Count-rates with Earth Limb Light and the WFC3/IR Count-rate Non-linearity

We report the results from the CAL/WFC3 program 11933 to constrain the effect of count-rate non-linearity on the WFC3/IR detector by observing stars in NGC 1850 with and without superposed scattered Earth light. The scattered Earth light raises the apparent count rate of the stars by a measurable amount, allowing us to measure its impact on photometry. After accounting for a time varying sky level we find a small effect at the level expected, about a 1% decrease in measured count rate per factor of 10 reduction in the true count rate. The precision of the measurement is limited by the maximum Earth light at the minimum bright Earth limb angle allowable under FGS guiding, ~2-4 electrons/sec/pixel, 2 to 3 orders of magnitude less than the lamp used on NICMOS for the same purpose.