7.2 Gain

Gain, a fundamental scaling parameter of the IR detector, relates the registered number of analog-to-digital units (ADUs) or equivalently, data numbers (DNs), to the number of accumulated photoelectrons. Like the UVIS channel, the Analog to Digital Converter (ADC) outputs a 16-bit number, allowing output signal values ranging from 0 to 65535 (i.e. 2¹⁶ -1) for each pixel.

While it is possible to operate the WFC3/IR detector at gain settings of 2.0, 2.5, 3.0, and 4.0 e-/DN, only a gain setting of 2.5 e-/DN is supported. This gain provides a complete sampling of the full-well of the IR detector (~80,000 electrons, which is ~32,000 DN at the 2.5 e-/DN setting). Due to slight hardware differences, effective gain values in each quadrant of the detector deviate slightly from the nominal gain setting. Twice yearly, the effective gain is measured via the mean-variance technique on pairs of internal flat fields: the mean signal level is plotted against the variance in signal, and the gain is the inverse of the slope. See WFC3 ISR 2015-14 for details on this method and the status of the IR gain from 2010-2015. Table 7.1 summarizes the current values of the effective gain measured in each quadrant of the IR detector. The WFC3 group will continue to monitor the effective gain of the detector twice yearly to evaluate its stability over time.

Table 7.1: Gain measurements for each quadrant, in e-/DN, averaged over all epochs of observations. Quadrants 1, 2, 3, and 4 are the 512 × 512 pixel detector quadrants that are read out independently from the outer corners (see Section 2.2.2).