|WFC3 Data Handbook v.4|
5.1.1 GainPhotoelectrons that accumulate in a CCD pixel are read out (as voltage) and converted to data numbers (DN), often called Analog-to-Digital Units (ADUs), by the analog-to-digital converter (ADC). The ADC output is a 16-bit number, capable of producing a maximum of 65,535 DN for each pixel. A straightforward scheme in which one DN corresponds to one electron would make it impossible to measure signals larger than 65,535 electrons.The conversion gain, defined as the number of electrons per DN, provides a way of changing the dynamic range of electrons that can be measured. Although it is possible to operate the WFC3 CCD detector at gains of 1, 1.5, 2, and 4 e-/DN, only a gain of 1.5 e-/DN is supported. This gain permits sampling of the entire dynamic range of the detectors, with negligible impact on the readout noise. The gains for the WFC3 CCDs, routinely measured every year, have been stable to within 1-2% since launch in May 2009 ( WFC3 ISR 2013-02, WFC3 ISR 2015-05 and WFC3 ISR 2016-13). The current values are summarized in Table 5.1. The gains were measured in flat-field images via the standard mean-variance technique: the inverse slope of the mean signal level plotted versus the variance yields the gain.
5.1.2 Read NoiseThe average read noise level measured in the bias frame science area pixels at the default gain setting (1.5e-/DN) are summarized in Table 5.2. Scatter in the measurements based on data acquired between June 2009 and July 2015 is less than 1%, depending on the amplifier (WFC3 ISRs 2017-17, 2015-13, 2009-26)
An analysis of the statistical behavior of the WFC3 ADCs shows some tendency for the least significant bit to be slightly biased at the readout speed adopted by the WFC3 electronics (see WFC3-ISR 2005-27). This minor effect should not degrade the photometric and noise characteristics of the WFC3/UVIS images.