|Space Telescope Science Institute|
|WFC3 Data Handbook v. 3.0|
5.1.1 GainElectrons that accumulate in the CCD wells are read out and converted to data numbers (DNs), often called Analog-to-Digital Units (ADUs), by the analog-to-digital converter (ADC). The ADC output is a 16-bit number, producing a maximum of 65,535 DN for each pixel. A straightforward scheme in which one DN corresponded to one electron would make it impossible to measure signals larger than 65,535 electrons. Hence the conversion gain parameter provides a way of adjusting the scale so that multiple counts correspond to a single DN, allowing larger numbers of electrons to be measured. The conversion gain is defined as the number of electrons per DN. 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 within 1-2% since launch in May 2009 (WFC3 ISR 2013-02 and WFC3 ISR 2015-05). The current values are summarized in Table 5.1. The gains were measured via the standard mean-variance technique: the inverse slope of the mean signal level plotted versus the variance yields the gain.Table 5.1: WFC3-UVIS Gains
5.1.2 Read NoiseThe average read noise level measured in the science area pixels of bias frames at the default gain setting are summarized in Table 5.2. Scatter in the measurements, based on data acquired between June 2009 and July 2015, is ~1% or better, depending on the amplifier. More recent correction for 1x1 readnoise values are in WFC3 ISR 2015-13, other values are from WFC3 ISR 2013
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.