Electrons 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 measured during Cycle 17 are summarized in Table 5.1
(from WFC3 ISR Borders in prep). 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.
The read noise level in the science area pixels of bias frames for all of the
amplifiers at the default gain setting was measured during SMOV (WFC3 ISR 2009-26
). Table 5.2
shows the results obtained at the default gain setting of 1.5 e–
/DN. The read noise was found to be stable to 1%, 0.4%, 0.7%, and 0.8%, for amps A,B,C, & D, respectively (based on measurements through the end of August 2009).
A preliminary 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.