CCD Performance

4.12 Read Noise and Gain Settings

The CCDs and their associated signal chains have readout noise levels (in the absence of signal shot noise or interference) of approximately 5e-. The analog-to-digital converter is highly accurate, and makes virtually no contribution to the read noise, other than the normal information loss caused by digitization of the signal.

The conversion factors from detected electrons (QE x number of incident photons) to data numbers (DN) are tabulated in Table 4.3, as are read noise and linearity ("gamma" is the power law index relating detected DN to input flux). Note that all calculations of sensitivity in this manual assume gains of 7 and 14 for two gain channels, choices very close to the measured gains. The photometric calibration is based on an assumed exact gain of 14 in all CCDs. The measurements given here were derived from thermal vacuum testing. On-orbit measurements have confirmed that the gain ratios are correct to within a possible systematic error of 1%--which will feed directly into a photometric calibration error for gain 7 data, as most of the photometric calibration was done with gain 14 data. Note that the gain ratios are known much more accurately than the individual gains; they are derived from flat field ratios instead. Also, note that the Phase II proposal instructions refer the ~14 e- DN-1 setting as ATD-GAIN=15.

Table 4.3: Signal Chain Gains.