A major disadvantage of the 1X mode is that it allows pixels to saturate near and perhaps beyond the full well of the detector, possibly allowing non-linear effects to occur. For these reasons, the UFOE is only operated in 4X mode. Spectra with very high photon signal to noise levels are achieved with multiple readouts. The full-well of each pixel is approximately 300,000 electrons. Figure C.1 shows a display of a single bias frame taken in 4X mode.
Figure shows a histogram of bias levels in the same bias frame.
The pixels on the CCD are 24 x 24 micro m, and the entire detector is 24.6 x 24.6 mm. Histogram of pixel intensity values in a single bias frame, taken in the 4X gain mode at HET on June 14, 1997. The y-scale is logarithmic above 1 and linear below 1.