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In principle, observing with the CCD detector is just a matter of integrating long enough on a given object until the desired signal/noise ratio is obtained. Figure 5 shows theoretically obtainable s/n ratios vs. count rates (ADU's) assuming a readout noise value of 12 e- per pixel for TI 5 (with preflash) and 8 e- per pixel for F3KB. Note that the values given on the graph are per pixel and there are usually several pixels perpendicular to the dispersion to extract. Therefore the final signal to noise obtained is higher than that obtained from just the count rate of the maximum intensity pixel. While dark count rates are usually negligible, radiation event spikes will show up at a minimum rate of about 1 - 2 per hour for TI5, and 4 - 8 per hour for F3KB, per spectrum.
Figure 5: Theoretical S/N Values per Pixel for CCDs.