System noise directly affects spectral resolution, especially at the lowest X-ray energies. This dependence is illustrated in Table 3. We have adopted a requirement of 5 RMS, as a level which is readily achievable, and which minimizes the contribution of system noise to energy resolution. At this noise level, photoionization statistics, rather than readout noise, is the dominant contribution to spectral resolution for energies above 500 eV.
CTI is the measure of how efficiently a CCD transfers a charge packet from the image array to the output node. Devices with high CTI will not transfer all of the photo electrons associated with an X-ray event to the output node. This charge loss results in position dependent energy scale which in turn leads to degradation in the spectral resolution for the entire device. These undesirable effects can be negated by demanding that the CTI fall below a critical level. We have chosen a threshold of , measured at 5.9 keV, for parallel CTI. At this level, CTI makes no measurable contribution to device energy resolution.
For front-illuminated (FI) CCDs the spectral resolution of a single pixel event is limited by the RMS noise of the system and the Fano factor of silicon. The theoretical prediction for resolution is given by the well known relation:
where N is the RMS noise in electrons, is the energy needed to create one electron-hole pair (3.65 eV/pair), F is the Fano factor for silicon (F=.12, and E is the photon energy in eV).  The thinning process used in fabricating back-illuminated (BI) CCDs results in poorer spectral response, forcing a relaxation of the threshold values for BI devices. Table 3 lists the acceptance criteria for FI and BI CCDs and theoretical limits for single pixel events in FI devices.
Table 3: Spectral Resolution Values from .5 - 6 keV for Single Pixel Events
Dark current is the measure of thermally excited free charge in the CCD. The screening is conducted at the expected flight temperature of -120 C, and at these temperatures the dark current is so low that is has no significant effect on the performance of the CCD. The acceptance criteria requires devices to have dark currents below .3 electrons/pixel/second.
Cosmetics is a blanket term used to describe device morphology, including hot columns, bad columns, hot pixels, and edge glow (contiguous regions of hot pixels along the edge of the chip created when the silicon wafer is cut). A CCD with gross defects (regions on order of 100 pixels square) is immediately removed from consideration. A device with a few bad or hot columns or tens of hot pixels that meets the other screening criteria is fully calibrated. The acceptance criteria for cosmetics is that a CCD have fewer than twenty total defective columns, with no greater than ten defective columns occurring in any quadrant.