The ACIS instrument uses MIT Lincoln Laboratory CCID 17 Devices. These CCDs consist of a 1024 x 1026 pixel imaging section, with a separate 1024 x 1026 shielded frame store region. The pixels are 24m square, and when coupled with the focusing optics, will provide resolution of 0.5 arcseconds. The frame store for these devices has a novel architecture, with two separate sections that feed into two independent serial registers (see Figure 1). This design permits charge to be clocked in either direction and makes possible non-standard readout configurations. For example output nodes A and B can be both be readout through output A, while nodes C and D are both readout through C (AC mode).
Both front-illuminated (incident photons pass through the gate structure) and back-illuminated (incident photons incident directly into the depleted silicon) devices have been made for ACIS. Back-illuminated devices (BI) offer the advantage of higher quantum efficiency at lower energies, but have significantly lower spectral resolution than the front-illuminated (FI) devices.
Figure 1: Schematic of CCID 17