CCD clocking modes

The SIS CCDs are operated in what is known as ``framestore" configuration. Each CCD is divided into two areas, the imaging area, which is exposed to radiation, and the readout area, which is shielded. Each imaging-area pixel has a corresponding pixel in the readout area to which the charge is transferred (in 16 ms). Once transferred, the charge resides in the readout pixel until the pixel takes its turn to be read out. Readout is effected by applying pulses of voltage (``clock" pulses) across the CCD which move the charge packets--row by row, pixel by pixel--to the amplifier. Reading out the whole CCD in this way takes 4 s (the slower the readout, the lower the noise). Each SIS camera has four CCDs, and the time resolution is determined by the ordering of the imaging/read-out cycles of the four CCDs. The various CCD clocking modes are:

• 4-CCD mode: All four CCDs are exposed at the same time. The time resolution (in high-bit rate, 32768 bps) is 16 s.
• 2-CCD mode: Two CCDs are exposed, so the active area is half that for 4-CCD mode, and the time resolution (in high bit rate) is 8 s.
• 1-CCD mode: One CCD is exposed. The active area is a quarter of that for 4-CCD mode, and the time resolution (in high bit rate) is 4 s.
• Parallel sum mode: One CCD is exposed, as in 1-CCD mode, but there is no spatial information. Events are sent directly to amplifier without being stored. The time resolution (in high bit rate) is 15.625 ms

To get the time resolution at medium (4096 bps) and low (1024 bps) bit rates, multiply the high-bit-rate figures by 8 and 32, respectively.