3.1.2 Continuous or Timed Exposure

Choices selected under this section reflect decisions to select different CCD clocking waveforms. Choices selected under the next section (On-board Processing Choices) reflect decisions to select different digital event processing algorithms in the DPA.

Timed exposure readout mode

Expose 0.2 to 10 seconds (typically), transfer imaging array to frame store array, then simultaneously start another exposure and read the frame store data. Note that the minimum time to readout and process every pixel in a CCD requires 3.3 seconds. Exposures shorter than this time must reduce the number of pixels processed by selecting a smaller `sub-frame window' on the CCD. (Sec. 3.1.3.5.) Another alternative is to collect a very short exposure (less than the 3.3 second full frame readout limit), then take the full readout time to process the complete CCD frame, but discard the incident photons while the readout is occurring. This allows short exposure times and full frame readouts, but suffers the penalty of `dead-time', time in which the CCD is inactive for X-ray detection.

Continuous readout mode

allows greater time resolution, but at the expense of angular and spectral resolution. The CCD is read continuously; each output pixel represents the integrated flux received as the charge crosses the array. The observer can `tune' the clocking rate to accommodate the expected counting rate from a given source, but at higher rates it becomes necessary to more severely degrade the angular resolution by ignoring or combining together CCD pixels in the readout process. Two pixel handling schemes are possible in continuous readout mode: Normal rate, in which each pixel of each row is read out; and Summed, which does on-chip summation of pixels; spectral resolution is degraded in this mode. Table 3.1 shows the timing and resolution of each of these modes.

 

 

Readout Mode Time Resolution Field-of-View Spatial Resolution Spectral

(seconds) (arc-min²) (pixels) Resolution

Timed integration 0.2-10 280 1x,1y maximum Continuous

$$6\times10^{-3} 2\times15.2$$   Normal 1x,1024y maximum

$$54\times10^{-6} 2\times15.2$$   Summed 1024x,256y minimum