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Readout and integration time (SW)

The readout of the SW array is an elaborated form of CID readout. Measurements of the pixel charge is done by sensing the voltages of the 32 output lines after sequentially injecting the pixel charges in the substrate through column voltage gates. The analog chain uses an adaptive filter followed by high gain preamplifiers. The filtering scheme gives a periodic signal with 32 peaks corresponding to the pixels intensities. The baseline is measured before and after each injection, giving 65 measurements. The true value of the pixel charge is provided by the difference between the peak measurement and the mean value between the 2 adjacent baseline measures. This scheme holds for the 32 photosensitive lines, and also for 2 blind reference lines used for removing the correlated pick up noise. The data are stored in a frame for the baseline measures, and a frame for the peak measures.

This array is little affected by glitches induced by charged particles, therefore long integration times are permitted. They allow to overcome the high readout noise. In flight, the integration times allowed are: 0.28s, 2s, 6s, 20s and 60s.

Glitches can easily be removed from the observations using filtering techniques. For this purpose, it is necessary to take at least four consecutive exposures at every pointing. When the integration time is equal to 0.28 sec, as the telemetry rate only allows to send one image every second, four images are averaged on board before downlink. Therefore, it is necessary to take four times four images, for deglitching purposes so the shortest observation will last four seconds.



ISOCAM Observer's Manual - V1.0
Tue Oct 31 12:06:23 MET 1995