3.1.5 Bias Determination

In general operation ACIS will produce far too much raw pixel data to allow all pixel values to be telemetered to the ground. Because of this, the on-board processing is required to filter the data by actively recognizing the pixels which contain information relevant to an X-ray detection (and hence allowing the information about other pixels to be discarded).

In practice this requires that ACIS internally maintain a set of values for each pixel in every CCD chip which corresponds to a `dark' level, i.e. the signal level which corresponds to no incident X-ray produced electrical charge. This image is called a `bias' frame, and is normally subtracted from a data image prior to any further processing.

Currently three bias algorithms are available in the flight software consideration and each has merit for some applications (see Sect. 3.1.5.1).

Bias frames will have to be recalculated after most changes of mode. In particular, changes from continuous to timed, changes in timed exposure durations, and changes to sub-frame windowing sizes will require new bias calculations. Furthermore, the size of the bias frame information precludes storing this information in radiation hard memory, but instead radiation tolerant memory is used. As a result of this we expect that the bias frame will acquire errors due to radiation induced bit errors during perigee crossings of the satellite. Thus we recommend new bias maps be routinely calculated after each perigee crossing, and after each initialization of the instrument following a radiation induced shut-down.

Because the calculated bias directly affects the inferred X-ray signal, it is important to telemeter the bias values to the ground. There are several options for this, typically involving the choice of on-board data processing program chosen (see Section 3.1.3).