3.1.3.6 `Smear' Exposure Times

Because ACIS operates without a shutter every Timed Exposure Mode observation CCD frame contains 41 milliseconds of data collected while the CCD is being clocked out. This has several implications which should be considered.

Images of bright point sources will have faint vertical trails which extend to the top and bottom of the CCD frame. This can profitably be used to extend the dynamic range of the CCD, because the effective exposure time gets as short as in Continuous Clocking Mode. So for a full frame exposure 3.3 seconds of integration time occurs while the target is stationary, and 41 milliseconds occurs with the target image smeared over 1024 vertical rows of pixels with the width of the point spread function of the telescope.

Of course any point or diffuse sources within the smear band will also be smeared and the contributions from each will be confused.

Note that the image smear may be particularly useful for gratings observations as the zeroth order image will be frequently piled-up or saturated, but the `smear' image can be used to find the centroid of the zeroth order image in the dispersion direction which is critical for establishing the wavelength zero point.

Note also that for the shorter frametimes coming from Sub-Array readout and Stagger readout, that the smear data will be relatively stronger than in full frame mode.

Care must also be taken in calculating integration times for observations due to the smear time. For a point source the integration time per CCD frame within a PSF sized region is equal to the dwell time (for a full frame this is 3.3 seconds). The repetition rate of frames includes the smear time (for a full frame this is 3.34 seconds). For a diffuse source, or if a larger spatial region on the CCD is selected, then the integration time per frame should be changed appropriately.