7.2.3 Extrapolated or corrupted stim-minus-background measurements
As mentioned above, long-term transients are tracked using the stim flashes. In scan maps, necessarily the first set of DCEs has an extrapolated stim calibration since there is no background for the first stim DCE. Of course, this implies that these first DCEs are of lower reliability; except for fast scan, these DCEs are generally part of the ''overscan'' region, and do not count for coverage of your requested area. This also means that you should not just grab all of the DCEs and blindly work with them, especially when you have scans over bright regions or scans that end in bright regions; see Figure 7.22, which shows a portion of a fast scan over a bright region. The artifacts apparent in the left panel are largely omitted in the right panel when the DCEs with extrapolated stims are dropped. Note that the white streak in the right is still a bright object artifact, but the effects of the bright object are substantially reduced in the final product on the right.
Conversely, however, the trailing DCEs have plenty of stim flashes, and are quite reliable. DCEs with extrapolated stim solutions can automatically be rejected by the SSC mosaicking software MOPEX. There is a bit in the bmask which indicates extrapolated stim solutions so that you can tell the software to ignore those frames.
Bad stim calibration can occur not only for extrapolated solutions, but in cases where the background DCE for the stim is on a bright region, which due to the latents from the source, can yield a bad stim-background measurement. You should try to avoid this when planning your observations (e.g., by taking a long enough scan to get enough background). You may need to correct these cases by deriving a correction based on the surrounding valid stim-background measurement; in some cases, the observation may not produce optimal results. This is a difficult problem, and if you think you have encountered it, your data may not be easily calibrated. Consider using the GeRT (see section 8.2.3).
Figure 7.22: Effects of extrapolated stim calibration. On the left, the artifacts from an extrapolated stim are clearly apparent; on the right, the DCEs with extrapolated stims have been omitted.