When a pixel is saturated, a responsivity change occurs in the corresponding readout. This results in a ''jailbar'' pattern occurring every 4th row/column for saturated sources (including some cosmic rays); see Figure 7.7. Only the portion of the array read out after the saturated source is read is affected. Typical effects of jailbars from point sources saturated only in the core produce a decrease in the flux of affected pixels at about the 2 MJy/sr (50 microJy/arcsec2) level, but the magnitude of the effect may vary with source flux.
The jailbar effect can occur in a number of different ways. For extended sources, often the saturated regions will cover all 4 or more contiguous readouts and thus an altered responsivity level is seen but no obvious jailbars. One may also see strong jailbars where there are no apparent sources. We believe this effect is due to a highly saturated cosmic ray hit. Faint jailbars are often seen in lower-coverage regions of photometry mosaics where the dither pattern stacks the jailbars. Many of these effects can be mitigated in the post-BCD stage using outlier rejection.
The fix for strong jailbars caused by saturated sources is additive - we recommend that you adjust the appropriate column (or portion of the column) additively to match the median level of the unaffected region of the column; see Figure 7.9 and Figure 7.10. There is contributed software for the correction of these strong jailbars available on the Data Analysis section of the website. Weak jailbars are corrected in the BCD pipeline.
Figure 7.7: Example of ''jailbar'' effect from a saturated point source; note that the jailbars are stronger after the bright object. Bright latents (above the source) can also be seen. The central pixel of this bright object is hard saturated (no replacement is available), so it appears black.
