All CCD sensors have a maximum ”well capacity”. If the number of photons detected exceeds this amount, the pixel will overflow into adjacent wells. Normally the photoelectrons are held in place by electric fields generated by the overlying electrodes. But when too many electrons accumulate, their own electric field swamps out that from the electrodes. The electrons are still constrained horizontally, because the columns are defined by insulating oxide, so they migrate vertically. In addition, some electrons become stuck in ”charge traps” and do not leave the pixel until after a number of clocking operations. These effects cause the overflow to ”bloom” vertically.
For science observations, the data in these pixels is lost. On the other hand, for general imaging purposes the blooms can be simply edited out. This is very tedious to do by hand, but the Bloom Removal tool can do this quickly and effectively. First the bloom is identified. Then the bloom is deleted and replaced by interpolated data. Finally the star image is cleaned up to make it round.
To ensure a clean removal, the tool automatically adds noise to the pixels it is replacing. This emulates the appearance of the background present in adjacent areas. The background statistics are obtained by looking at pixels near the bloom; but they can be manually adjusted for best effect.