The OTA CCDs have known artifacts and anomalies. A lot of work has gone into characterizing these artifacts, and removing them if possible. Pixels affected by these artifacts or anomalies are kept tracked of in the mask images with pixel flags.
Row-by-row bias correction
Certain CCDs suffer from an issue whereby the bias level varies between rows. A software fix based on a linear fit to the bias in each row (check if this is correct?) has been applied on CCDs prone to this effect, however
- sometimes CCDs are not identified as problematic so this issue is not corrected.
- in areas of strongly varying background (such as large galaxies) the fix itself can introduce extra problems as the bias level cannot be correctly estimated.
Example of problems on a warp caused around a bright galaxy by the row-by-row bias correction.
An example of uncorrected row-by-row bias effects imprinting through onto a stack.
The sky subtraction routine is known to over-subtract around large galaxies or areas of nebulosity. There is currently no fix for this.
On some stacks, if one or more input warps has poor astrometry this can imprint through to the stack (in particular, supposedly masked areas such as the centres of bright stars can show a background star field poking through). This is quite rare and mostly seen in crowded fields.