HRC-I Background Vignetting
When making an exposure map for HRC-I data, the normalized image may contain a pattern of rings. Viewing the image with ds9, using color map i8 makes these rings most evident.
The rings are the result of applying the HRMA vignetting correction in the exposure map to the un-vignetted particle background.
The effect can be mitigated by background-subtracting the data image before dividing by the exposure map.
Additional examples of the correction are on the How to Create a Background Map for an Observation page.