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.
![[Rings in HRC-I Background]](imgs/rings.png)
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.