Last modified: 11 October 2018

URL: http://cxc.harvard.edu/ciao/why/hrc_bg.html

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]

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.