Now define a 2nd region file that encloses a representative area of background emission in the image. This will be subtracted from the source spectrum during the spectral fitting process. If you do not want to subtract any background emission from the source spectrum, simply click the 'Exit' button on the 'Edit Region' window.

Choose as large a background region as is practical while still excluding other contaminating sources. The intensity of the background spectrum will be normalized to correspond to the area of the source region.

The background is typically defined by a number of small regions (circles, ellipses or boxes), avoiding any obvious emission sources. The emission within all these regions will be added together to form the background spectrum.

An alternate method is to draw a single large background region and then exclude smaller regions within it around any contaminating sources (like swiss cheese). In this case you may need to temporarily move the large 'included' region to one side in order to draw the excluded regions.

More region help.