  In the framework of TRACE coronal observations, we compare the analysis and
diagnostics of a loop after subtracting the background with two different and
independent methods. The dataset includes sequences of images in the 171 A, 195
A filter bands of TRACE. One background subtraction method consists in taking
as background values those obtained from interpolation between concentric
strips around the analyzed loop. The other method is a pixel-to-pixel
subtraction of the final image when the loop had completely faded out, already
used by Reale & Ciaravella 2006. We compare the emission distributions along
the loop obtained with the two methods and find that they are considerably
different. We find differences as well in the related derive filter ratio and
temperature profiles. In particular, the pixel-to-pixel subtraction leads to
coherent diagnostics of a cooling loop. With the other subtraction the
diagnostics are much less clear. The background subtraction is a delicate issue
in the analysis of a loop. The pixel-to-pixel subtraction appears to be more
reliable, but its application is not always possible. Subtraction from
interpolation between surrounding regions can produce higher systematic errors,
because of intersecting structures and of the large amount of subtracted
emission in TRACE observations.
