A Pixon is a generalization of a pixel. It represents a single degree of
freedom in the construction. Pixons are used to control the number of
degrees of freedom by locally smoothing the image while still satisfying
the constraints imposed by the data.
(Metcalf, 1999)
The Pixon method is the slowest of the RHESSI reconstruction algorithms, mainly because it has to discover the best Pixon basis for the map (a computationally intensive process) as well as minimize the difference between the observed count rate profiles and the model countrate profiles.