Multi-Scale Clean: A Comparison of Its Performance Against Classical Clean on Galaxies Using Things

A practical evaluation of the multi-scale CLEAN algorithm is presented. The data used in the comparisons are taken from The H I Nearby Galaxy Survey. The implementation of multi-scale CLEAN in the CASA software package is used, although comparisons are made against the very similar multi-resolution CLEAN algorithm implemented in AIPS. Both are compared against the classical CLEAN algorithm (as implemented in AIPS). The results of this comparison show that several of the well-known characteristics and issues of using classical CLEAN are significantly lessened (or eliminated completely) when using the multi-scale CLEAN algorithm. Importantly, multi-scale CLEAN significantly reduces the effects of the clean "bowl" that is caused by missing short-spacings, and the "pedestal" of low-level un-cleaned flux (which affects flux scales and resolution). Multi-scale CLEAN can clean down to the noise level without the divergence suffered by classical CLEAN. We discuss practical applications of the added contrast provided by multi-scale CLEAN using two selected astronomical examples: H I holes in the interstellar medium and anomalous gas structures outside the main galactic disk.