In particular, for a point source the residual left on the dirty image is . In typical VLA applications a reasonable compromise lies in the range 0.1 0.25. Lower loop gains may be required if the u , v coverage is poor, but the improvements in deconvolution for 0.01 are generally minimal. The choice of the number of iterations depends upon the amount of real emission in the dirty image.
Possible culprits are the gridding process, aliasing, and finite precision arithmetic.
In the case of the Clark or the Cotton-Schwab algorithms, the truncated dirty beam patch that is used in the minor cycles of these algorithms must violate Schwarz's conditions. Therefore both may be subject to instability or divergence if the minor cycle is prolonged unduly.