ctools

I suppose in order to get an uncorelated skymap in RING bkg method the roiradius user parameter can be chosen to be <=1/2 of the specified pixel extension.
Anyway for much smaller values (e.g. zero but starting for values ~<1e-9) artefacts are produced in the maps. These are certainly due to numerics present in the pixel selection process. Hence a small fix in compute_ring_values() solves that for me:

// ... otherwise check if pixel is inside source region
else if ((position.cos_dist(skydir) >= m_cos_roiradius) || (ipixel==i)) {

instead of

// ... otherwise check if pixel is inside source region
else if (position.cos_dist(skydir) >= m_cos_roiradius) {