[Shiraz (Ray)]

Took some lunar images recently and noticed (again) slight ringing artefacts in the dark regions of craters (see example attached), where there is a thin white line inside the core region and the central dark part is not black. The dark crater and rille regions also throw slight shadow lines into the brighter zones and the small craters look more like pimples than depressions.

In an attempt to understand what is going on, I made the synthetic black and white image shown as the upper section of the insert in the image below and;
1. convolved it with an "Aberrator" point spread function for my scope to simulate the image formation process
2. sharpened the resulting blurred image using a VanCittert deconvolution. this algorithm is typical of sharpening operators in that it boosts image content at high spatial frequencies.

The result is shown in the lower section of the insert (with dynamic range reduced) - all of the features of the real image are reproduced (I didn't adjust the scales yet), from the the thin white lines in the dark regions to the lightening of the central zone and the external dark lines. I could not get this result with any other approach.

Thus, it seems that the artefacts are a normal outcome of applying sharpening to images that have been blurred by an Airy pattern. Probably have to live with this, but there may be implications for optical design and quality - eg do SCTs or mirrors with low Strehl, which both have stronger Airy ring structures, produce more pronounced artefacts? And how much damage does this do in lower contrast images where there are fewer step changes in intensity? ...And maybe it is possible to tailor a sharpening algorithm to take account of the real PSF of the scope that took the image. Food for more thought. regards Ray