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People don't tend to use Mewlons for DSO imaging, at least I haven't seen many DSO images taken through one. At f/12 it has quite a narrow FOV and while Tak do make a reducer for it it only takes it down to about f/9. The other issue is that while Dall-Kirkhams are extremely sharp on-axis they do have a problem with coma towards the edge of the field. Anything longer than about a 20mm EP in my scope and you will see that coma start to creep in.
The Mewlon does make a nice visual DSO scope - at least on any target that's not wider than about 40 minutes. Because it is so sharp and contrasty DSO tend to stand out very well against the background.
I think the apparent focus depth in that image is an illusion. I suspect its because the terrain in the foreground is more cratered and pitted than the smoother terrain around Pythagoras (which I guess is molten ejecta from the impact) and because the foreground is closer to the terminator the shadows stand out more. The other relevant factor might be that when you do multi-alignment point stacking and wavelet sharpening in Registax the regions of higher contrast do seem to benefit more than low contrast regions.
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