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How to get rid of the hard edge?

Started by awesley, 09/24/2005 06:07PM
Posted 09/24/2005 06:07PM Opening Post
All my mars images have a hard, bright edge on the sunlit side. I tried a lot of different things yesterday but I'm ^%$#&^ if I can stop it from forming.

It shows up in the stack, and even very gentle wavelets in registax make it start to appear, and it's all downhill from there - every processing step seems to make it more evident.

Does anyone have the secret about stopping or removing this?

regards, Anthony
Posted 09/25/2005 04:37AM #1
Hi Anthony,

well, I fight this effect on all my mars image this opposition. I guess it's caused by the "triple edge effect" at the hard sunlit rim showing the edge inside and outside the real edge again. I can even observe this 'overshooting' visually so it's not a pure photographic phenomena. The best explanation I heared so far are turbulences in the lower atmosphere. It's been observed at all planets with a distinctive phase - mars, mercury, venus. Of course all sharpening processings enhance the edge.
According to my experience the effect can be more dramatic in better seeing when the overshooting at the edge is smaller and the contrast of the ghost edge is harder defined, in bad seeing with a wide overshooting the contrast is lower.
I took a look at your images again and the hard inside edge and the ghost edge outside are visible but not really that dramatic compared to other images.
Alan Friedman proposed a processing variant some time ago to minimize the enhancement during processing/sharpening - I guess it can't be avoided at all if it's already present in the raw frames. IIRC he uses a layer mask or selective processing to sharpen only the inside of the planet's disk and to avoid sharpening the edge. I tried this on one of my hard edge images, selected the planet and applied some "feather" to the selection to have it fade out towards the edge and applied the sharpening only to the selection. It helped a bit but wasn't that sucecssfull - I guess the effect was too strong in my source image. I also tried to select a reference frame in Registax without the effect visible and use a harsh difference-cutoff to use only similar frames but that's not been too succcessfull either.
I've seen some people removing the hard edge in the final image (inside and outside) but I don't like this kind "repainting" the original image. The phenomena can be seen on many excellent images e.g. Damian Peach's stunning captures so I think it's not a shame to have it in an image.

Best edgeless regards,