Hi
In a recent post, Satchmo questioned the level of sharpening used by Paul Haese on a Saturn image. That raised some interesting questions and the following is my take on the process used for sharpening planetary images. It hopefully shows why Paul and others end up with more realistic levels of detail than can be seen through a scope. Would welcome any discussion.
Regards Ray
The attached top left image shows Saturn as seen by Hubble some time ago (this is what it actually looks like). The blurred image below it shows what happens when a low pass filter (representing the point spread function of my 12 inch telescope) is passed over the image. This is about the best visual image I could see if I was in space (or in perfect seeing), but it looks flat compared to the Hubble image and doesn’t capture the crisp nature of the real thing or show much of the true ring detail.
Unlike visual, a digital image can be manipulated to restore some of the detail that has been reduced in contrast by the optics MTF – to “crisp up” the image again. The central image shows the blurred image after sharpening, where the enhancement has been sufficient to restore much of the detail in the globe (by my judgement) and make it look about as “crisp” as the original, although lacking the finest detail. Along with enhancement comes artefacts – Cassini is a bit wider and there is lightening of the B ring near Cassini and the A ring outside Encke. This is unavoidable - by definition, you always get artefacts when you sharpen. The tradeoff is to get the best possible restoration of fine detail while minimising the unavoidable artefacts – that is a judgement call. The central image represents about the very best image I could expect from my scope if there was no atmosphere – it is significantly more realistic than the visible view through the same scope.
The rightmost image is from a good imaging session in the real world (note colours are different as these vary from time to time). The atmosphere added blurring and the noise is high because the majority of the AVI frames were rejected for low quality. I sharpened fairly heavily to restore some of the finer detail and ended up with significant artefacts at the bright/dark boundaries. However, the main ring features are there, including Encke, the structure in the B ring and a C ring of appropriate brightness, complete with darkening near the B–C junction – the worst of the associated artefacts is the lightening of the B ring at the Cassini-B junction and at the ring/planet junction. It is not a perfect image, but it is close to the best representation I could get from the available data (in my judgement) – erring slightly on the side of having too much sharpening.
So in summary, the lucky imaging process lets us get rid of much of the atmospheric degradation and the sharpening lets us restore some of the fine detail contrast lost in the combined optics and residual atmosphere MTF. The results are different to what can be seen through a telescope, even in perfect conditions – they are actually significantly closer to reality.
Ref:
http://hubblesite.org/gallery/album/...system/saturn/