what should Saturn look like?

[Shiraz]

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/solar_system/saturn/

[von Tom]

Nice comparison and analysis Ray. Thanks for posting. I know when I do my processing (especially sharpening) of Saturn I'm aware of trying to avoid introducing artefacts, especially in the rings as too much sharpening can add false rings etc. the images and posts on this forum have encouraged me to get the most I can from my images, particularly because they show detail that I never otherwise would have looked for. Images like yours help me judge what is really there and what is not.

Imaging and presenting Saturn can be a little subjective as sometimes ring fidelity/sharpness/colour is sacrificed for cloud feature details and vice versa. It depends upon what the imager is trying to depict or highlight. When I first started with Saturn I was mainly concerned with capturing the storm on the disc, so the rings were secondary to what I thought was of interest. I traded disc contrast for the C ring so that is wasn't visible, for example. At the moment I'm trying for better definition in the rings as the only cloud detail I can pick up on with my equipment is the hexagon (again, without others posting here probably wouldn't've have looked for it).

Cheers,
Tom

[Shiraz]

thanks for the interesting comments Tom.

We all face a balancing act depending on what we are trying to do and what we end up with is a matter of interpretation. As a group though, we seem to be unwittingly using evolutionary techniques to converge on a "true" representation, so Mark's question was a valuable sanity check.

Until Mark's post, I hadn't fully realised that we are actually trying to get reasonably close to what a planet looks like from space - without an intervening atmosphere or scope. It is unrealistic to compare what is seen in a scope with what can be dug out of an image with sharpening, since there is no way to boost the contrast at high spatial frequencies in visual viewing - processed images should have much more detail than can be seen in a scope.

Regards Ray

[bird]

Saturn is a very difficult target to process - the high contrast edges everywhere around the rings are artifact-magnets, even the slightest drop in seeing (or seeing "cells" that drift across in front of the scope) will cause ghosting and double-edges.

Pretty much any enhancement you try, if applied to the image as a whole, will also cause the edges to over-sharpen and look nasty.

On the whole, it's damn near impossible to get pristine rings and still bring up enough detail on the globe to look interesting.

Bring back Jupiter, it's a whole lot easier...

cheers, Bird

[Shiraz]

I assume that seeing cells are the source of the ghost images that can appear in individual avi frames - no way to process them out.

re Jupiter - it's not so easy at 30 deg alt :)

regards Ray

[bird]

Hi Ray, yes that's my assumption as well, the cassini division in particular seems easily affected by this, I guess due to the sharp edges.

We have to wait a few years, but we'll get Jupiter back at high altitude again soon...

cheers, Bird

[Paul Haese]

My aim is always to produce an image that looks like it was taken from space. I have a body of work dating back to 2004 and can say my processing has undergone many changes in technique over that time. However I have always tried to maintain fine detail with as low noise as possible in my images.

I do however think that people who know nothing about imaging should keep their opinions to themselves as they have no idea what is involved.

The view through a telescope is vastly different on most occassions to what you can image and extract from data. There is only one occassion that both Anthony and I know showed almost picture perfect views of Jupiter. That was in 2006 during our visit to the Grampians. Detail was so nice visually one could see tiny cyclones and very fine detail in my 18" SDM. I have not seen seeing like this again.

[clive milne]

Your level of honesty is reflected in your work.

[barry2718]

A fascinating read. This explains my difficulty in seeing the ring planet in my Celestron (8"). Last night as it rose the distortions where very evident and reduced as it got higher. At 10pm the moons where evident close to the planet in my 8" SCT but i could still see the image tearing. The best viewing was with a 21mm eye piece on the 2000mm focal length scope. With the 6mm eye piece it was very blurry and distorted. I guess I can only dream about seeing Saturn as good as shown here. Barry - Macarthur Astro.

[Quark]

Hi Ray, very well put. I have often heard people remark that Saturn should be depicted as it appears in the eyepiece, I generally say to them, why would they want to aim so low. My 16" F4.5 Newt has a F/L of 1836mm but I image at a F/L of 9669.6mm.

With a state of the art CCD and high frame rates, the ability to stack many thousands of frames, thus improving the signal to noise ratio means that the detail that can be resolved now by amateurs is right up there, better than the professionals could produce 10 years ago. Birds Jupiter from Exmouth, in my opinion, is the best Jupiter image obtained from Earth.

The professionals are becoming ever more aware of the quality and resolution of amateur work and are actively seeking out amateurs to support their research. A very current example of this involves both Bird & I. We are supporting a research team from JPL, supplying possible target features along with drift charts created from our own data that accurately predict the positions of these features. The team will have Subaru on April 30th and then move over to the IRTF May 1st.

To show the detail that is of scientific interest means, on occasions, processing deeper than normally we would to just create an esthetically pleasing image. The researchers are not interested in any way with artifacts that may be created in the rings due to the level of processing applied to resolve the target detail or structure that they are interested in across the disk of the planet.

It takes me a lot of hours to process my data from a good session, all of it is sent to the various reseach groups I am involved with and my processing is always done with the view to hi-lighting the detail & structure I know they will be interested in. The same data is posted here on IIS, I simply dont have enough time to do another version of it.

[Shiraz]

thanks for those comments Paul - things sure have changed a lot over the decade since you guys first started using high frame rates for planetary imaging.

Really good visual viewing conditions are to be treasured since they seem to be so rare.



thanks very much Clive.



Hi Barry. FWIW I find that seeing is generally best when the jetstream at 200hPa is below about 40 kts - that does not guarantee good seeing, but a fast jetstream generally rules it out.



Hi Trevor. Hope all went well with your graft.

You raise many extra issues in your post, in particular the lack of interest of the science community in how pretty an image is - when trying to dig out a particular feature, who cares what artefacts appear elsewhere. Your use of animation to find persistent features allows quite low contrast regions to be confidently enhanced for science purposes.

It would seem that, for this sort of planetary imaging, scopes around 14-16 inches currently hit a sweet spot. Bigger scopes (eg the 1m at Pic du Midi) produce some excellent results, but they do not seem to be all that much better (if at all) than a good C14 or Newt (eg Bird's standout Jupiter). Maybe there is an underlying level of atmospheric blurring that limits what can be done with larger apertures and/or higher framerates. What do you think?



Thanks all for the iteresting discussions. Regards ray

[Quark]

Hi Trevor. Hope all went well with your graft.

You raise many extra issues in your post, in particular the lack of interest of the science community in how pretty an image is - when trying to dig out a particular feature, who cares what artefacts appear elsewhere. Your use of animation to find persistent features allows quite low contrast regions to be confidently enhanced for science purposes.

It would seem that, for this sort of planetary imaging, scopes around 14-16 inches currently hit a sweet spot. Bigger scopes (eg the 1m at Pic du Midi) produce some excellent results, but they do not seem to be all that much better (if at all) than a good C14 or Newt (eg Bird's standout Jupiter). Maybe there is an underlying level of atmospheric blurring that limits what can be done with larger apertures and/or higher framerates. What do you think?



Thanks all for the iteresting discussions. Regards ray[/QUOTE]

Hi Ray, well, my hand and forearm don't look pretty but the wires have been successfully removed. Reckon it is a waiting game now to see how it all pans out regarding the success or not of the graft, my problems fade into insignificance compared with what has just happened in Boston.

Regarding the effects of aperture on image quality, for sure the 1 metre scope at Pic du Midi has produced some very fine planetary images and there is no disputing that.

I think the major thing we would take from a comparison of Birds exceptional Jupiter from Exmouth is that given exceptional seeing a smaller scope can produce images as good or better than a larger scope can in a bit lesser quality seeing. My point being that the seeing Bird experienced on that particular occasion was likely better than the seeing for the images that I have seen from Pic du Midi.

Now I am not for one minute saying that the seeing at Pic du Midi is inferior, only that in at least one instance it has been better at Exmouth.

I would conclude therefore that of all the contributing factors involved in producing high class images, the quality of the seeing is far and away the most significant one.