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Double limb on planets

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#1 The_8_Bit_Zombie

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Posted 09 July 2020 - 09:07 PM

Lately I've been noticing that my planetary images taken through my C8 have had double limbs. Here are a few examples:

 

v9t5UvY.jpg

 

This does not show up in my C11, so I know it's an issue with the C8 and not the rest of the optical train. It's just weird because my C8 looks pretty well collimated so I'm not sure why this is happening. What do you all think?


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#2 deepwoods1

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Posted 09 July 2020 - 09:19 PM

While I don’t have an answer as I don’t image. I just admire. As such, I’ve seen this effect on many images. Hope you get your answer! 



#3 John_K

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Posted 09 July 2020 - 09:37 PM

Probably a combination of focus and poor seeing that's all. Also if the optics have not cooled down then I would say the effect will be similar.

 

Clear skies.

 

John K.



#4 TOMDEY

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Posted 09 July 2020 - 10:48 PM

I'm convinced it's your processing, not the telescope. Notice the phase shifts and ringing from top to bottom. That's characteristic of a very aggressive sharpening filter in software... either overtly applied, or (more common) imbedded in some other stock or toggled processing that you are using. Check your truly raw images... I'll bet they look soft, and without that processing artifact. But the images really need to be true raw or equivalent... no processing at all.    Tom

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  • 36 unsharp mask kernel ringing.jpg


#5 Tom Glenn

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Posted 09 July 2020 - 11:43 PM

This is an often asked, and often answered question here, yet unfortunately many misconceptions remain.  This is caused by diffraction, and is inherent to the telescope itself.  Nothing you can do about it.  People that don't frequently image planets (with amateur sized apertures) or read about such have the misconception that it's caused by processing.  It can be exaggerated by processing, but it is present in the raw data and is NOT caused by sharpening.  Oversharpening causes other artifacts, but the ones you are showing are routine diffraction rings.  They vary in size with the aperture and the wavelength, and are present in the raw images.  Large aperture telescopes can minimize their presence in the final image depending on presentation scale.  Another source of confusion is that many imagers will remove the artifacts in processing, giving the false impression that they were not there to begin with.

 

Many good sources about this, but it is most commonly observed on the bright limbs of planets, usually Mars, Venus, and Mercury, but also occurs within lunar craters.  See below for more information.  

 

http://www.skyinspec...s-edge-artefact

http://www.astro-ima...iffraction.html

https://www.cloudyni...ple-mitigation/

https://www.cloudyni...n-i-image-mars/


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#6 TOMDEY

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Posted 11 July 2020 - 09:05 AM

Interesting! One would expect the ringing to come in at about the Rayleigh Freq / lateral spatial wavelength, if it is due to inherent telescope-induced diffraction... For an 8-inch scope that is ~0.7 arc-sec, and my eyeball-estimated grid there is at ~0.8arc-sec. So, I'll modify my claim --- that it is the interaction of over-aggressive sharpening with the inherently affected image. Certainly something that one would want to either not enhance, or possibly suppress in processing. That is, this artifact shows up most severely, when the selected sharpening kernel reinforces the diffraction... which is what happened here. The sharpening, rather than teasing out structure in the object (Mars), is teasing out structure in the telescope's diffraction response. In that sense, Tom and I are both onto something.

 

This might also relate to why observers sometimes see canals on Mars... frequently doubled. And that debate had raged for a good hundred years... explanations ranging from true lines, to strings of craters, to wishful thinking, to visual physiology, to... diffraction!  Tom



#7 Sunspot

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Posted 11 July 2020 - 09:45 AM

Everybody talks about this, but nobody does anything about it (To paraphrase Charles Dudley Warner).  lol.giflol.giflol.gif


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#8 Tom Glenn

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Posted 11 July 2020 - 12:47 PM

So, I'll modify my claim --- that it is the interaction of over-aggressive sharpening with the inherently affected image. 

This we can agree on!  With commonly used amateur sized apertures, typically in the 8-14 inch range for planetary imaging, the angular size of the diffraction ring is very similar to other ringing artifacts, and so the two can be easily confused.  Additionally, as you point out, any sort of sharpening scheme will tend to enhance a diffraction artifact, thereby making it worse.  One of the benefits of very large aperture telescopes (which most of us don't have!), is that the angular size of the diffraction edge artifact is so small, that it could essentially become invisible in the final image, especially if the image is downsampled, or undersampled to begin with.  



#9 AstroDan2015

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Posted 11 July 2020 - 01:15 PM

Double limbs on planets can easily be remove with proper processing and post processing. You just have to know what your doing. You can work magic with different processing software, it's almost endless what can be achieved by using modern freeware. You don't even need Photoshop, if you can't afford it. Those are fine images of Mars and Venus.

 

Cheers, Dan cool.gif


Edited by AstroDan2015, 11 July 2020 - 01:17 PM.

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#10 TOMDEY

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Posted 11 July 2020 - 01:51 PM

This we can agree on!  With commonly used amateur sized apertures, typically in the 8-14 inch range for planetary imaging, the angular size of the diffraction ring is very similar to other ringing artifacts, and so the two can be easily confused.  Additionally, as you point out, any sort of sharpening scheme will tend to enhance a diffraction artifact, thereby making it worse.  One of the benefits of very large aperture telescopes (which most of us don't have!), is that the angular size of the diffraction edge artifact is so small, that it could essentially become invisible in the final image, especially if the image is downsampled, or undersampled to begin with.  

Yes, thanx for that reminder. What I had failed to realize is that the planets are "small" and our telescopes are "small". At work, we built big scopes (some ground-based, but most space-based) with apertures generally in the 1-4 meter class. So the ringing was still potentially there, but at a much finer scale.

 

Double limbs on planets can easily be remove with proper processing and post processing. You just have to know what your doing. You can work magic with different processing software, it's almost endless what can be achieved by using modern freeware. You don't even need Photoshop, if you can't afford it. Those are fine images of Mars and Venus.

 

Cheers, Dan cool.gif

I have indeed seen some wonderful planetary images that substantially avoid ringing at the scope-characteristic spatial freq. Come to think of it... sharpening right at that freq would also account for the phase-change as on traces along the ringing edges (where light becomes dark and dark becomes light) most often seen in e.g. Saturn's rings, when structure just happens to be competing with the ringing... an aliasing kinda manifetation between the inherent scope freq and that of the true object.    Tom


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#11 Tom Glenn

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Posted 11 July 2020 - 02:45 PM

One needs to be careful when interpreting the absence of ringing artifacts in amateur images.  This has become a pet peeve of mine, because many newcomers attribute the lack of artifacts to superior imaging skills, or having some magic formula for sharpening.  The reality is a bit more complicated.  While certainly good techniques in the above areas improve image quality, many imagers "clean up" the diffraction edge artifact by using methods that manufacture data.  These include a variety of tools, including "spot healing" brushes, "clone stamp", and "content aware fill" tools.  These tools all sample nearby pixels and then replace a chosen area with a different set of pixels.  These methods are very useful in terrestrial photography for removing an unwanted background element (such as a random person), from an otherwise perfect photograph.  BUT, this is airbrushing, and is generally frowned upon in many areas of photography. Nevertheless, these methods are used by some to eliminate the edge artifacts in images.  Other methods, such as non-destructive dodge and burn, can also be effectively used to ameliorate the edge artifacts without the same artificial procedure as the clone stamp methods, but even this method is not universally accepted, because it does involve the subjective manual alteration of tonal values in the image, and so it would not be acceptable in many situations (such as scientific publications).  However, as a general matter, most alterations in images are acceptable as long as one is forthcoming about the methods used.  


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#12 TOMDEY

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Posted 11 July 2020 - 10:37 PM

One needs to be careful when interpreting the absence of ringing artifacts in amateur images.  This has become a pet peeve of mine, because many newcomers attribute the lack of artifacts to superior imaging skills, or having some magic formula for sharpening.  The reality is a bit more complicated.  While certainly good techniques in the above areas improve image quality, many imagers "clean up" the diffraction edge artifact by using methods that manufacture data.  These include a variety of tools, including "spot healing" brushes, "clone stamp", and "content aware fill" tools.  These tools all sample nearby pixels and then replace a chosen area with a different set of pixels.  These methods are very useful in terrestrial photography for removing an unwanted background element (such as a random person), from an otherwise perfect photograph.  BUT, this is airbrushing, and is generally frowned upon in many areas of photography. Nevertheless, these methods are used by some to eliminate the edge artifacts in images.  Other methods, such as non-destructive dodge and burn, can also be effectively used to ameliorate the edge artifacts without the same artificial procedure as the clone stamp methods, but even this method is not universally accepted, because it does involve the subjective manual alteration of tonal values in the image, and so it would not be acceptable in many situations (such as scientific publications).  However, as a general matter, most alterations in images are acceptable as long as one is forthcoming about the methods used.  

Yep! At work, I helped build the imagers, but frequently visited the ground station analysts and got to discuss and see the ~product~ They rightfully obsessed over "target truth"  even if the rendition didn't look as pretty as the overprocessed press-release stuff. They used the enhancements that we are familiar with (and far more!), but also knocked themselves out to keep artifactuals from bleeding into the image. When given a choice... hold back on the makeup and perfume... a little goes a long way, usually too far.    Tom



#13 The_8_Bit_Zombie

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Posted 13 July 2020 - 04:04 AM

Thank you all for the replies! Really informative stuff, I've learned a lot.



#14 Jeff B1

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Posted 13 July 2020 - 08:07 AM

Maybe a discussion with our CN friend, Kokatha man, about how he processes Mars images would help.  After looking at images of Mars by Clyde Foster, Antonio Lasala, Ed Grafton, Damian Peach, Kokatha man, the late Don Parker, and several others may reveal some methods that will help.   I see very little of this “artifact” in their images.  

 

The only image of Mars I dared publish was taken with a Toucam on my 16” f/6.9 scope and except for my focusing problem and lack of a decent laptop my images of Mars did not have the defect.  Parker helped me stack and process the images and concluded that nothing could be done to make my images better because it was a little defocused and seeing was not all that great when I took the AVI.  I used Paint Shop Pro and then Photoshop but could never make my image any better.  Parker used my telescope and camera once and produced a very good Mars image; he processed it and it looked just like his typical great Mars images.  Guess the conclusion was I was the problem. cool.gif

 

 



#15 Jeff B1

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Posted 13 July 2020 - 08:57 AM

A couple great articles on the subject;  By our image guru, Darryl:  https://www.momilika.net/Index.html

 

And by the late Parker:  https://skyandtelesc...anetary-images/

 

Unfortunately, the links on the S&T article are no longer available so you'll have to dig out the magazines that info is located.  Guess the new owners haven't caught up with links yet.


Edited by Jeff B1, 13 July 2020 - 09:02 AM.


#16 Tom Glenn

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Posted 13 July 2020 - 10:12 AM

Maybe a discussion with our CN friend, Kokatha man, about how he processes Mars images would help.  After looking at images of Mars by Clyde Foster, Antonio Lasala, Ed Grafton, Damian Peach, Kokatha man, the late Don Parker, and several others may reveal some methods that will help.   I see very little of this “artifact” in their images.  

 

Jeff, the diffraction "rind" artifact on Mars is well accepted, and has been discussed many times on this forum.  Darryl has published tutorials on his "non-destructive dodge and burn" method for removal.  Other imagers use various methods for removal (some involving clone stamping and other airbrushing methods).  And as I said above, larger telescopes will keep the artifact to a minimum to begin with, and it's presence in raw data also varies from imaging sessions, likely because of the way that atmospheric turbulence obscures the diffraction effects from being recorded, especially with larger instruments.  If we were publishing rigorous scientific reports here, all of the raw video files would be uploaded into a server so that anyone could have access to the raw data.  Also, manual selection of pixels to manipulate would not be allowed in most circumstances.  But this is amateur photography, so anything goes.  My main point was that when you look at a published image, you can't be sure of what was done to it, unless you had access to the raw data, and a detailed description of methods.  



#17 Jeff B1

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Posted 13 July 2020 - 10:29 AM

Tom, I hear you loud and clear.  I wish Parker’s advice to me would have been more productive but I lost interest. Back when we used film, 2415 mostly, and worked in the darkroom for hours it was fun, but after he started with the CCD stuff it just got boring for me.  One would laugh at us having fun developing film seeing how hard it was to produce decent images.  One may think that me being a technician and/or engineer in electronics the CCD stuff would be fun, but for some reason it just did not interest me.  

 

Also, my inability to focus well played a part in it, so my forte was counting clouds and keeping track of dust storms.  I'm sure his images would be much better than mine, so that too affected my interest. lol.gif

 

He did use plenty of dodging and clever processing of his images to make them look like the real Mars.  He did not consider it cheating because he did not alter the images all that much.  I think using a 16” scope helped.


Edited by Jeff B1, 13 July 2020 - 10:30 AM.

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#18 moonwatching ferret

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Posted 13 July 2020 - 10:31 AM

Iv had that happen this year only with mars a Saturn in bad seeing . My last image of mars was on a steady night and had no rind effect whatsoever 



#19 Jeff B1

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Posted 13 July 2020 - 10:38 AM

Iv had that happen this year only with mars a Saturn in bad seeing . My last image of mars was on a steady night and had no rind effect whatsoever 

That is what I first though the rings were from; bad seeing.  


Edited by Jeff B1, 13 July 2020 - 10:39 AM.


#20 KiwiRay

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Posted 13 July 2020 - 11:17 AM

That is what I first though the rings were from; bad seeing.  

My experience is that it's worse in good seeing.



#21 Jeff B1

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Posted 13 July 2020 - 12:12 PM

My experience is that it's worse in good seeing.

ok



#22 Tom Glenn

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Posted 13 July 2020 - 01:07 PM

The relationship between the artifact and seeing is somewhat enigmatic.  We've all encountered situations in which the "rind" is better or worse, and it doesn't always correlate with the image quality.  The article from Martin Lewis that I linked to earlier is one of the best summaries I have encountered on this topic.  

 

http://www.skyinspec...s-edge-artefact

 

Notably, he makes reference to seeing conditions with the following statement:

 

"There seems to be a correlation of the phenomenon with the prevalent seeing conditions with some seeing conditions favouring its occurrence. The defect seems to be absent in images taken poor seeing or in excellent seeing."

 

​This doesn't leave many instances in which it doesn't occur! Near perfect seeing with a large instrument would minimize it greatly.  Most of us don't have that option.  


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#23 gfstallin

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Posted 13 July 2020 - 01:10 PM

This is an often asked, and often answered question here, yet unfortunately many misconceptions remain.  This is caused by diffraction, and is inherent to the telescope itself.  Nothing you can do about it.  People that don't frequently image planets (with amateur sized apertures) or read about such have the misconception that it's caused by processing.  It can be exaggerated by processing, but it is present in the raw data and is NOT caused by sharpening.  Oversharpening causes other artifacts, but the ones you are showing are routine diffraction rings.  They vary in size with the aperture and the wavelength, and are present in the raw images.  Large aperture telescopes can minimize their presence in the final image depending on presentation scale.  Another source of confusion is that many imagers will remove the artifacts in processing, giving the false impression that they were not there to begin with.

 

Many good sources about this, but it is most commonly observed on the bright limbs of planets, usually Mars, Venus, and Mercury, but also occurs within lunar craters.  See below for more information.  

 

http://www.skyinspec...s-edge-artefact

http://www.astro-ima...iffraction.html

https://www.cloudyni...ple-mitigation/

https://www.cloudyni...n-i-image-mars/

I see this often on my own lunar craters, sometimes more and sometimes less prominently. I had attributed it to likely poor seeing, but I've had it in images that otherwise attested to at least decent seeing. I wasn't even close to over-processing - any amount of processing made them more visually apparent. 

 

This is my first year imaging Mars with any real effort. I've been rewarded with the rind effect. It is good to know that there isn't anything that I'm doing to cause this in the first place. 

 

George



#24 Jeff B1

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Posted 14 July 2020 - 05:54 AM

Maybe there is a problem in the stacking software?  Just asking....




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