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Wikipedia:Featured picture candidates/Saturn's rings from Cassini

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Saturn's rings from Cassini

[edit]
Saturn's rings from Cassini. The outer rings are made visible when backlit by the sun.
Original file
Edit 1, by Fir0002 - a less extreme edit

I saw the image while browsing on the Internet, so I thought that it would be an excellent addition to our Saturn article. Well, when I was looking where to add it, it turns out it already was uploaded here, and was put on Rings of Saturn. It has a good caption, and is unique due to its rarity, has adequate resolution, is accurate, pleasing to the eye, in the public domain ({{PD-NASA}}), and of overall good quality. I believe it exemplifies Wikipedia's best work.

Reason is that the image nominated is not the original file as produced by NASA but an exaggerated edit. I've added the original which I feel is far superior. For interest, this is the original source of the nominated image I think: Image:Saturn eclipse exaggerated.jpg --Fir0002 21:44, 11 November 2006 (UTC)[reply]
I'm still a bit suspicious of how they managed to show the far side of Saturn illuminated. I understand that this sort of image is inherently a composite but the exposure difference between the sun-lit rings and the far side of the planet receiving only starlight should be enormous - far greater than it seems to illustrate. Diliff | (Talk) (Contribs) 13:24, 12 November 2006 (UTC)[reply]
I see what you mean, however (at least for the outside of the planet) being a gas giant would mean that a certain amount of light would be able to diffuse through the atmosphere. This could then presumeably be reflected so that the center is somewhat illuminated. The other thing is, I'd expect NASA to have some pretty high tech camera gear and the dynamic range they can capture (particularly since they are taking photos of objects in darkness) may well be much higher than what you (or I) have experienced with DSLRs. --Fir0002 06:00, 13 November 2006 (UTC)[reply]
I think that the dark side is illuminated by the light diffusely reflected by the rings. Olegivvit 11:43, 13 November 2006 (UTC)[reply]
I don't buy that - the ring is essentially a flat disc. Any light diffused by it would have to: 1> penetrate through the 'structure' of the disc for millions of kilometres to reach the area of the disc not exposed by the sun's rays then 2> diffuse again with enough light to reach the observer so that it is only minimally less bright than the directly exposed area of the rings. It just doesn't sound plausible to me. Compare diffused light to direct light and you will see just how little of it is actually reflected. Then consider it diffused twice over great distances (where only a small amount actually reaches a specific destination) and I can't see how there would be enough left to reach us in a visible amount. Diliff | (Talk) (Contribs) 12:17, 13 November 2006 (UTC)[reply]
Maybe you misunderstood me. I mean, the dark side of the planet surface is illuminated by the light diffusely reflected by the rings. Then we see the rings in transmission. I agree that this light is reflected twice, but in the image we compare it with the light reflected once, not with direct sunlight. Thus there is one order difference. The rings are quite large and can give enough light. Olegivvit 18:32, 13 November 2006 (UTC)[reply]
I don't think I misunderstood. I understand that there is only one order difference but you can't use their size as a reason for the brightness. The size of the reflective surface of the rings is not really relevent as the rings might be large but the area that they need to illuminate is also extremely large. Light would have to reflect/diffuse off the rings, penetrate through the rings for millions of kilometres (because aside from a second reflection off the planet itself, that is the only path that will reach the rings) before reaching the 'dark side' and then diffuse again in all directions before finally reaching the CCD on Cassini. There would simply be too much light lost in the process in my opinion. Notice the area at the bottom-right of Saturn where the rings meet the rear of the planet. There is a definite shadow on the ring where the sunlight is blocked by Saturn. The far side of Saturn is quite a lot brighter than that shadow, despite being much further away from the sun-lit rings than the shadow. That to me seems like pretty conclusive evidence that this image is a composite, possibly with very different exposures for the planet and for the rings. Diliff | (Talk) (Contribs) 11:33, 15 November 2006 (UTC)[reply]
...you can't use their size as a reason for the brightness I can. The amount of light received by a unit surface is determined by integration of incoming light over the solid angle above the surface. If the moon would have its visible area doubled (by being closer to the earth, for example), the illuminance (and therefore the brightness) of a surface illuminated by it is also doubled. The rings of Saturn are very close to its surface. They probably cover one-third of the sky when viewed from the Saturn's surface. About a half of the rings is in the shadow. So, I estimate the illuminance of the surface to be about 1/6 of the brightness of the rings. ...surface of the rings is not really relevent as the rings might be large but the area that they need to illuminate is also extremely large. Wrong. The illuminance of a surface by a source does not depend on what else is illuminated by this source. It depends on brightness of the source and the solid angle covered by the source. ...penetrate through the rings for millions of kilometres before reaching the 'dark side' First, the thickness of rings is 5-30 meters. Second, transmission through the rings is 8-95%. Third, there is no need to penetrate through the rings before reaching the surface. ...There is a definite shadow on the ring where the sunlight is blocked by Saturn. I see it. ...The far side of Saturn is quite a lot brighter than that shadow, despite being much further away from the sun-lit rings than the shadow. Distance does not matter in this case. The ring cannot be illuminated by the ring itself because it is in the same plane.
If, as you say, the light is reflecting off the rings, onto the surface of the planet, then back onto the ring not illuminated directly by the sun, then reflected again to Cassini, then WHY is there a black gap between the the directly illuminated rings, and the edge of Saturn (which you admit you see)? Surely it should be illuminated in exactly the same way as the rest of the ring receiving reflected light from Saturn? Why are there rings in front of Saturn that are quite a lot brighter than the equivalent point of the rings in the 'gap'? Diliff | (Talk) (Contribs) 13:26, 16 November 2006 (UTC)[reply]
I made an illustration.There is the first order reflection from rings and the second order from the planet surface. The rings in the gap are poorely illuminated because they can receive only light reflected from the surface. Thus, it is the third order. The rest of the ring in the shadow is visible in transmission, not reflection. Olegivvit 14:12, 16 November 2006 (UTC)[reply]
I see what you are saying now, and this may be the case, but then why do the rings correspond directly to the darker and lighter parts of the reflections, and why is the exception to this the area above the rings where there is a thick dark line and a transition to brighter above it? A non-transmissive ring? Diliff | (Talk) (Contribs) 14:35, 16 November 2006 (UTC)[reply]
Outside the shadow: Brightness of a ring in transmission scattering depends nonmonothonically on its density. A thin ring is poorely visible because there are few particles which can scatter the light. A thicker ring scatters more light and is brighter. When a ring is very thick (the central dark ring), the light is lost inside it. Inside the shadow: Darkness of a ring in transmission depends monothonically on its density. The thicker is a ring, the darker it is. The central ring is thick and dark. The internal ring is almost transparent. We see the unevenly illuminated surface of Saturn through it. The surface of Saturn is brighter below the equator (reflection scattering), dark on the equator (in the plane of rings), and less bright above the equator (transmission scattering).
I disagree with NASA having higher tech camera gear that we are used to now. The probe was launched almost ten years ago. Just think about what the cutting edge was back then (In 1995, a 1.3 megapixel Canon DSLR = ~USD$15k). And then there is the fact that as NASA's gear is not mass produced, and have to be built to withstand huge environmental stresses, it would be far more costly still. To be completely honest, I don't think NASA's technology is cutting edge in terms of electronics anyway really. Cutting edge in terms of space/aerospace engineering, perhaps, but not electronics. The consumer electronics industry simply has a greater market and budget. Diliff | (Talk) (Contribs) 12:01, 13 November 2006 (UTC)[reply]
Digital cameras used on imaging satellites are usually custom made. In terms of "higher tech", I don't think consumer cameras and satellite camera are on the same playing field. Consumer cameras are used for everyday shooting in visible light, whereas satellite imaging cameras need to have super high-resolution imaging sensor(s), have multiple filters for different wavelengths, and be extremely durable (and are the size of washing machines). =) Jumping cheese Cont@ct 10:47, 24 November 2006 (UTC)[reply]

Promoted Image:Saturn eclipse.jpg. The NASA original has it. Raven4x4x 05:28, 25 November 2006 (UTC)[reply]

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