Some thoughts on a Transit of Io

This is an engaging speculation. I really want to pursue it.
Pete

Speculation just ended for me, Pete, on Dec 2 at 1700UT. Ganymede, Io, and Europa all in a row trailing Jupiter on the Zenith in 9/10 seeing and 384x - all three in the same FOV.

Ganymede is easy to ID, I actually picked Io out of the line up because Europa was perfectly circular. Io was not. I would never have believed such a thing until my lying eyes showed it to be true. It can most certainly be done.

I did a little deeper research on this topic and I think that the elongation that you guys are observing might be a result of Io’s constant volcanism (apparently the most active in the solar system) and that Jupiter’s magnetic field is perpetually stripping away and absorbing the moon’s temporary, though constantly replenished, sulfur dioxide atmosphere.

One interesting thing that I read on Wikipedia (link below) is [that] “…this material escapes Io's gravitational pull and goes into orbit around Jupiter…these particles spread out from Io to form a banana-shaped, neutral cloud that can reach as far as 6 Jovian radii from Io, either inside Io's orbit and ahead of the satellite or outside Io's orbit and behind the satellite”. I’m certainly no expert on this topic, but this “might” explain it?

I’m wondering if I can see this effect through my 90mm (f/ 910) using a 6mm EP and a x2 Barlow. That would give me what…303x?

http://en.wikipedia....7s_magnetosp...

Hi, Stray, will read your link in a moment. Just wanted to post a sketch from last night, during opposition.

Ganymede seemed to sport a slightly less bright Jupiter-side limb. It might show in the sketch. No bright specks noted. Callisto is really more "beaver" color. Yes, that's a color very close to a grey brown. (Beaver #9F8170 R62%, G51%, B44%.)

http://en.wikipedia..../List_of_colors

Europa was strikingly yellow with a faint distinct diffraction ring. Io was more orange with a hint of reddish. It also sported a fain diffraction ring. But, here's the thing. Europa was distinctly circular, Io is not. In fact, it's diffraction ring was slightly distended.

I tried to capture as accurately as possible that difference. Seeing during opposition was 9/10 and Jupiter was on the zenith around 1630UT. This sketch represents about 1630UT, but leaves Jupiter as it was at 1430UT and only for reference to simulate the view.

A better look...

Cool looking.

Rich (RLTYS)

It is perfectly circular.

In the C14, I see it as distinctly circular. What I observed though is that there appears to be some albedo darkening at the north and south.

In a small scope, I think you are basically just getting some diffraction blurring of the central "strip" so that it is "Stretching" the central band.

I may have trouble describing this, but let me give it a try.

Suppose you have four or five Airy Disks in a line with the edges slightly overlapping. At the top and bottom of this "Bar" the light from the diffraction will tend slightly fatten this bar so that it will appear slighly wider than it is.

At the end of this bar though, the light will appear to make the bar stretch slightly.

Because in smaller scopes, the central part of Io's disk is only maybe a few Airy Disk diameters, you can see how a brighter equitorial region with darker northern and southern hemispheres could start to look almost like a football.

But in the C14, it looks very circular. I see albedo darkening at the hemispheres, but it still appears circular.

The effect though, contributes to the "Pearl" appearance. It is like there is a luster to the circle.

But it does appear like a circle. Diffraction I think could make it look more like a "Dash" in a smaller instrument.

If one wants to bother to compute the Airy Disk and firt ring diameter of their scope and super-impose these over the center of Io, I think you would see that Io only appears a few Airy Disk diameters wide, and a bright diffraction ring at either side would then show the "elongation.".

Eddgie, makes sense. I suspect you are correct, Io is indeed circular in larger apertures. I just could not "resolve" the polar albedo as you could. So, yes, the overlapping /spurious/ discs would tend to elongate Io in smaller apertures. One can easily imagine a C14 would better resolve Io showing even the polar regions giving it a circular appearance with a brighter equatorial region. One might argue that's more true resolution.

In a 6", Io at 1.7" is just a bit smaller than the Airy disc diameter (1.82") but also nearly twice the diameter of the (series of) spurious disc(s) at Io's magnitude with the CO considered. The combined diffraction rings would sum into a slightly elongated first ring much like a brighter, very close double of equal magnitude (72 Pegasi at 0.5" arc, for example.) It might be that the dimmer poles contribute less to the diffraction ring observed.

The idea Io simply "appeared" elongated, even though it is not, is perfectly sensible. While not true resolution in the Raleigh or Dawes sense, maybe the lack of observable polar spurious discs could be considered resolution in much the same way Cassini is resolved through lack of any light emitted from it while the brighter bordering regions offer such diffraction.

It is a stunning revelation, none-the-less. Never would have thunk it possible to see such variation in Jovian moons we normally think as simply "discs." Never would have had reason to believe otherwise until this thread.

An afterthought on Io's color. Reading Stray's article above, Io is sulfur and silicates, and high res images show a distinct yellow color. That actually confuses me, because I found Io to be more orange-slightly red and Europa to be yellow. Io's poles, apparently, are more orange than it's yellow equator. And I do no think I am seeing much light from the poles.

So, why is it orange? Not sure. In fact, that night observing the moons, I made the initial ID on Io based not on it's color, but that it was (indeed, appeared to be) more elongated than the yellow one further east. The red, elongated one had to be Io, and indeed it was. That, itself, was stunning.

I'm getting what Eddgie is saying and its what I guess would be the case and yes Io is truly round afterall but the meat and potatoes here is that this is a heretofore unheard of achievement in detection and resolution with a 6" aperture. Time waswhen the four moon's of Jupiter written about in books were noted on magnitude difference and by the way the shuttled around back and fourth. They were treated as ornamental things framing a more worthy object and that was that. Then it became written about that they could be resolved as discs and so that added something in the guidebooks. Beyond that some shadings were mentioned ad visible in only the larger Scopes and anyway Jupiter is the main subject so moving on. .. then some time in the 90s Gary T. Nowak of Vermont's Astronomical Society turns his ten inch Trischiefspeigler on Ganymede with the sole purpose of going out on a limb and looking for detail inwhats normally big scope territory. He sees the detail then goes a step further and even manages with a 6" apo. He notifies Alan MacRobert at Sky and Telescope who realizes the gravity of the achievement and publishes Nowaks finds. Gary redefined what the limits were. For having a visa to detect albedo shading and I for one NEVER looked the same way at the moons in general again. What's happened here is the next progressive step and it wad wild to see it unfold and so unassumingly. I truly wrote this off as seeing related.

Someone ought to pass this on to Gary.

Great stuff.

Pete

Well, I think amateurs have been reporting seeing albedo features on Ganymede using 6" instruments for many years. I know I have seen them in 6" refractors from time to time.

And I have done much better than this on Ganymede with the C14, having resolved Osiris and Galilee Regio as distict shaped features, and I know this has been duplicated by one European observer using an 8" APO.

Io is much harder though. There have been several good C14 images that show very clearly resolved surface structure, but visually this is very difficult. I have seen shadings hinting at structure on Io, but not what I would call "resolved features."

I think that it is good that people are reporting these things because it will encourage others to look.

But Norme I think has "the right stuff."

The "Right stuff" is patience and persistance.

I feel like I have done some very excellent observations too, and patience and persistance have been the most important tools... More important than what eyepiece you use for sure.

I often observe Jupiter for up to an hour waiting for moments of good seeing. And a funny thing happens... In moments of good seeing, your eye will catch a detail. A few minutes later, your eye will catch another detail.. And your brian I think starts "Building" a picture. And suddenly, even though seeing is not really any better, it seems like you are now seeing a lot of the detail that previously was difficult.

Some people refer to this as "Mental Stacking" and I am a believer in this description.

People should look. Even if conditions are not great, they should get comfortable and keep their eyes open (I binoview.. LOL). And with patience and dedication, it happens.

Eddgie is correct, partially, persistence is important. His own observations as does Jason's persistence resulting in this thread attest to that. Most of our observations require persistence, patients, and a desire to observe awesome and powerful natural beauty of places we cannot visit. And that Eddgie and Jason all report Io sightings is ground breaking fro amateur visual observing.

Observing is one thing, equally important is people like Pete pushing back the observing boundaries we read about in books. Forget Dawes, forget even Sparrow, go deeper and you will be surprised. He is absolutely correct. I never would have attempted 72 Pegasi at less than the Sparrow limit nor a myriad of other unheard of observations. Pete pushes the real telescope limits well into the realm where seeing sub arc second is required.

And that's the point, some observations are unheard of. I think it's absolutely wonderful guys are out observing such things and describing the universe we live in. Pete tells the story above like no other, we all know the Galilean moons are discs. We've know that for years and have been just a little bit wrong. Now we are telling a different story, one that says go and look at them. Sure, folks have been reporting albedo on Ganymede for years, but everyone should do it. It should be in all the observing books. It adds to the beauty of our observing.

For me, the right stuff could not be more right. Persistence and patients, desire, attention to detail, and folks like Pete driving it even deeper and all that under some excellent skies with a scope that is finely tuned and in it's niche`. It really get's no better, except maybe with some aperture.

Eddgie, mental stacking is how my sketches develop over time. Catch a detail here, then one there. Before long the entire NEB is rife with light and dark shading and dotted lines. There are times when Jupiter just "burns" into the retina. It really does look like an image (minus the finer features.) In those moments, color is much more pronounced and the detail is just "pencil dropping." It lands right next to your jaw.

In a 6", Io at 1.7" is just a bit smaller than the Airy disc diameter (1.82") but also nearly twice the diameter of the (series of) spurious disc(s) at Io's magnitude with the CO considered. The combined diffraction rings would sum into a slightly elongated first ring much like a brighter, very close double of equal magnitude (72 Pegasi at 0.5" arc, for example.) It might be that the dimmer poles contribute less to the diffraction ring observed.

The idea Io simply "appeared" elongated, even though it is not, is perfectly sensible.

Io is in fact only 1.2 arcsec currently. What is sensible here? Is it logical that a smaller telescope can show something elongated that isn't elongated?

Steve

Io is indeed 1.2" currently.

Io's colouring is not uniform. It is darker at the poles than at the equator. During transit, to me, it does appear to be elongated, squashed pole-to-pole, if you will. When not in transit, it appears to be a perfect disk.

I suspect that the poles are very close in albedo to the Jovian cloud band behind it, and thus are not resolved due to lack of contrast. I suspect that the bright equatorial region is resolved, as it has higher contrast with the background clouds than do the poles. It seems reasonable to me that a larger telescope might well be able to resolve those poles, returning Io's aspect to circular in the more capable instrument.

If a 1.2" disk can be resolved as a disk (and Io, to me, presents a clearly circular disk when not in transit), then it seems reasonable to me that a feature that is 1.2" by 1.0" (for example), might well be resolved as a non-circular disk. I believe this is the case for Io's equatorial region while in transit.

The effect is VERY well shown in many astrophotos of Io in transit, taken by scopes in the size range discussed. I guess that makes the question into one comparing the spacial and contrast resolutions of CCD's vs the practiced human eye.

I question the validity of my own observation. That was the purpose of this thread, to explore the possibilities of what I think I have seen. At least a couple other members have effectively duplicated my observation, from reading above. In beginning this thread, I'd have called my confidence in my observation at about 65%. I'd raise that now to 80% based on the reports of Norme and others here, though weather has prevented me from duplicating my observation.

I've very much enjoyed the discussion thus far.

J

Yes, possible.

An extended image is (in theory) made up of an infinite number of overlapping Airy Disks.

They are not laid "Edge to edge" as in my clumsy attempt to describe what I think may be at play but rather very slighly overlapping.

If the planet were perfectly illuminated, diffraction would make the planet look perfectly round.

But diffraction has this quality. A black line between two white lines will appear narrower because the light from the bright lines will diffract into the black line at either edge.

But a white line between two black lines will appear wider, again because the light is diffracting over the white dark lines.

I can see that there is albedo darkening on Io when conditions permit. I reported this in a post a couple of weeks ago when I reported that Io looked like a domed rivet in front of Jupiter when viewing a transit. I suggested that this and the pearl like look were a result of slight albedo effects.

And images show it clearly.

If there is a brighter equitorial region, and a darker limb on either side of that, then I think diffraction could indeed permit a 6" instrument to see that the "Length" of the "Line" (the brighter equitorial region) could appear to be stretched out because the very edges where the limb was darker would taper off before the ends of the line.

The result would be like maybe two very slightly displaced Airy Disks, which would indeed appear lengthend.

And this is exactly what a Sparrow Criterion spit is. There is not actual split, but becuse the Airy disks overlap but are not perfectly concentric, the elongation may be just sufficent to see the combined stars as something other than a singel point source.

And Io could be doing the same time. The theoretical Airy Disk "Pixel" on one end of the brightened region and the "Pixel" on the other end of the brightend region both generate Airy Disks that are seperated by 1.2 Arc Seconds (the width of Io's Disk). Now, you have two Airy disks that are spreat at the edge by this amount.

If the source were unevenely illuminated, being darker on one side and the other than at the center, then using the powers Norme is using, I think that it would be enough to perceive a very very tiny lengthening.

Again, recent images clearly show strong albedo and even surface resolution on Io, and I have seen albedo differences visually in the C14, so there is proof and there is at least one visual observation that supports the uneven illumination.

So, I think that theory would support that this effect could occur, and the fact that at least two people using similar apertures have reported it(6" and 8") seems conclusive to me personally.

But I see skeptisim on this forum from time to time (Banding on Uranus, yea or nay?) but I see people making observations that I have made using larger instruments that I think might within the reach of their own instruments.

And I think this is the case here. I think it is within reach of Normes instrument.

But he and I had a variation on this conversation in the past.

I suggested that to see albedo features was different than having "Resolved" a detail, and this is a great example (and one that I think may cause him to re-evaluate our previous conversation).

In this contect, I think Norme has "detected" that there must be some shading on Io. Otherwise Io would be perfectly circular (as it is in my C14). I can see though that there is distinct albedo shading. He can only "Infer" it because of the diffraction making the disk appear slightly elongated.

But that doesn't at all distract from the observation. In fact it stands in stark testominy to the laws of physics, resolution, and contrast transfer. As the aperture increases, the scope will have a better chance of showing direct evidence of uneven illumination, but the smaller apertue might only show an effect that could be described and attributed to diffraction effects caused by viewing an unevenly illuminated extended object.

Bottom line? I think it is a slam dunk.. He says he saw it, I know it is there (the albedo shading), and diffraction can explain it.

You're correct, according to JPL sim it is 1.2" arc. That's probably accurate enough for all intents and purposes. And in fact, I am trying to make sense of it myself through studying extended object diffraction. I offered one possible explanation above. If you have a better explanation, I'm all ears.

The power of suggestion can be pretty powerful tool. Was it suggestion that ruled the observation? It's possible, especially for those with a healthy dose of skepticism. Nothing wrong with that. But, don't let skepticism rule out attempting the observation, as crazy as it sounds. It's probably going to depend greatly on seeing, but there are always those good moments.

So, can a 6" do it. I believe it can, having actually undertaken the observation in nearly perfect seeing, cooled and perfectly collimated. And the timing could not have been better, at opposition no less. All the advantages were in favor of a successful attempt. If it could be done, the conditions on or about Dec 2 were ideal for attempting it.

Personally, I am no longer restrained by Dawes or Raleigh limits because of experience with point sources smaller and tighter than even Sparrow. A 6" can easily elongate 72 Pegasi at 0.5" arc and well below the Sparrow limit. So, because this violates Raleigh in a big way, and Dawes pretty much the same, neither in any way invalidates seeing something that tiny - to the best of my knowledge and recent experience.

The whole idea is to get others to attempt it, to validate and repeat the claim, or to refute it. You game? I feel you might actually be surprised. It's not like we're looking at something in the microwave frequencies, it's all visible light observation of an extended object. But, if there is an explanation as to why this is impossible, again I am all ears.

My cheese might well have slipped off my cracker and I would have no way of knowing it without someone stating so clearly. Or sending the white van to pick me up. But, until being absolutely insane is proven, I saw what I saw. And would have never believed it, either.

If it were not for Pete pushing back the theoretical, textbook limits and Jason suspecting something was strange with Io, I would have been busily humming along in life conversing with my invisible friend. Look, I really think there is something to the observation, and would urge anyone interested to see for themselves.

I just cannot get two images out of my head. One was a couple nights earlier when Io was approaching the preceding limb. I cranked up the power and looked at it. It certainly struck me as a tiny, bright dash mark. The second time, I identified Io immediately because it just did not appear right. I had two moons in the FOV to choose from, Io and Europa. I identified Europa because it was a perfectly circular disc. The other, less that perfect disc was Io. The color confirmed it, and so did every other source. It was Europa furthest out and Io next one in. That convinced me I am not nuts, or that I am...can't remember.

So, what's sensible? Well, what is not?

I should add to my post above that not only will white line look wider on a black background, but a short white line will appear longer on a black background because the ends will be extended due to diffraction. This is the critical element involved here. that the "Line" that is Io's brighter center region I think is being "Lengthened".

Anyone could see this effect using Abberator. Overlap two stars seperated by 1.2 arc seconds in a 6" apperture. Assuming the stars represent diffraction from the edges of a brighter central band, it should be easy to see that they are elongated.

And that was my point. Io is not evenly illumninated. Pictures show this and I can see it at the eyepeice. So, if it is illuminated unevenly, especially if it has a central band that is more illuminated than the limbs, you will be seeing diffraction effects extend out slightly further at the ends of the "Band" than at the top and bottom.

And the spreading does not start at the center of an Airy Disk that is represented by the center of Io, but by the edge of the disk. that is what the theory of image formation for an extended object says.. Every point on the object generates its own Airy pattern, so the spread of the Airy Pattern for points on either end of the brighter band will spread further than points from the edges that are darker (well, they will spread the same but be dimmer and it will be harder to see this spread).

Bottom line? Diffraction accounts perfectly for being able to see Io at 300x as slighly elongated if the disk is not perfectly illuminated.

In fact this is the exact result theory says you will get. So rather than be surprised, I would think that this would be the "Expected" result.

And Norme, this is kind of in tune with our previous conversation regarding "detecting" and "resolving".

You can infer that Io is not evenly illuminated because if it were, the disk would appear slightly larger than an Airy Disk and it would be perfectly round. It would have to be this way.

But if it appears elongated, you can infer that the disk is not evenly illuminated because diffraction clearly will cause the light at a dimmer limb to not be as bright as light from the darker limb. And of two limbs are dim, then you get a more distict elongation.

So, you have clearly detected that there is albedo shding, but have you "Resolved" it? In this case, if you could not tell which limb was actually dark (assuming that one limb was not fully illuminated for example) but the only evidence is that the diffraction pattern is not round, then you have clearly "detected' the presence of some detail.

But you haven't resolved it because you can't see where it is and how large it is relative to the disk itself.

And that is what I was trying to describe in our previous post. Sometimes I detect detail because of uneven illumination, but it is indistinct. I call this "detecting" that a detail is present. And you are doing this with Io.

But to resolve it, you would have to tell me what side it was on, and how much of the ark of the limb it occopies, and this would be just a bit blow the capability of the scope.

We can say though that the scope has sufficent resolving power to detect the presence though, the same way that it is done with double stars. Many people feel that a double is only resolved with a Daws split, but the Sparrow criterion clearly shows the presence of a double even though the star is not truely split. It is detected though, and that is why I think the term "Sparrow Criterion is used" rather than "Sparrow Split." It is a subtle difference. We detect it is a double because it is not round, but we can't actually spit the intensity curve, which seems more of a "Resolved" feature.

Anyway, perhaps now you can see why I prefer to use "detected" and "resoved." as different discriptions.

Yea, Eddgie, I am still chewing on what it means to resolve something that small. But, if I may...

...both generate Airy Disks that are seperated by 1.2 Arc Seconds (the width of Io's Disk).

He can only "Infer" it because of the diffraction making the disk appear slightly elongated.

Exactly. Not easy by any stretch. In fact, I am wondering if a 4" could do it. That would be one for the books.

If there is a brighter equitorial region, and a darker limb on either side of that, then I think diffraction could indeed permit a 6" instrument to see that the "Length" of the "Line" (the brighter equitorial region) could appear to be stretched out because the very edges where the limb was darker would taper off before the ends of the line.

You know, resolution depends on contrast. And not just the length of several spurious discs in a row that end against a dark sky (devoid of any point source diffraction), but also the height of Io from pole to pole. I am not saying I saw any spurious discs from the poles, though, as you have.

In Raleigh, resolution means two white (half lines) and one dark. That is 1 line pair/one or cycle within the Raleigh limit. It could mean two dark (Io's poles) and one white (the equatorial region.) As long as those line pairs are resolvable at Raleigh (28% contrast?) or Dawes (5% contrast), then true resolution is said to occur. And those spatial frequencies are not even at the limit of the MTF, theoretically speaking. Dawes is not even at the maximum spacial frequency, but stops just short at 5% contrast. And that far out the curve, the spurious discs in an obstructed scope are smaller. Brighter on Io, but smaller, too. I think Suiter uses Abbe limit as maximum resolution. But those contrasts are, by convention, said to be between two bright lines (or technically point sources for double stars.) I am not sure I actually resolved the equatorial region in the pure sense. Interesting thinking about it, though.

Io is different in that resolution is exactly like the one white line between two darks you describe. So, even at 1.2" arc, that frequency is still well (outside) the Raleigh limit (half the Airy disc, a dark space, then half the other Airy disc with the centers separated by 0.92" arc. Right?)

You might be able to go even smaller to the Dawes limit which is an even tighter line pair. But at 5% contrast at the poles, one might imagine it get's a lot more difficult at this point and beyond.

Then there is the complicated math governing multiple point sources with decreasing optical path distance. At zero OPD, the Airy disc actually expands both in diameter and in intensity. At greater OPD, such as Io's multiple Airy patterns, the Airy (an spurious) discs returns to pretty much normal in both aspects.

I still don't fully understand how to apply the latter.

..In fact it stands in stark testimony to the laws of physics, resolution..

I think were we might talk past each other is in the way resolution is "defined." Resolution, per se, is not a law in that it can only occur the way Raleigh, Dawes, or Mr. Sparrow say. They define resolution as some level of contrast between two points and then develop their math from that definition. But, I guess the Airy disc is a law of physics, Dawes really is not. But, it helps if we all use the same definition.

-----------------Off topic------------------but related.

I was skeptical of Uranus observations, well, because 2% (low) contrast on the planet is just not modulated through the scope efficiently enough at that spacial frequency. It would be visible on the focal plane, where a CCD could capture it. But the human eye just could not resolve such low contrasts. Not average vision, anyway. The other thing that bugged me was the color of the bands so far into the red.

But, hey, that's theory. Today I take it a bit easier on those claims and offer the benefit of the doubt. He might just be able to do it, and a few others with exceptional abilities as well. But, again, that's the skeptic mind set, to shrug it off as impossible when some document somewhere proves it can't be done. Well, here we are...

I no longer totally doubt the Uranus observations. Still have to overcome some skepticism, though. But, an open (and suggestible mind) helps immensely.

I was not commenting on Io while in transit. I was commenting on the observation of Io looking like a boxcar floating in space. I have spent enough time at the eyepiece to know better than that.

Steve

I've never observed Io to appear non-circular when not in transit. To me, it (and the other Galilean moons) have always appeared to be purely circular against the blackness of space.

J

Well, it is a semantic difference to be sure.

You might see a bright spot at the edge of he limb of Ganymede and look at a simulation and say "that is Osiris" and be right.

I would look at Osiris in my scope and see that it is not on the limb, but in fact a measurable distance inside the limb and have given it a size, shape, and position.

We both see Osiris, and we are only debating my own defintion of "resolved" (shape, size, and position) with yours (see something that is in generally in the right place at the right time, so must be Osiris).

Again, we are debating the samantics of it, but I for my own records, I tend to differentiate this. The fact that I could seee the size and position of Osiris in the C14 and could never see it in any of my other scopes made me redefine these terms for my own use so when I look back at my logs, I can see that "Resolving Osiris" for me personally had a much more secific meaning than having seen it as an albedo feature (a slightly brightend area on the limb with no distint shape to indicate it was a seperate feature) in my 6" scope.

So, perhaps we are just debating the "Relative" nature of our resolution of these objects. And to me, they are far more "realatively" resolved at 14" than a 6".

And this is why I am so adament that the best planetary result is going to be had with aperture. It is the single most important attribute of a good planetary scope. The bigger the aperture, the "Better" the resolution.

And for me, the meaning of resolution evolved as I started seeing more granularity in the shapes and positions of details on the surface of planets and moons.

So, no point in arguing it because it is semantics, but i don't at all doubt that in a small scope, Io could appear less than perfectly round due to albedo shading and diffraction. It is all to easy to explain it using this mechanism.

If disk does not appear as perfectly round, it can only be because there is a difference in brightness present on the surface, and this would exactly explain it.

Supose that Io were seen in a Half-moon type lighting. One edge would be "Flat" and the other "Round"

If the half of the disk that was illuminated was 1.2 arc seconds in diameter, the light from the edge of that disk would form a perfectly straight line that was 1.2 arc seconds long and displaced the width of the radius of the Airy Disk.

If you magnified this image 300 times, you would see a circle of light that was flat on one side extrending 6 arc seconds in the eyepiece on one side of what would appear to be a very slightly flattened circle of light.

This is 100% consistent with how extened objects are formed. It would be easy to see if conditions were perfect because the amount of flattening would be significent enough that the other side would bulge away with sufficient curvature to be seen. It would actaually be much larger than the half disk that formed it, but again, this is what diffraction does. It takes the energy from the point that originated it and spreads it out past the point.

You are working at the extreme edge of visible perception, but honestly, I personally think that your observation is not only possible, but probable if the limb darkeinging is extreme enough.

I mean after all, Sparrow did it with a Double Star! (though he measured it using a camera, but that was clearly a case where he detected the presence of a doube star simply because the Airy Pattern he observed was not round anymore).

And here is another picture that shows the extreme contrast variation in Io.

I could easily see this in the C14 during a recent transit and reported it on this forum. I reported that Io looked like a rivet standing out against the planet and attributed it to albedo dimming at the north and south when the planet was in the sky.

And agian, I said it made the moon look like a pearl.

And someone else posted and said that they thought I was over-stating the situation until they observed it in their 8" scope..


Hmmm. Is it impossible to believe that it could not be detected at 6"?

Clearly there is no magic cut-off where this just suddenly stops. It gets progressivly harder and harder to see as the aperture gets smaller and smaller.

At 14" it was easy. At 8" it was easy enough that someone that looked for it saw it and reported it here.

Why should we be surprised that someone using a 6" scope noted the effect when two other people have reported it, and we have a picture that shows it clearly, and diffraction can easily explain why it looked elongated.

Great picture. Clear to see that there is strong contrast drop-off at the north and south. Saw it in my C14.

http://www.cloudynig...5553738/page...

Anyone that cares to can step back and tell me if it doesn't appear to be like a little dash... In fact the further you step back from the monitor, the more obvious it becomes.... Hmmmm. From four feet, it does kind of look like a little box car..

Above post, I think I said five arc seconds, and of course that is silly. It would make a soft, straight line 5 arc minutes long at 300x. Easy to see a 5 arc minute long detail as a line.

But again, this was just an example of how the image could form if the top and bottom of the moon were darker than the center. You would essenitally have two gently curved lines 5 arc minutes in length that would look, well, like a football.

I have observed a number of transits during this Jupiter season. My eyes start playing tricks on me after staring through the telescope for a while. I was watching a bright/shadow transit of Europa and I swear I saw a momentary flash of bright light in the transiting shadow of Europa. Was it really there? Impossible to say, but I definitely perceived it and enjoyed it greatly. You may never know if what you perceived was real, even if others confirm that such a phenomenon is possible to observe. Remember the canals of Mars? I'm sure Giovanni Schiaparelli enjoyed seeing them, no matter the ultimate outcome of the observation.

/Ira

I have spent enough time at the eyepiece to know better than that.

Steve

So, we should take that at face value because you say so? Why should we believe you? I'm skeptical of your're claim, or could be if I were serious (but am not.)

Not trying to be contentious, just driving home a point. You made a claim that is not substantiated in any way, not even in theory nor with any data to prove otherwise. I have no reason to doubt you have plenty of eyepiece time, so I accept what you say at face value.

The point is, there is something strange with Io and I wish you could see it, too. And don't think for a moment those doubts don't come creeping back after a day or so. They do.

So, perhaps we are just debating the "Relative" nature of our resolution of these objects. And to me, they are far more "relatively" resolved at 14" than a 6".

No doubt. You know, there was a thread Pete did asking about resolution, I'd hate to repeat it here...but the urge is overwhelming. Raleigh and Dawes both offer resolution in terms of a contrast between two point sources. This is the standard by which they say two stars can be seen distinctly as such and separate from one another.

However, if you think about it, they are not differentiating contrast from one star to the other as both have equal brightness hence no distinct contrast from one another. The real contrast is from the bright point and the black of space. In their case, they say that blackness must exist between the centers. But, that blackness contrasts with the brightness on all sides. So, really, if one star's bright pattern protrudes into the blackness of space, it can be said to resolve if we stretch the semantics a bit.

In other words, there is enough contrast around the star to see it. And because we know diffraction patterns are circular (cheating a bit) an elongated star must be comprised of at least two diffracted points. In a sense, this realization is resolution by contrast, too, just not between the centers. If the star is so dim, say less than 5% brighter than the blackness of space, we'd never see nor resolve it no matter the separation.

But, yes, in the classic sense, nothing was resolved on Io, best I can tell, in a 6". Certainly a C14 culd do a much better job on Io and Ganymede. On Ganymede, as you say correctly, nothing was distinctly seen like a well defined, large lunar crater. But, the brighter "spot" indeed represented one half of a line pair with a low contrast darker (opposite) limb comprising the other. And all of that within a space larger than the Raleigh limit.

Anyway, we're having that discussion, again, ain't we.

But, I agree with you. If Io had a brighter equator, and it does, and it was a certain angular dimension dependent on aperture and Raleigh's laws, then the spurious discs produced should appear elongated. And if they are darker at the poles, it would not appear circular nor resolved in the classic sense. We both understand that pretty clearly.

I also agree with you that it's not only probable, but it is the likely outcome. That's part of what makes this an exciting thing, it should be possible and yet such an observation is not common knowledge. Maybe it should be. And it's certainly not an easy one, either. It requires sub arc second seeing, even just a fleeting moment of it would suffice given, as you say, the right stuff (perseverance.)

I applaud Jason for bringing this up. I applaud you for having made the observation more convincingly and reporting it in the face of those who will argue otherwise. I applaud Pete for busting through those theoretical barriers (kind of described above) making the realization such things are, indeed, possible despite the Dawes limit. I applaud anyone willing to see for themselves and report back. And I applaud myself for being excited about all this.


What a fascinating topic, Jason. This just added another dimension to this year's opposition, like no other. Gotta run, my pills have arrived.