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Eddgie
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Re: Some thoughts on a Transit of Io new [Re: Eddgie]
      #5553806 - 12/04/12 11:35 AM

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.


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Asbytec
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Re: Some thoughts on a Transit of Io new [Re: Asbytec]
      #5553809 - 12/04/12 11:38 AM

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

Quote:

...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.

Quote:

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.

Quote:

..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.

Edited by Asbytec (12/04/12 11:55 AM)


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sqrlman
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Re: Some thoughts on a Transit of Io new [Re: Eddgie]
      #5553819 - 12/04/12 11:41 AM

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


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JasonBurry
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Re: Some thoughts on a Transit of Io new [Re: sqrlman]
      #5553859 - 12/04/12 12:07 PM

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


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Eddgie
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Re: Some thoughts on a Transit of Io new [Re: Asbytec]
      #5553881 - 12/04/12 12:26 PM

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).


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Eddgie
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Re: Some thoughts on a Transit of Io new [Re: Eddgie]
      #5553893 - 12/04/12 12:34 PM

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.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/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..

Edited by Eddgie (12/04/12 12:37 PM)


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Eddgie
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Re: Some thoughts on a Transit of Io new [Re: Eddgie]
      #5554179 - 12/04/12 03:31 PM

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.


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Ira
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Re: Some thoughts on a Transit of Io new [Re: JasonBurry]
      #5554474 - 12/04/12 07:14 PM

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

Edited by Ira (12/04/12 07:17 PM)


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Asbytec
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Re: Some thoughts on a Transit of Io new [Re: sqrlman]
      #5554493 - 12/04/12 07:25 PM

Quote:

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.

Edited by Asbytec (12/04/12 08:56 PM)


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Asbytec
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Re: Some thoughts on a Transit of Io new [Re: Eddgie]
      #5554667 - 12/04/12 09:25 PM

Quote:

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.

Edited by Asbytec (12/04/12 09:26 PM)


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Asbytec
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Re: Some thoughts on a Transit of Io new [Re: Ira]
      #5554829 - 12/04/12 11:12 PM

Ira, certainly good points. They eye and the brain can do amazing things, whether we like it or not. Perception is reality, I guess. But is it? I can see my cell phone sitting right in front of me, so I am pretty sure it's there.

Yea, can't argue the canal mystery. Some thoughts, though. It was definitely shown canals do not exist on Mars. Not that anyone doubted they were perceived, but they do not exist. Images of Io, however, clearly show the phenomenon exists. That may not be conclusive in the strictest sense, but it is supportive of the observation. This is support the Canal mystery did not have, outside of a few Martian features that might have been seen as canals.


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azure1961p
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Re: Some thoughts on a Transit of Io new [Re: Ira]
      #5554895 - 12/05/12 12:12 AM

Quote:

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




To that end why observe at all?

Fact is his visual o servations are supported by electronic imaging. Too he's got a record of success and failure meaning if something's not showing he's not dreaming it into being. Frankly at this point it'd be irresponsible to ignore his observations.

I know where your going with your argument but there is compelling testimony this is real.


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Asbytec
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Re: Some thoughts on a Transit of Io new [Re: azure1961p]
      #5554992 - 12/05/12 01:58 AM

Pete, I still owe you Titan's disc, right? You say it's possible, theory says it's possible. But, I'll be darned if I have seen anything but a point of light with a faint ring. Maybe it's just so small an image. Gotta keep at it. It's fun trying.

I know I saw Enceladus once. It took a few hours on good nights over a week to actually do it again.

Do I owe you Alpine ridge, as well? I forget. You drive us to observe the seemingly impossible. Quite often you are right, in fact surprisingly often. Your infatuation with blowing past stated resolution limits is well founded, IMO. The results are staggering.

Observing is an adventure, not a quick weekend warrior look-see at Jove that reveals little.


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Asbytec
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Re: Some thoughts on a Transit of Io new [Re: Asbytec]
      #5555076 - 12/05/12 04:08 AM

Pete, thinking of Titan and trying to stay related to the topic of difficult observations.

You know, Titan is about 8th magnitude and right at ~.9" arc. That's exactly the size of a 6" Airy disc, if that matters. Being that dim, it appears smaller with a brighter center. Not only that, each point on Titan is a little dimmer than it's stated magnitude. So, really we we have is a series of, oh, 9th magnitude(?) point sources at best. Of course dimmer along the limb.

So, we have, say a 9th magnitude point source at the center and dimmer ones toward the limb. But the bright central spurious disc will not completely overlap the others. Not like it would if Titan were a 6th magnitude star nearly 0.9" arc across. So, the limb sources should poke out. Question is, how much and how bright.

So, to see Titan's disc, I have to see something that IS the size of the Airy disc (which is larger than the point-like spurious disc I do see) and much fainter than Titan itself. If I can get below limited magnitude of about 11 or 12 (about the limit so far this year), then it might be doable. Those dim limb point sources have to stand out against the sky with at least 5% contrast. So far, seems only the central spurious disc, or two, does so.

You know, given dark enough sky, good transparency, dark adaption, and some steady seeing those limb point sources might just stand out a little better than previous experience. And each should be a bit brighter than Enceladus (~12 mag), I think. It should not be as difficult as Enceladus (hours of observing it's location waiting for that brief speck to pop once, maybe twice.) So, I wonder why I have not been able to resolve a disc, yet.

Maybe I am overestimating its surface brightness. If memory serves, it does appear like a faint, fuzzy spot - nothing like Europa, for example. So, no clearly defined disc. It really does look like a faint star. But, there is enough energy from that spot to produce a faint ring, further reinforcing it's star-like appearance.

Gaaa! I cannot decide if it's doable or not. Sounds like it should be. I mean, come on, we should be able to see discs of the solar system's largest moons. How hard can that be?

Maybe this deserves it's own thread when the time comes.

Edited by Asbytec (12/05/12 04:09 AM)


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azure1961p
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Re: Some thoughts on a Transit of Io new [Re: Asbytec]
      #5555211 - 12/05/12 07:22 AM

This past summer I gave Saturn a gander on a night with 5-6 seeing and looking at TItan at 200x I was kinda put back a bit. It IS definstrly fainter in the 6" sct. It took 364x with the 8" juusttt to eek out an orb. I couldn't see it being any easier for a sct of that size then given how it was at 200x. I don't know about this one. Its the dimness that's really ramping up the difficulty.

Thanks for the kind words about challenging objects but its actually been my wish and your reality at times like the Catspaw!!! Ill see it sooner or later but probably with a setting sun angle as you have as its easier for me to observe later than earlier.

OK off to work.

Pete


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azure1961p
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Re: Some thoughts on a Transit of Io new [Re: azure1961p]
      #5555298 - 12/05/12 08:52 AM

I think the issue that often rears its head in astronomy is the term resolution being used as an absolute value for true recognition of a feature or albedo shading. The problem is it becomes a very relative term with each each increase in aperture revealing how little truly was resolved prior. A 1 meter scope on pic d midi could leave the C14. Views interpreted as detected but not resolved. There's nothing absolute about the term resolved that is anything but a relative term. Detection is merely the beginning of resolution but its still resolved... just on the most basic level. I understand the diffraction line effect here but it is an effect born out of the reality of the physical state of its albedo or reflectivety. Something not born out of a true physical state or shape are diffraction rings. Strictly diffraction generated. In the case of Io the line is a simplified representation of reality. Yes there's effect there but its a close link to the actual albedo patterning.



Pete

Edited by azure1961p (12/05/12 08:54 AM)


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Asbytec
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Re: Some thoughts on a Transit of Io new [Re: azure1961p]
      #5555351 - 12/05/12 09:36 AM

I really want to revisit Io in the coming days, soon as the late season typhoon passes.

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azure1961p
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Re: Some thoughts on a Transit of Io new [Re: azure1961p]
      #5555649 - 12/05/12 01:04 PM

Typhoon.... wow and I get bumped if its partly cloudy

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wky46
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Re: Some thoughts on a Transit of Io new [Re: JasonBurry]
      #5556213 - 12/05/12 07:03 PM

My thoughts are one of excitement now Great looking skies (in a relative kinda way). Much like waiting for a good concert or good movie to start

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JasonBurry
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Re: Some thoughts on a Transit of Io new [Re: wky46]
      #5556978 - 12/06/12 07:28 AM

LOL, I've seen some blue skies in the past couple weeks, but precious few starry nights.

I was poking around into the early visual explorations of Io yesterday. I ran across several descriptions of the earliest observations of surface detail on Io. The following passage, pilfered from Wikipedia, struck me, especially the conditions under which Io's albedo variations poles vs equatorial region were first noted - While in transit.

http://en.wikipedia.org/wiki/Exploration_of_Io

"Beginning in the 1890s, larger telescopes allowed astronomers to directly observe large scale features on the surfaces of the Galilean satellites including Io. In 1892, William Pickering measured Io's shape using a micrometer, and similar to his measurement of Ganymede, found it to have an elliptical outline aligned with the direction of its orbital motion.[23] Other astronomers between 1850 and 1895 noted Io's elliptical shape.[21] Edward Barnard observed Io while it transited across the face of Jupiter, finding the poles of Io to be dark compared to a brighter equatorial band.[24] Initially, Barnard concluded that Io was in fact a binary of two dark bodies, but observations of additional transits against Jovian cloud bands of different brightness and the round shape of Io's shadow on the Jovian cloud tops caused him to change his interpretation.[25] The egg-shape of Io reported by Pickering was the result of measuring only the bright equatorial band of Io, and mistaking the dark poles for background space.[21] Later telescopic observations confirmed Io's distinct reddish-brown polar regions and yellow-white equatorial band."

I believe EE Barnard's observations were made on the 36" Lick Refractor.

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