Some thoughts on a Transit of Io
Posted 26 November 2012 - 10:55 AM
I did the bulk of the observing with my 8", the 4.5 was just there because the transit would be a fun test of its contrast and resolving abilities, with its near-spherical primary.... The 8" dob was running a 12.5mm plossl and 2x barlow, for about 200x. The 4.5" had a 4mm Huygens of very low quality, giving 225x.
The GRS turned out of view a little before Io's shadow contacted Jupiter's disk. The seeing was uncommonly good, with all 4 of the moons showing clean disks, with a faint spray of glare around each. Knowing Io's angular dimension, the seeing was generally well under 1".
As the shadow bit Jupiter's limb, I was astounded by the "3d-ness" of the spectacle. Fascenating to see the limb reach around the back side of the shadow, until it was a black spot on Jupiter's face rather than the bite it had been moments before. A look through the 4.5" newt showed the shadow quite well too, not quite so vividly, but easily observable. An pleasing view, when the planet was well centered in the FOV, as the cheap eyepiece had tons of false colour off-axis.
The real magic hadn't yet begun, however. Back to the 8". As Io approached Jove's limb, it seemed to take on a 3d aspect in its own right, a yellowish pearl. Its glide into the face of Jupiter's disk was astounding.... I was surprised how clearly the tiny disk stood out from the otherwise similar colourscape of the Jovian cloudtops. I was also able to see Io's disk in transit thru the 4.5" scope, but it was much harder to resolve.
Over the next hour or so, I began to notice something very peculiar about Io, and it is this that is the real subject of this post.
Now, Io has a diameter in the 1.5" range (I don't have the figure here), and an 8" scope has a resolution on the order of 0.5".
I could swear that in my telescope, while in transit, Io appeared to be elongated in the equatorial direction. It was my impression that it appeared to be about 3:2, seemingly squashed pole to pole.
Now I know that Io's effectively spherical. I also have read (in the imaging forum), that Io often appears "football" shaped in their images, aparently due to albedo (seems Io's poles are darker, and blend into the Jovian cloudtops more).
So, did I genuinely see this effect, or am I (once again, LOL) a victim of the "aperature of my own mind"? I was aware of this effect (the "Io Football") before I made my observation, but one cannot put the genie back into the bottle.
Anyway, it was a great blast watching the first half of this transit (before the skies went to pot), and one of the more memorable sights I've seen thru my scopes. What a beautiful season this has been for Jupiter. I've never seen it like this!
Posted 26 November 2012 - 12:22 PM
If the football effect is visible to the human eye, contrast wise, then you very well could have seen it. As to whether you are engaged in "aperture of the mind" or not, I have no way of knowing without evaluating your response to some ink blots.
That you know of the effect or not may not matter. Question is, did you see the elongation or not? You'll have to tell us. I don't know, but judging from the highly processed, yet distinct contrast, it looks doable under very good conditions necessary on that scale - IMO.
What a beautiful season this has been for Jupiter. I've never seen it like this!
Edit: Sorry, image credit goes to...
Posted 26 November 2012 - 12:36 PM
I tend to think I DID see it...
Does that make it my first successfull detection of a surface feature (ie the dark poles of Io) on a Jovian satellite? I'm gonna contend that it does!
- Asbytec likes this
Posted 26 November 2012 - 01:06 PM
It would be interesting to know more about the surface of Io. Does it indeed have darker poles? Or is it some sort of effect caused by uneven illumination of a sphere. Or is it an image processing effect? (I hope not.)
Again, just accounting for the image above, the contrast between the poles and the equator looks significant. Possibly enough to differentiate visually. But, maybe another observation or two would confirm it. Or if you held the elongation for a period of time, then it seems more real.
Another question that just popped up, does Io exhibit the same football effect against the black of space. If so, or if not, what does the answer mean? Does this effect require Jovian could tops to pull off. And if so, what does THAT mean? Are there images of Io against the black of space that show this pole darkening? Or is the pole darkening simply an "effect" requiring a Jovian backdrop.
In any case, it's a stunning observation. I am inclined to believe, if you saw elongation...well, then you saw elongation. I've never seen it, but I have never tried. Maybe I should, others should.
Posted 26 November 2012 - 02:04 PM
Once Io had made it about 1/4 of the way across the disk, I went inside for a bit, and put away the 4.5" instrument. An hour or so later, when I returned to the eyepiece, Io was nearly halfway across the planet's disk, and much more difficult to resolve against the clouds. The seeing had deteriorated, the wind was rising and the clouds were thickening.
Just looking at some HST images of Io, and the poles do seem to be subtly darker than the equatorial region in many photos. I'd guess that what I may have observed might be a subtle colouring difference between the poles and equator that is all but blown away from my eye when seen against the blackness of space. But Io and Jupiter's colours are quite similar, so against the giant's disk, the more subtle detail in Io is more easily noted.
Or, I'm a victim (again) of the complex interaction between eye and brain. It's easy to have an idea in one's head, and to force the "eye" to see supporting evidence. I'm not sure if I saw what I saw or not, but I'm certainly gonna try to repeat the observation.
Posted 26 November 2012 - 02:21 PM
Posted 26 November 2012 - 03:17 PM
a yellowish pearl.
This is almost exactly how I described Io during the night about two weeks agon when Ios shadow transited the GRS. I saw Io as a pale pearl. Interesting that we both came up with the same analogy, but like you, I found that the similarity was striking!
And in that post, I attributed it to some very slight albedo effects!
And I noted the exact same oblate-ness but again, felt that both the pearl texture and the oblate appearance were due to albedo features on Io.
Anyway, I am of course quite sure that they would be visible to you.. I was using a C14 but my seeing was not great that night, so with better seeing it would not at all be a surprise to have someone report this using an 8" aperture.
And yes, the shadow transits have been great. Norme caught on where the shadow was on the Oval BA, and there have been images of both of these events.
Looking at images posted on CN though you can see that Io is indeed showing albedo features in most of the images these day.
But the "Pearl" analogy is again exactly what I reported, so that says that the effect is very real and more than ust an "Impression." When two different observers on two different nights use the same term to describe it, that kind of says that it isn't their imagination.
Posted 26 November 2012 - 03:26 PM
"By the gods, it does look like a pearl!"
Posted 26 November 2012 - 08:33 PM
It's interesting, however, that Io is football shaped against the planet. Thinking out loud, it if has the same color scheme, more or less, it might appear camouflaged against Jupiter except for it's brightness.
Could be volcanic activity that gives Io it's perceivable contrast. It probably does look pearl-like. Need to observe it more closely.
Posted 26 November 2012 - 09:58 PM
But I am thrilled that someone else not only enjoyed the sight the way I did, but agreed that the "Pearl" look I reported was in fact visiable.
Very happy that you saw it and reported it....
Cool, isn't it? Way cool. And really beautiful to see.
Posted 26 November 2012 - 10:18 PM
Id like to think this is a toe hold on Io discerning albedo in its own peculiar way or effect but Id be more in line with seeing vagueries and the manipulation these fine orbs are subject to through atmospheric lensing.
Posted 27 November 2012 - 03:16 AM
Posted 27 November 2012 - 12:07 PM
Okay, just observed Io and Jupiter for nearly 3 hours from 27/1330 to 1630UT. My scope was perfectly collimated. I know because in 9/10 seeing I tweaked the Poisson spot about half an arc second to dead center at about 1 wave de-focus. In focus was perfect.
If observing this elongation had a chance, it was tonight as Jupiter was approaching the zenith. Europa was on the other side of Jupiter, but Callisto was just 1' arc trailing Io. So, I had two good references for what round looks like. One in the same high power field of view.
Normally, I'd make you read the entire account, but I gotta say Jason might be onto something. I'll explain why below. But, this brings one question to mind. Why is this not common knowledge already? I mean, surely we're not doing ground breaking amateur observations, here. Surely someone has reported this before.
Okay, now the boring account. A bit before Io's shadow touched Jupiter's limb, I observed it at 320x (6mm TMBII.) I wanted to observe it against the black of space. At this magnification, nothing really struck me but I did get a curious feeling about it. Not curiosity, I already had that. But a, "hey, hmmm, that's curious moment."
So zipping up to 380x (8mm TMB II and 1.6x Barlow) there sat Io against a pretty dark background, for all intents and purposes. My first impression was, "Hey, that does kind of look like a tiny bright dash." But, always on guard for the mind's aperture effect, I noted it and kept observing. Over time, Io bounced and blurred slightly. It was sometimes circular, sometimes not. Inconclusive on the elongation, but that nagging feeling it might be hung in there. Glancing at Callisto, then back to Io several times gave the impression Io might not be exactly circular. But I wasn't sure.
Okay, well, let's see if atmosphere is causing this tiny elongated impression. Slewed over to observe Europa. In the same seeing conditions, it was pretty much a clean disc. It bounced and blurred a bit, but it never gave any hint of elongation. Okay, so Io gave some hint of elongation, Europa nor Callisto did. Now, I am becoming somewhat convinced Io might actually be, and just barely detectable as elongated. The same observations were made at "stupid" 520x (6mm TMB II and 1.6x barlow.) Same results, but bigger disc.
The shadow drug itself over the limb, and Io struggled to keep up. It made contact with Jupiter's limb. And you know what? The elongation effect seemed a touch more convincing. Still not enough to really call it with utter confidence of a WWF wrestler picking a match in a school playground. But, interesting never the less.
Okay, not convinced, but cannot deny it's not elongated. That was the verdict at this point. By the way, I managed to follow Io all the way across the planet. It was in the darker SEB southern edge just preceding the GRS. In fact, Io with it's shadow just ahead of it (a few arc seconds) was just preceding Oval BA and it's companion. The whole entourage (BA, Dark spot, GRS, Io and shadow) marched across the disc. I got a sketch of it. Sweet.
So, anyway, after the sketch was done and Io approached the preceding limb, it deserved another look. Back up to stupid 520x for another look. Now! This was the most convincing elongation. In fact, I'd almost bet my Christmas gifts it's elongated. It just looked like a tiny bright dash mark.
So, yea, I really think Jason and Eddgie are onto something. I think it certainly can be glimpsed as elliptical or kind of like a small bright dash. It was way to small for me to even say whether it was 3:2 or any other ratio. Just that it did not really look circular all the time.
In fact, 72 Pegasi was clearly elongated at 0.56" arc (380x), and Io was a tougher to see elongation. Just to give you some idea, some scale of how hard this observation really was.
That's my report. Your turn! (Forgive the lack of editing...tired.)
Posted 27 November 2012 - 12:30 PM
When I was observing it, at 200x, the seeing was such that the moon's disk was quite clean and round, though it would jump about by about half its diameter once in a while, presenting a reasonably clean and stable image for maybe 50-70% of the time. As good a night as I can hope for, here at sea level on the east coast.
I'm going to remain skeptical of my observation, but with some cautious optimism regarding it. It seems now we've got 2 folk who are seeing this curious effect, and that breeds hope! I'd love to hear some more accounts, successful or not. Skepticism is entirely warranted, I think, when we're attempting to observe some form of detail/albedo on a disk that is a little more than 2x our scope's resolving limit.
Last night held 50km winds for me, and Orion's bright stars were twinkling mightily when I looked out. With luck, the coming days may afford me another chance to observe this.
Thanks for your thoughts!
Posted 27 November 2012 - 12:48 PM
Posted 27 November 2012 - 12:51 PM
Posted 27 November 2012 - 12:59 PM
Then it dawned on me, is this even possible in a 6" with an Airy disc 1.84" arc? Io, according to JPL sim is only 1.2" arc. But, it is not a point source, it's an extended object. So, maybe more than a few tiny spurious discs overlapping at magnitude 5 might give it the edge into doable territory. I really don't know.
By no means was my observation conclusive, either. I was equally convinced Io was not circular as I was unconvinced it was elongated. Something like that. I was torn to whether it was or wasn't. The thing that struck me was, I was not torn about Europa in the same way. And not nearby Callisto either. It was that close, it was that difficult. Maybe the power of suggestion runs deep.
I dunno what to make of it. Like I mentioned, there was no doubt 72 Peg was elongated. Absolutely none. It's 0.5" arc. It was easy in comparison to detecting elongation in Io.
Surely it becomes easier in larger apertures, and I remain convinced it's doable, as you and Eddgie reported. I am convinced you might well have seen it. I mean, I am that close to believing I did, too. I just hope it's theoretically and practically even possible with 150mm aperture. If not, well, I must have experienced the power of suggestion. Surely that's always a possibility, especially when you are operating at the very absolute limits of our senses...pushing the envelop. You can bet I'll look again.
Thanks for pointing Io's effect out. I noticed it on images, but just thought it was an artifact of motion.
Posted 27 November 2012 - 01:03 PM
"What are you doing, Pete?" Any more powers of suggestion you want to inflict in my head? Next time I sketch that spot (tomorrow), look for some very tiny corners on it. And maybe an ape nearby.
I might just add them for fun, though. A small monolith would be easy. I might can draw a monkey using about 8 pixels...
Edit: On a sad note, I /think/ I discovered a small cataract in my left (dominate) eye. I am of age to have them. Thankfully it's not directly on axis, but very occasionally, if I look at Jupiter at some specific angle it blurs pretty bad and has a small diffuse darkening imposed on it. My observing eye seems to be clear in this regard. And it seems a bit sharper and more sensitive. Gotta stick with it, I guess. Anyone else run across this?
Posted 27 November 2012 - 01:31 PM
My right eye never turns in a good performance with optics, despite it being dominant, generally floater free and not much worse in astig than my left. No cataracts yet, thankfully, though my mother has developed'em.
Posted 27 November 2012 - 09:40 PM
Doing the math last night in my head, and wagging it while half asleep, I figure the spurious discs on Io should be about the same size as a 6th magnitude spurious disc in a 150mm unobstructed scope. A little larger because it's brighter, then smaller a bit due to the CO. Anyway, the spurious discs formed on Io should be smaller than Io itself, at least currently.
So, if that is true (I believe it is), then at least one spurious disc on the eastern limb could be seen as elongated from one on the western limb. And, of course, with many more between. This would hold true for one on the northern limb elongated from one on the south. And, if they are a bit less bright, they may appear darker even if overlapped by some brighter ones along Io's equator.
It would be much like observing 72 Pegasi where the companion is in tight but a tad fainter (~5.7 and 6.1, respectively.) And elongation was visible with the fainter companion as a hump on the companion's disc at just 0.5" arc. That's smaller than the separation at Io's equator (and poles.) And that separation is tighter than the Sparrow limit at 107/Dmm. But there was no contrast between the spurious disc centers, just elongation (very evident in the diffraction ring) and a slightly fainter "hump."
Trick is, are Io's poles darker by enough to be detected as being dimmer than it's equator, hence some elongation effect? That get's complicated with numerous spurious discs superimposed on one another, unlike the two easily elongated by 72 Pegasi. Observing Io might be like separating two 5th mag stars separated by Io's diameter: two 5th mag stars at 1.2" arc. Bright when PA is along the equator, and dimmer from pole to pole at that same separation. At that mag and sep, it would be like splitting 32 Ori (slightly unequal pair at ~1.2" arc) with no dark space, of course. It is obscured by other points.
Processed and enhanced images show pole darkening quite easily, they eye not so. Hence the PITA you and I have calling it elongated. But, again, I think it's doable in 6" and 8" apertures based on the math and reports observing elongation - or hints of it. Not easy by any stretch, but Eddgie could do it with a higher degree of certainty.
Posted 01 December 2012 - 02:16 PM
- Asbytec likes this
Posted 02 December 2012 - 03:16 AM
I'm thinking that the elongation effect might be caused by sunlight reflecting on gas along Io's equator. Looking at the photo that Norme posted we can see that while the moon appears elongated, its shadow does not. This effect would probably be apparent whether Io was in transit or not...maybe...possibly...?
Posted 02 December 2012 - 04:46 AM
This effect would probably be apparent whether Io was in transit or not...maybe...possibly...?
Don't be confused, it just might.
Personally, I found having something comparable in size that is known to be spherical for comparison introduced a small bit of doubt about Io being circular or spherical. The shadow might serve that purpose. Callisto was in the same FOV for comparison before Io transited. That was very convenient, but not conclusive.
It was a very tight call with Io against the black of space even with Callisto nearby. Over the cloud tops, it was more convincing. It really did take on a tiny bit of a bright dash-like appearance, whether exclusionary or not. It was still very hard to say for sure. I'm convinced it's more readily doable in greater apertures, even against the black of space.
Posted 02 December 2012 - 08:52 AM