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Detail on any of the other Jovian Moons?

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#1 azure1961p

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Posted 17 November 2012 - 10:43 AM

I've confirmed Ganymedes details on several occaisions but I haven't even tried for Callisto or the others. And you know how it is, eventhough they are seen its quite another thing to mount a concerted efffort for a quest like this. Still I've seen large reflectors 14-18" actually image details even on Io and Europa.

Anyone ever make visual details on the other moons?

#2 Eddgie

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Posted 17 November 2012 - 03:42 PM

I will qualify my terminology before I answer.

To me, there is a difference between "Detecting" a detail and "Resolving" a detail.

In my smaller scopes, I would often "Detect" detail that was visible on Ganymede. In other words, I could see that there was albedo shading on Ganymede in my smaller scopes, but these details would appear more or less like shading differences across the face of the disk and not exibit specific shapes. I might see one limb as appearing darker or brighter, but not actually be able to detect the shape of the feature that made it. I could use a simulator to determine what features were visible at the time, and based on that knowledge I could infer that I had "Detected" their presence.

But it was hard to "Resolve" those features. By this, I mean that if there was a specific shape to a feature, I could usually not actually see it that way. Only some change in reflectance without any defined borders or shapes.

Using my C14, I have actually resolved features on Ganymede. I have seen Osiris as a tiny white point of light just inside the limb on one side of the disk, and I have seen the flat top "C" shape of Galilee Regio at the other limb a couple of times.

The reason I point these out is because they are about the only two features that I have actually resoved becuse they are about the only features that have a meaningful "Shape" or albedo contrast high enough to allow them to be resolved visually at a 14" aperture. Easy in an image as I will show you later...

For Io, the case is different. I have not really had any success detecting features on Io using my 6" scope. At best, in a 6" aperture, I see Io only as a tiny, pale sphere with no albedo shading at all. It has always appeared featureless and with little color.

In the C14, it is little better. The color is a bit more intense, and it often displays an almost pearl-like luster. I am going to assume that this is the effect of surface detail very slighly altering the albedo, but the amount of albedo change is so small that once again, I can only detect that there is perhaps some feature present, but cannot come close to identifying it. This is pretty common though (to see the pearlesent nature).

At least visually. I have seen C14 images that have resolved a fair amount of detail on both Ganymede and Io.

One of them did win an award and I am including the link. It is unmistakeble that there is resolution there, so I I feel 100% confidence that I have indeed detected detail on Io, but never really resolved it like the shapes shown in the image.

But Ganymede is a different story, and I have had rare occasions to see specific well resolved surface detail on Ganymede.

And doesn't Io bear a strinking resemblence to Mars as seen in a small telescope in this picture???

C14 image showing fully resolved features on Ganymede and Io. But hard to do...

Just the other night, I reported that I saw Io and its shadow transiting the GRS, and I mentioned that Io was pearl-like, and speculated that it was an albedo feature.

And an image posted by someone else the next day taken at the same time confirmed it. In the image I could see the the disk of Io was not evenly illuminated.

So, detected is the word I would use here and not resolved. It is within the range of the C14, but not visually I think. Only with a camera.

I could be wrong though, and maybe a night of perfect seeing would allow it. I just never get perfect seeing.

#3 Eddgie

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Posted 17 November 2012 - 03:50 PM

Regarding my previous post, if you stand back from the monitor some distance while looking at Io, you can see what I mean. When the angular size is very small (as it is in the eyepiece) the eye itself is incapable of seeing the shape (it has to be a couple of arc minutes of true field for this to happen).

If you magnify the image so that the angular size is bigger, it starts to get dimmer and I find that this works against me.

My best observations of Io and Ganymede have been made at 340x (slighly bigger than 1mm Exit Pupil in the C14). I have made the image bigger of course, but for the most difficult features, I find that they are often easier to detect when they are smaller but brighter.

No mistake though that I have detected albedo features on Io using the C14. It just take patience.

And increasingly, I am finding these details truely are easier to see using binoviewers.

Mark V binoviewer arrives in about 10 days. I can't wait.

#4 azure1961p

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Posted 17 November 2012 - 05:47 PM



That is without a doubt one of the most stunning C14 images I have ever seen. The details are off the chart - on everything! Its crazy good.

I understand what you mean by resolving specific features and generally noting albedo shadings. My views have been of the latter as I have needed better resolution in terms of seeing even in the good seeing that presented it. Those were my pre-fan days however so it'll be interesting to see what comes.

Io does look Mars like to be sure. For that matter an HST image of Ceres looked like a relative of Mars as well. The image I'm thinking of looks for all the world like. "MARS II - The OTHER Planet". Its different ofcourse in a number of classifications but that HST pic is uncanny. Again Edd you provided a stunning portrait of several things. Its so good its depressing LOL.

Callisto is one I've gandered and while its distinct color and value plus size is noted, it was always smooth in the 8" - tho its muddy kind of "look" was glimpsed as odd even with my 70mm. I'm holding out hope something may pop in terms of an albedo muddle with the cooled reflector.

Thanks for your comments and qualifiers. And again that image.... like it was shot in the vaccum of space with no seeing softening. Wildly good.


Pete

#5 MikeBOKC

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Posted 17 November 2012 - 06:41 PM

I have wondered about this. Never have detected or resolved any Galilean moon details in my 11 inch, though on rare nignts I get a distinct reddish hue from Io, and can note the different disc sizes. Hoping to add a 14 or 16 inch Dob to the arsenal soon and this will be a definite observing target.

#6 Asbytec

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Posted 17 November 2012 - 08:52 PM

I concur with Edggie's account of the 6". Ganymede is nothing like the image posted above, simply a bright spot and some limb shading. Its very difficult in anything less than darn good seeing and, for me, about 383x. (Pete, sorry, due to some errors in my focal length, I misrepresented 480x, earlier.)

Eddgie, something I am trying to understand better, but maybe one can call it resolution in a 6". The Raleigh limit is about 0.92 " arc (Dawes near to 0.78" arc.) That's about half of Ganymede's diameter. In that space, there should actually be 1 line pair of resolution: one bright (Osiris) and one dark (Galilee Regio.) So, while not well defined as a lunar crater, it is technically resolved. It is a bright, high contrast feature.

The other moons appear as tiny featureless discs, so far. Well, I did notice elongation in 72 Peg at 0.566" arc. Maybe Io will exhibit some surface feature "elongation", too. Christ, Pete... :lol:

#7 azure1961p

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Posted 17 November 2012 - 09:19 PM

LOL.

I think when you cut it down fine enough the resolved versus unresolved becomes a leap of faith. Not that in theory Rayleigh and such is belief system founded but there is a point where the fact of resolution is more apparent than the actual visual perception of it. On either side of ray and Dawes and sparrow - when the fractions are small enough a feature or disc could be resolved on paper but at the eyepiece its there only in fact but not in anyway apparent that its different in seperation at all.

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#8 Asbytec

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Posted 17 November 2012 - 09:34 PM

Sure, theory is cold and calculated with set initial conditions that rarely exist in the eyepiece. Theory on paper is not subject to seeing, either.

#9 Eddgie

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Posted 17 November 2012 - 09:55 PM

Eddgie, something I am trying to understand better, but maybe one can call it resolution in a 6". The Raleigh limit is about 0.92 " arc (Dawes near to 0.78" arc.)



This is where MTF comes in. While the scope could resolve a black and white line pair this size, these line pair starts out at 100% contrast.

And of course when you start with a feature that only has 30% contrast at the target, and you loose 80% of that available contrast, you are left with a contrast at the eyepeice of 6%. And that is about the bare edge of the mesoptic eyes ability to detect.

Also, a line will still show when a dot no longer shows. The line continues across the focal plane and has no "End", but a square would be softened not only on either side, but on the top and the bottom.

As I mentioned, I would call a feature seen only as a shading as being "Detected" but not necessarily "Resolved."

But that is my own personal classification. I see albedo features on Ganymede at 6" with a bit of difficulty, but never much more than soft blurs that do not appear to have any irregular shape.

Again, this is why understanding MTF is important. All telescopes loose contrast, and all telescopes loose 100% contrast at their max spacial frequence. Thin black and white lines will dissapear into a uniform gray.

But small irregular features will disappear (visually) at about .8 of the max spatial frequency unless they start with very high contrast at the target. Cassini's division is a classic example. It is easily visible in small scopes because it starts with almost 100% contrast. And yet if you looked for a similar size low contast feature on the surface of Jupiter or Mars, you would not be able to see it unless it starts with 30% to 40% contrast.

#10 Eddgie

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Posted 17 November 2012 - 11:01 PM

This is indeed one of the best Jupiter images I have seen from a C14.

I believe this was taken several years ago after the Comet Shoemaker Levy impacted. The sequence of black markings following the GRS I beleive are where the comet broke up and struck in a chain.

I could be wrong though, but I think that is what it is.

It is a superb image though, and this is a great demonstration of the resolution and contrast transfer capabilites of a modestly large aperture telescope.

#11 Asbytec

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Posted 17 November 2012 - 11:16 PM

I see albedo features on Ganymede at 6" with a bit of difficulty, but never much more than soft blurs that do not appear to have any irregular shape.


Yes, exactly.

Still, 6% contrast can be seen, so at the Raleigh frequency (1 line pair) per 0.92" arc, you will get a white spot (the bright line, or spot sized segment of that bright line) and next to it a shadow (not a dark line feature like Cassini) at 6% contrast. Ganymede's disc is twice as large (0.92 * 2 ~ 1.8" arc.)

So, really, one should get 2 line pairs across Ganymede using Raleigh, no? The "center" of Osiris is offset within Ganymede's disc by enough space to allow another bright feature's "center" to discriminate a darker feature between (or very near) them. For example, Ganymede's brighter limb.
Of course, magnitude plays a role, so it might even be more applicable to apply Dawes and a slightly higher frequency...line pair spacing...I think.

Actually, it might be a little more contrast, guessing. Osiris is pretty bright, relatively speaking, and that darker feature is not all that tough to see considering the size of the disc. This is why higher power works better, for me, I believe. In a 6", that spatial frequency has to be a bit lower than 50 to 60 cycles where the eye can just detect 100% contrast...I think.

Also, a line will still show when a dot no longer shows.

Yes, but that is the edge equation, not PSF of a bright object. It really does not apply to Galilee Regio, eh? Cassini, yes, that is a dark line feature with really no light of it's own emitted therefore is a "lack" of diffraction. That's different. Galilee Regio actually reflects some light and will exhibit diffraction that has to be "resolved" from Osiris. If you can imagine a dimmer star (at least one point in Galilee Regio) sitting on or near Osiris's first ring, then it's resolved, the way I see it.

But, you know this stuff inside and out, I am still trying to "apply" it. It stumps my brain every time.

#12 Astrojensen

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Posted 18 November 2012 - 06:19 AM

I believe this was taken several years ago after the Comet Shoemaker Levy impacted. The sequence of black markings following the GRS I beleive are where the comet broke up and struck in a chain.

I could be wrong though, but I think that is what it is.


No. Shoemaker-Levy 9 struck much further south, near the south pole and with much greater distance between impact spots. Also, the traces had almost completely vanished after a few months.


Clear skies!
Thomas, Denmark

#13 azure1961p

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Posted 18 November 2012 - 08:05 AM

I think that image was from last year when the SEB. Was beginning to reemerge first as a chain of spots then well all the grandeur we see now.
Also no one had the technology to image that cleanly with a c14 back then. Boy it was a helluva show tho when those impact scars began coming around.

Pete






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