Detail on Ganymede ?

[John]

A few nights ago I was playing around with high magnifications with my 12" dobsonain. At one point I was using 450x on Jupiter. While the contrast and sharpness of the features was better defined at lower magnifications, the image held up quite well considering. I also noticed that the moons of Jupiter really showed noticeable variations in the apparent diameters, and subtle tint, of their disks.

I have read that some observers have discerned, while not details as such, albedo variations on Ganymede and Callisto. To do this the conditions have obviously been excellent and I've seen this reported with scopes from 11" and upwards where 400x - 500x magnification was usable to good effect.

I did not detect any noticeable contrast variations on the disk of Ganymede on the above occasion but it's something I'm going to pay some attention too when the seeing conditions are kind to me again. I believe Ganymede's disk is around 1.5 arc seconds in apparent diameter.

While I'm waiting, I'm wondering if any SGL members have managed to spot any trace of details on the larger Jovian moons ?

I'm talking about visual observations here - I have seen images which do capture some detail on these worlds.

[voivod]

How much magnification do you think is needed to show the moons as disks?

The maximum I can get is 300x, and the moons have always just been starlike points to me.

[John]

I've seen them as apparently different sized disks at 250x with my ED120 refractor. I say apparently because I'm not sure if I was actually seeing the moons apparent disk or their airy disks. In any event I could tell which moon was which by the size of the apparent disk I was observing. I made a note of the moons position and it's respective size compared to the others and this tallied with the positions of the various moons when I later checked them out.

[voivod]

So at 300x a disk should be fairly noticeable?

Never really spent much time observing the moons because they just looked like stars to me, and because I would spend most of my time just looking at Jupiter. I will definitely check them out next chance I get though.

[John]

If your scope and the seeing conditions are capable of supporting that sort of magnification, you should notice that the moons have different size disks.

If the conditions are not so good and the scope not up to that sort of power then you won't get clear disks I think.

[jimmyjamjoejoe]

I've seen images on here of people getting very rough detail on the jovian moons, so i suppose its possible. They werent massive details, just coloured hemispheres etc. I never thought it was anywhere near possible until i saw those.

[alan potts]

John,

A nice one to think about trying but at the moment I I am having the upmost problems with seeing the GRS, I think someone has nicked it, call the Jovian Police. I can never remember going above X250 on Juptier which is a bit shortsighted of me really.

Alan.

[cotterless45]

Just size difference and crescent on Ganymede. In a Newt you collimation should be very accurate and you'll need a spectacular night,

Nick.

[lukebl]

Just size difference and crescent on Ganymede. .....

A crescent's not possible wih the position of Jupiter and its moons. They will always appear at the same phase as Jupiter, i.e. no less than about 99%.

However, Ganymede has an apparent diameter of 1.8" at opposition, so I guess it's do-able with big optics and good seeing, and plenty of people here have managed to image some detail.

[Stu_2011]

Hi

If you look at some of Damian Peaches' pictures of Jupiter then he sometimes resolves detail on the moons. I think the really good ones (and this is relative - his pictures are all good) are taken from somewhere like Barbados with his 14 inch SCT. Amazing pictures....

Thanks

Sent from my iPad using Tapatalk

[Astro_Baby]

I have had a single night with my 8" newt where I could see the moons as small disks. Like John I am unsure whether I REALLY saw that or whether it could have been an airey disk or even my own imagination. the eye/brain combo are fantastic image peocessors capable of creating perfect images from scatter and hazy conditions.

I have also had a single night with the Skymax 180 using much overpower where I could discen a single moon showing a disklike appearance but again it was hard to know whether this was true or merely a visual defect in the scope or my own brain applyimg some tricks to the image.

Both times the seeing conditions were near perfect. Cold, stable air, no wind and good transparancy, very cold on both nights.

[Macavity]

Hmm... Back of envelope calculation (Do check figures!):

Diam ratio (Gan / Jup) times (Angular size Jup) arc sec?

5000 / 140,000 * 40 = 1.4" (arc) --> Sounds possible?

But not until I collimate my MAK150 / get better seeing!

[chiltonstar]

With a humble 5" Mak, the moons do have different apparent sizes which is I believe diameter rather than airy disk size (edit: it is both of course, diameter convolved with the airy disk diameter); in nights of exceptional seeing at x240, I am sure I've made out albedo differences on the larger moons - more obvious when they are against a dark area of Jupiter itself than against a sky background.

Chris

[michael.h.f.wilkinson]

Under good conditions I have seen the disks of one or two moons at 290x, and even at 234x. Io does always strike me as yellow, but otherwise I need a bigger scope (don't we all )

[John]

Interesting reading your thoughts and experiences folks, thanks for posting them

Jupiter is such an interesting object - always something to be looking out for

[paul mc c]

I had a quick session on the 5th when the seeing was excellent,probably the best i have seen,i was using powers of x352 a combination of a 9mm BGO and my x2 barlow and Ganymede showed as a disk very sharp and steady and a was a tan/brown colour,never noticed any variations on the disk,then i didnt look for any.It was a quick session(Man U match was coming on)

Will have to look if i ever get as good seeing again.

[Macavity]

<Begin Wibble /> If I'm right, and the Galilean satellites are (say) 1-1.5" arc in diameter...

Resolution of even the most basic scope (60mm objective), is better than one arc second? The eye's resolution is about one minute of an arc. So, once you get (a modest bit!) above a (Tasco?) 60x60 refractor, you can see "many" visual double stars. IIRC, this is more limited by "seeing" - Worse than a FEW arc seconds on a BAD night?

What intrigues me: The ability to see 2-dimensional objects, less than an one arc second in one dimension? You can't see a penny at one mile, but you can see electricity transmission lines, right? Do the maths! I suppose this is something to do with "pattern recognition". Or maybe I just got the calculation wrong?

It seems a feature of *extended* objects - They rarely conform

to the (simple) theory. But such is (observational) Astronomy?

[earth titan]

I've only ever really been able to determine very subtle colour differences, not what I would call detail. Even on the best of nights.

Typed by me on my fone, using fumms... Excuse eny speling errurs.

[John]

I had a quick session on the 5th when the seeing was excellent,probably the best i have seen,i was using powers of x352 a combination of a 9mm BGO and my x2 barlow and Ganymede showed as a disk very sharp and steady and a was a tan/brown colour,never noticed any variations on the disk,then i didnt look for any.It was a quick session(Man U match was coming on)

Will have to look if i ever get as good seeing again.

Thanks Paul - thats pretty much the best I've managed to do with it as well, so far

[steveb1977]

I have been able to relsolve the moons to tiny discs and managed to pick them out as they transit Jupiter, but have never been able detail or variations in contrast.

[jimmyjamjoejoe]

You can't see a penny at one mile, but you can see electricity transmission lines, right? Do the maths!

Mind = blown.

Also, you left your wibble open.

[Macavity]

Also, you left your wibble open.

I did, I did. Where is the documentation, when you need it...

[michael.h.f.wilkinson]

I regularly do the math

You can see (i.e. detect their presence) structures smaller than the limit of resolutions, you cannot resolve them (i.e. measure their size or determine their exact shape). What you see is their shape blurred by the diffraction pattern of the optics and the seeing disc. As the object is blurred more, the contrast reduces, so for a very tiny object to be detected it needs a great deal of contrast. Blurring is also slightly less pronounced for linear structures than for points. Given the contrast of a star (like the sun) with respect to the black background is pretty high (try looking at the sun ) you can see it clearly. The Cassini Division is another object which can be seen without resolving it fully.

[Macavity]

I regularly do the math ... You can see...

I think that says it all for me! Thanks, Michael - An important contribution.

[michael.h.f.wilkinson]

I posted on the topic of seeing objects smaller than the limit of resolution here:

include illustrations of the point