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Jupiter's Moons: Visual differences in apparent diameter


John
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Before clouds spoiled things I was observing Jupiter last night with my 4 inch Vixen ED refractor.

At 166x magnification Jupiter showed a nice large, if slightly flattened, disk. It's apparent diameter last night was a generous 48.2 arc seconds. 

The four Galilean moons (Io, Ganymede, Europa and Callisto) were strung out nicely in a line to one side of the planet. Observing them carefully at the same magnification showed each as a tiny but clear apparent disk.

Each time I observe these moons at a reasonable level of magnification I can see a noticeable difference in the size of their disks. Last night, as on other occasions, I tried to work out which moon was which simply by noting the relative apparent diameters of their disks, sketching their positions and my "predictions" of which moon is which and then checking using Stellarium or Cartes du Ciel software to see the actual respective positions of the moons.

Last night I got 2 out of 4 right, confusing Callisto and Io but finding Ganymede (the largest) and Europa (the smallest) clearly to show their identities through their apparent size in the eyepiece.

What interested me last night was why and how we, as observers with modest telescopes, can actually do this :icon_scratch:

I don't claim to be an exceptional observer and I have read reports from others quite often who spot these apparent size differences between Jupiter's major moons.

Last night the actual angular sizes and brightness of the Galilean moons, according to Stellarium, were as follows:

Ganymede = 01.78 arc seconds / magnitude 4.6

Callisto = 01.62 arc seconds / magnitude 5.8

Io = 01.23 arc seconds / magnitude 5.4

Europa = 01.05 arc seconds / magnitude 5.3

To put these in perspective, the maximum resolution of a 4 inch aperture scope is around 1.14 arc seconds I believe so all of these moons are far too small to really be resolved in such a scope. 

So why do we see differences in the apparent size of the moons with comparatively modest aperture scopes ?. Why is it quite obvious, with a little observation, which moon is Ganymede ?. Why does the somewhat smaller Europa appear just as that through the eyepiece ?

My guess is that we are seeing a form of optical effect rather like the airy disk produced by a star but produced by a combination of the moons brightness, it's comparative apparent diameter at the time of observing and the aperture of the scope rather than the "true" disk of the moons.

I'm not 100% sure of this at all though so would be very interested to hear other views :icon_biggrin:  

 

 

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I'm not an expert but my guess is that the image of a non-point light source is the integration of the airy discs of its points.

So for a small circular light source the image will have two parts: the circle itself which will be the brightest part, and a fainter ''halo''  around it from the contribution of the airy discs near the boundary.

As usual, an unobstructed scope e.g. a refractor will show a fainter halo because the diffraction rings contain less energy than in obstructed scopes. This must be why refractors produce tighter images of the moons. But in all cases the diameter of the image will be larger than the apparent diameter of the moon. As an approximation I guess we have to add the apparent moon diameter to the diameter of the airy disc of the telescope. It's probably not so simple because the ''halo'' is fainter but without calculations this is the best I can think of.

If we accept this then in a 4 inch telescope Ganymede looks like a 2.9'' disc while Europa is 2.2''. At say x120 magnification these will look like 5.8 arcminutes and 4.4 arcminutes, so well within the average observer ability to distinguish them as discs rather than stars and also to spot the difference in size.   

Edited by Nik271
typos corrected
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Interesting observations John. I shall try the same experiment next time I am out with my 4”. I did try to perceive discs when I was out with my 8” a few nights ago - I would say there were definite size differences but certainly not sharply defined. I can’t help with any theories about the disc visibility in a 4” but it’s interesting to see that disc diameter does not necessarily correlate to brightness, presumably due to differing albedo? 

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20 hours ago, John said:

Before clouds spoiled things I was observing Jupiter last night with my 4 inch Vixen ED refractor.

At 166x magnification Jupiter showed a nice large, if slightly flattened, disk. It's apparent diameter last night was a generous 48.2 arc seconds. 

The four Galilean moons (Io, Ganymede, Europa and Callisto) were strung out nicely in a line to one side of the planet. Observing them carefully at the same magnification showed each as a tiny but clear apparent disk.

Each time I observe these moons at a reasonable level of magnification I can see a noticeable difference in the size of their disks. Last night, as on other occasions, I tried to work out which moon was which simply by noting the relative apparent diameters of their disks, sketching their positions and my "predictions" of which moon is which and then checking using Stellarium or Cartes du Ciel software to see the actual respective positions of the moons.

Last night I got 2 out of 4 right, confusing Callisto and Io but finding Ganymede (the largest) and Europa (the smallest) clearly to show their identities through their apparent size in the eyepiece.

What interested me last night was why and how we, as observers with modest telescopes, can actually do this :icon_scratch:

I don't claim to be an exceptional observer and I have read reports from others quite often who spot these apparent size differences between Jupiter's major moons.

Last night the actual angular sizes and brightness of the Galilean moons, according to Stellarium, were as follows:

Ganymede = 01.78 arc seconds / magnitude 4.6

Callisto = 01.62 arc seconds / magnitude 5.8

Io = 01.23 arc seconds / magnitude 5.4

Europa = 01.05 arc seconds / magnitude 5.3

To put these in perspective, the maximum resolution of a 4 inch aperture scope is around 1.14 arc seconds I believe so all of these moons are far too small to really be resolved in such a scope. 

So why do we see differences in the apparent size of the moons with comparatively modest aperture scopes ?. Why is it quite obvious, with a little observation, which moon is Ganymede ?. Why does the somewhat smaller Europa appear just as that through the eyepiece ?

My guess is that we are seeing a form of optical effect rather like the airy disk produced by a star but produced by a combination of the moons brightness, it's comparative apparent diameter at the time of observing and the aperture of the scope rather than the "true" disk of the moons.

I'm not 100% sure of this at all though so would be very interested to hear other views :icon_biggrin:  

 

 

Another interesting experiment might be to image the moons, with different size scopes, and see the effect resolving them.  I could do it. i have a 102. 120. 127mm refractors here to attempt it. Not sure if i will try. but a interesting idea. They say the camera never lies ? 

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15 hours ago, neil phillips said:

Another interesting experiment might be to image the moons, with different size scopes, and see the effect resolving them.  I could do it. i have a 102. 120. 127mm refractors here to attempt it. Not sure if i will try. but a interesting idea. They say the camera never lies ? 

That's a good idea.

I have a similar range of refractors so could do a similar comparison sometime.

 

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Great observation John.

Like you I can also detect the different sizes of the Moons in my 4” scope - to be honest I just accepted it without thinking about resolution levels !

They are always a nice sight 🙂

Edited by dweller25
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6 hours ago, John said:

That's a good idea.

I have a similar range of refractors so could do a similar comparison sometime.

 

It certainly is a interesting idea. It will take the subjective quality away from understanding the limits of resolution you are discussing. Although scope differences could also affect this question. Meaning quality of scope, How much i am unsure. 

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2 minutes ago, neil phillips said:

It certainly is a interesting idea. It will take the subjective quality away from understanding the limits of resolution you are discussing. Although scope differences could also affect this question. Meaning quality of scope, How much i am unsure. 

I was thinking of comparing my ED120 Skywatcher and my Tak FC100-DL. Both are doublets working at F/9. More practically, I can get them both on my Skytee II mount !

 

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I believe its because the moon's are not true point sources, and so being extended objects they play a different game. In my experience telescopes that are supposedly incapable of resolving two stellar point sources are quite capable of resolving an extended object. When any of the Galilean's begin to cross the face of Jupiter and while they are still in the shaded limb area, they each display a definite disc, just as they do on leaving the disc. And as the moon's make 1st or 4th contact they display a definite disc in a 4" scope. 

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27 minutes ago, John said:

I was thinking of comparing my ED120 Skywatcher and my Tak FC100-DL. Both are doublets working at F/9. More practically, I can get them both on my Skytee II mount !

 

Good idea. The excellent quality of your scopes. will help remove or minimize the variable of quality. On the question of resolution resolving the moons John. 

Think i will leave this project to you. And check for any updates if you attempt it. 

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I remembered this thread in mind and I tried yesterday . I was easily able to tell Ganymede and Callisto were much larger than background stars ( and other 2 moons for that matter ) . I later checked SkySafari and confirmed that both the semi-point sources were indeed Ganymede and Callisto. It was a bit difficult to tell the extended nature of Io and Europa as one would expect from their sizes . 

It's fun and I recommend you try to see which are the moons comparing them to background stars before checking it with the software ! 

I used just 96× as seeing was bad but the lower power helped mitigating it . GRS too was visible ! 

Edited by Voyager 3
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A very interesting question you raise John,
It also again demonstrates that a 4" refractor can provide plenty to keep us amused with as Visual Observers.

Interesting on folks thoughts on why you can decern the differences based on the scope resolution,
but us Refractor lovers all know they come with a sprinkle of magic and that's what makes the differnce.

So my official reply is 'It's down to Magic Refractor Dust, coming into play'

 

 

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On 12/09/2021 at 02:45, John said:

Before clouds spoiled things I was observing Jupiter last night with my 4 inch Vixen ED refractor.

At 166x magnification Jupiter showed a nice large, if slightly flattened, disk. It's apparent diameter last night was a generous 48.2 arc seconds. 

The four Galilean moons (Io, Ganymede, Europa and Callisto) were strung out nicely in a line to one side of the planet. Observing them carefully at the same magnification showed each as a tiny but clear apparent disk.

Each time I observe these moons at a reasonable level of magnification I can see a noticeable difference in the size of their disks. Last night, as on other occasions, I tried to work out which moon was which simply by noting the relative apparent diameters of their disks, sketching their positions and my "predictions" of which moon is which and then checking using Stellarium or Cartes du Ciel software to see the actual respective positions of the moons.

Last night I got 2 out of 4 right, confusing Callisto and Io but finding Ganymede (the largest) and Europa (the smallest) clearly to show their identities through their apparent size in the eyepiece.

What interested me last night was why and how we, as observers with modest telescopes, can actually do this :icon_scratch:

I don't claim to be an exceptional observer and I have read reports from others quite often who spot these apparent size differences between Jupiter's major moons.

Last night the actual angular sizes and brightness of the Galilean moons, according to Stellarium, were as follows:

Ganymede = 01.78 arc seconds / magnitude 4.6

Callisto = 01.62 arc seconds / magnitude 5.8

Io = 01.23 arc seconds / magnitude 5.4

Europa = 01.05 arc seconds / magnitude 5.3

To put these in perspective, the maximum resolution of a 4 inch aperture scope is around 1.14 arc seconds I believe so all of these moons are far too small to really be resolved in such a scope. 

So why do we see differences in the apparent size of the moons with comparatively modest aperture scopes ?. Why is it quite obvious, with a little observation, which moon is Ganymede ?. Why does the somewhat smaller Europa appear just as that through the eyepiece ?

My guess is that we are seeing a form of optical effect rather like the airy disk produced by a star but produced by a combination of the moons brightness, it's comparative apparent diameter at the time of observing and the aperture of the scope rather than the "true" disk of the moons.

I'm not 100% sure of this at all though so would be very interested to hear other views :icon_biggrin:  

 

 

As a scientist and statistician I would say that maybe its not possible and that you just got lucky in your guessing. Unless you can see a clear pattern over multiple observation sessions in which  you average more than 2/4 correctly identified then it is just too easy to fluke 2 out of 4 (incidentally the rate of correct ID that you would expect on average through random chance alone is 2/4 correct). 

 

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6 hours ago, Adam J said:

As a scientist and statistician I would say that maybe its not possible and that you just got lucky in your guessing. Unless you can see a clear pattern over multiple observation sessions in which  you average more than 2/4 correctly identified then it is just too easy to fluke 2 out of 4 (incidentally the rate of correct ID that you would expect on average through random chance alone is 2/4 correct). 

 

When I've done this before I've got better results - a couple of times 4/4.

I was a little disappointed with 2/4 if I'm honest :rolleyes2:

Edit: Of course 3/4 correct is actually 4/4 of course ! (Doh !!! :rolleyes2:)

Edited by John
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Just trying this now with the 80mm, it’s quite clear which moon is Ganymede but I can’t tell the others apart.

Ganymede is reported on SkySafari as 1.8 arcseconds, Callisto 1.6, Io 1.2 & Europa 1.0. Despite Callisto being closer in apparent size to Ganymede, I’m just not able to differentiate it from the smaller two moons tonight.

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1 hour ago, IB20 said:

Just trying this now with the 80mm, it’s quite clear which moon is Ganymede but I can’t tell the others apart.

Ganymede is reported on SkySafari as 1.8 arcseconds, Callisto 1.6, Io 1.2 & Europa 1.0. Despite Callisto being closer in apparent size to Ganymede, I’m just not able to differentiate it from the smaller two moons tonight.

Similar result here tonight with the ED120 at 225x. Callisto just does not look as close to Ganymede's apparent diameter as it's listed figure suggest that it should :icon_scratch:

No chance to compare the 100mm / 120mm views tonight - too much cloud about !

 

 

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4 hours ago, John said:

When I've done this before I've got better results - a couple of times 4/4.

I was a little disappointed with 2/4 if I'm honest :rolleyes2:

Edit: Of course 3/4 correct is actually 4/4 of course ! (Doh !!! :rolleyes2:)

Lol yes 3/4 is not possible unless you guessed two of them as the same moon. Conversely if you guessed they are all Callisto then you are guaranteed to get at least 1/4 correct. 😉

Edited by Adam J
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Interesting topic! They do look quite different through a 4" and certainly through my 180 Mak. I had always put this down to the fact that the Airy disk is not "flat-topped" but a pseudo parabola (ie with a "point") and therefore resolution on some objects can be apparently slightly better than theory (one point Nik, shouldn't the Airy disk diameter and the apparent moon diameter be added as the SQRT of the sum of the squares rather than linearly?).

Differences in brightness will change the apparent visual diameter as well I believe - look at how large a bright star appears to be cpd with eg a mag 8 star.

Chris

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I had a short session under pretty good seeing last night and thought I'd see if I could definitely make out a disc in my 4". I homed in on the brightest moon, which turned out to be Ganymede with a diameter of 1.8", and went up to the max magnification I could manage which was around 320x. Ganymede appeared to be a definite disc, but I have noticed that with only 4" of aperture, stars at high magifications also show a pronounced disc (less noticable in my 6" and 8" obstructed apertures). So as a comparison, I looked at a nearby star of similar magnitude (around mag 5) at the same magnification. It was an interesting exercise - the star disc was slightly smaller, better defined with a clear diffraction ring, and appeared to 'move around' more in the slight air turbulence - I guess this would be expected from a point source? Going back to the Ganymede, it was slightly but clearly larger, less well defined edges, didn't really move around and didn't have a noticable diffraction ring - pretty much what you'd expect from a extended source? Does any of this mean I have definitely seen Ganymede's disc? What do folks think?   

Edited by RobertI
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It certainly sound like it @RobertI, nice way of establishing it. That’s not something I’ve done, just compared the different apparent size between Ganymede and Io I think it was, which are the two easy ones I think.

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18 minutes ago, RobertI said:

Thanks Stu. I forgot to mention that Jupiter itself was awesome, the GRS finally showed up for me!

Nice one! Is that a first for you? Really good when you do see it finally 👍👍

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I'm observing Jupiter with my 100mm refractor tonight through cloud that is just thin enough to make it worth observing, but I can't make out Ganymede's disk .......... it's still in Jupiter's shadow at the moment !

The 3 Galilean's that are showing do show slight variation in the apparent disk sizes at 225x though.

I'll wait for Ganymede to pop into view before packing in I think.

 

Edited by John
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18 minutes ago, Stu said:

Nice one! Is that a first for you? Really good when you do see it finally 👍👍

I’ve seen it quite a few times over the years Stu, but never with the new 102EDR, so was really interested to see what it looked like through a 4” frac. It was pretty small tbh and although very obvious, not that easy to make out the pink oval. Next time I’ll get the C8 to compare. 👍

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The GRS was nicely on show last night. It is smaller than I recall from previous years though.

Interesting to think that the GRS has an apparent diameter at it's widest point of around 7-8 arc seconds.

 

 

Edited by John
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