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Jupiter and twilight: prolonging the experience


John

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I can certainly see the moons as disks against dark sky, but you do need high mag to dim them down a bit. Once or twice, I have seen Ganymede as definitely darker than the other moons, but not seen visually any albedo detail.

Chris

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I tried an experiment recently to see if I could identify the moons just from their appearance. And i could, kinda. Ganymede does look bigger and is by far the easiest to identify, Callisto is second and the little two can generally be distinguished by their colour. I'd expect Io to be redish but actually to my eyes, Io is slightly blue and Europa slightly red. I probably have about a 50% success rate in identifying all 4 in a 7" refractor. I have convinced myself that I could see some markings on Ganymede on occasions but then again, I've also convinced myself that after a couple of brandies my singing's quite good :D

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I do see the Jovian Galilean moons as disks with my scopes, even when against the sky. I sometimes play a little game trying to ID the moons from the apparent diameter of their disks as seen visually. Most of the time, with some concentration, I can get it right :smiley:

I've not seen clear details on any of them though although Ganymede was showing quite a well defined disk at 600x with my 130 refractor the other night. My eyes are not sharp enough to spot any shading or other variations though (at least not any that I'd trust ! :rolleyes2:)

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Thinking about it, Ganymede definitely shows as larger than the others, I'll have to have a better look next time out and compare it with a similar brightness star to convince myself....

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Very interesting comments - yes I definitely do see the moons as different sizes and colours, and can generally have a good shot at identifying them visually. What I see is a bit of 'hairy-ness' along the disc edges against the black of space.  However, what I noticed that night was that while appearing against the backdrop of Jupiter, Io's disc was very sharply defined - it was a wonderful little sphere (@ 430x; planetary detail holding up very well, so it was a very good night). I watched it slowly clear the limb, and the sharpness in disc edge clarity that I perceived faded, while sharp detail on Jupiter remained - that's why I was wondering if it was a contrast thing, and not seeing.  I was wondering if the 'Airy diffraction' effects around the edges, and other imperfections due to my optics and eyeballs, where not being perceived by my brain due to the brighter backdrop of Jupiter or something.

Tips like looking at a bright light/white sheet sound intriguing, and I think MikeDNight mentioned trying a blue filter when observing a planet at low altitude to reduce atmospheric chromatic dispersion effects.  I've also been contemplating getting a variable polarising filter to reduce light on say Jupiter/the Moon.

Sorry for straying a little off topic John - I'm very interested in how these effects of contrast might impact how our eyes are registering detail :)

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I remember that Callisto can look very dark when in transit across Jupiter's disc (which it doesn't seem to do very often) and has been mistaken for a shadow. 

I often think that if these moons were independent minor planets, say, then we would pay them more attention and try to tease out detail and colour like we do when Mars is very small. But the attractions of Big J are too great!   

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9 hours ago, Paul73 said:

I've only seen discs against the planet. Don't you loose most of the planetary detail at those sort of mags?

Paul

 

Jupiter itself does not benefit from super high magnification, I agree.

It's fund to play around sometimes though and see what happens :smiley:

According to Cartes du Ciel, the apparent diameters of the 4 Galilean moons are 1.57", 1.43", 1.07" and .93". While very small, they are certainly discernable as a definite disks with a 4" or larger aperture scope IMHO. We see Neptune as a definite disk at high power and that is just 2.4" currently.

 

 

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18 minutes ago, John said:

Jupiter itself does not benefit from super high magnification, I agree.

It's fund to play around sometimes though and see what happens :smiley:

According to Cartes du Ciel, the apparent diameters of the 4 Galilean moons are 1.57", 1.43", 1.07" and .93". While very small, they are certainly discernable as a definite disks with a 4" or larger aperture scope IMHO. We see Neptune as a definite disk at high power and that is just 2.4" currently.

 

 

I'm certainly going to give these more attention in future. I guess I look for transits/shadows etc but don't pay so much attention to the moons when away from the disk.

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2 hours ago, Stu said:

I'm certainly going to give these more attention in future. I guess I look for transits/shadows etc but don't pay so much attention to the moons when away from the disk.

I did see detail on one occasion, but only when the brightest spot on Ganymede was in the right place, viz:-

Chris

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Following John's report, I setup as soon as bath duties had been completed tonight. It takes ok me a little while to pick Jupiter up as there is some hazy cloud around and I wasn't sure on focus position, but once found in binos it was easy enough.

I've just come in from a nice 15 minutes with some lovely views just proving John's point. The glare was reduced and I had lovely detail coming through including the soon to disappear GRS.

As for seeing the moons as little disks? Yeah, I'll give you that, I reckon they are :) 

EDIT Got my timings wrong, GRS transitting at 21.44. Sky still clear!!

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Very steady seeing tonight in unexpectedly clear skies. As Stu says, GRS and detail very good on Jupiter and I made a note of confirming all the moons as discs with some variation in colour - Callisto and Ganymede  looking yellowish compared with the others.

Got Comet Johnson too before the moon rose - and split Zeta Herculis for the first time! An enjoyable session with the 100ED on the Ioptron 

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I observed Jupiter's Moons for the last hour (twilight) at 150x, 225x and around 400x with the Vixen HR 2.4mm and Baader VIP. As I was concerned that a potential moon enlargement was the consequence of the Airy disk at very short exit pupils, I used a star as a control. Thankfully this star, HIP 62915 (mag 6.45), was right below Jupiter and its magnitude was just slightly lower than the four moons. 

Ganymede was clearly larger than the other Moons at all these magnifications. In particular it was noticeably larger than the control star. Callisto and Europa appeared a bit larger than the star at 225x, and clearly larger at 400x. Io was larger at 400x. At 225x Callisto was slightly darker than Ganymede. No colour or feature was visible on their surface.

Now at 400x, Ganymede, Callisto, and Europa appeared like small disks. In comparison, the control star was about 1/4 their size. Io was just a bit larger than the control star. After checking on S&T - Jupiter's Moons application, I found out that Io was behind Jupiter two hours earlier. 

 

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As well as the size difference, I find that the moons "shimmer" differently with the seeing, presumably because unlike stars, they are not point images. Even with my wee ED80 tonight, I could see Ganymede clearly as a disk.

Chris

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