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First Light on Uranus and Neptune!

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#26 Sheol

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Posted 10 November 2020 - 07:20 PM

      I've never tried to boost my magnification up enough to see Ganymede as a real disk. Never mind see details on it. I doubt my seeing as well as clarity would prevent that from being possible here at home.

 

         Matt.



#27 Redbetter

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Posted 12 November 2020 - 01:48 AM

      I've never tried to boost my magnification up enough to see Ganymede as a real disk. Never mind see details on it. I doubt my seeing as well as clarity would prevent that from being possible here at home.

 

Shouldn't take all that much magnification with an 8".  200x should be more than enough.  That is a pretty normal planetary magnification range with my 8" SCT.

 

For reference, the diameters of planetary/moon disks:

  • Uranus ~3.7 arc seconds
  • Neptune ~2.4 arc seconds
  • Ganymede ~1.7 to 1.8 arc seconds during opposition, ~1.3 arc seconds now, ~1.2 arc seconds approaching solar conjunction.  

Even my 80ED begins to reveal Neptune as a disk at 100x, despite Neptune's angular size being less than that of airy disk diameter (~3.46" by my estimate in that aperture).  While the resolution of the scope is bordering on marginal for this, further magnification reveals some things.  The spurious disk of an 8th mag star is perhaps half of the airy disk diameter (which is measured from minima to minima) in an 80mm.  Neptune as an extended object, near the resolution limit of the aperture, has fuzzier edges and lacks an observable diffraction pattern.  The result is that it is noticeably larger in apparent size than the same magnitude star. 

 

Incrementally larger apertures more clearly resolve the edges of the planet, while the airy disk (spurious) disk pattern shrinks at the same magnification as aperture increases.  A 100-127mm scope makes things considerably more recognizable than the 80.    

 

Ganymede is brighter and closer than Neptune, helping its resolution.   This is offset by being about 30% smaller in apparent size near opposition.  If the brightness of the planet was the same as Ganymede it would take about 40% more aperture and resolution to reveal Ganymede the same way. 

 

A general rule of thumb is that modest brightness objects generally begin to appear non-stellar in the eyepiece as they reach roughly 3 arc minutes of apparent size.  (Brighter objects and/or better eyes will reduce this to 2 arc minutes or less from what I recall and full daylight conditions can narrow this down more.)    So if one has the aperture to resolve the disk sharply, it should only take about 3*60/1.8 = 100x to begin to see Ganymede as a tiny disk.  200x makes the difference more noticeable.   


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#28 Sheol

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Posted 12 November 2020 - 07:44 PM

            Hmm, I should say I have never used really high magnification on Jupiter for some reason. I certainly have for Saturn, to see ring details & reveal moons other than Titan. I do have a 3x Barlow but it is only used on Luna, as a rule. And rarely at that, the light hurts my eyes from our moon, which is why I'm a DSO person. Never thought to use it on Jupiter, but definitely would show the "Big 4" Jovian moons as disks, if used with my 10 mm Sirius Plossl ep. 

 

         Clear Skies,

            Matt.



#29 Redbetter

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Posted 13 November 2020 - 01:26 AM

Sounds like your accessories were not chosen with reasonable high power viewing in mind, and instead skipped over the normal planetary viewing range.  I believe your scope is an 8" ~1200mm focal length, f/5.9.  The 10mm is only getting you to a mid range exit pupil of 1.7mm and, at 120x, it isn't quite at the planetary magnification level either.  It would be just entering it for a scope of half the aperture.  My preferred planetary magnification with an 80mm is about 150x, and with a 60mm it is 100-120x.  It is worth mentioning that once Plossls drop into the 10mm range and below they become more of a pain to use with eye relief of about 2/3 of their focal length, and the narrow field of view.    

 

You could get into a better magnification range with a 2x Barlow for the 10mm, but a 3x is jumping to the far end of the planetary range with an 8" Dob.  The 3x is 360x with a 0.56mm exit pupil.  There aren't likely to be many nights that is a good combo unless you have very steady skies, and your optics are quite good (and thermally equilibrated.)  However, one thing the 3x, 10mm pairing should be good for is star testing/getting a feel for the appearance of the airy disk pattern.  The latter is a prerequisite for pursuing very close doubles and it will also help you appreciated difference between a 5th mag star and a moon of the same magnitude that has some actual physical size.    

 

To equip my son's 10" f/5 (1250mm focal length) I chose a 6.7mm and 4.7mm eyepiece for the high power end (both for planets and some DSO's, particularly globulars.)  The 6.7 at 187x is used on nights when the seeing is mediocre or poor, while the 4.7mm at 266x is for nights with good seeing.  He uses the 4.7 a lot, often on mediocre nights that I would drop down to the 6.7--but young eyes better handle the focus fluctuations of mediocre seeing.  On very good nights I loan him a 3.5 (or 7 with a 2x Barlow) to provide 357x--but this is infrequent.  

 

Good luck.



#30 Sheol

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Posted 13 November 2020 - 07:17 PM

    LOL. I have a much larger range of EPs than that would sugges these days, but remember I am more of a DSO hunter than Planetary viewer anyway. I do have a 2X shorty Barlow that works extremely well. Also, I have a Plossl smaller than 10mm. Just one, but with the barlows, I actually have 8 available magnifications. Not counting my 2 inch EP for sweeping large areas at low power, apart from those. No, that isn't right this is... 4 Oculars plus a 2X shorty Barlow & a 3X Barlow. 

      Unfortunately, these eyes aren't young anymore. I hit 58 this year. Not ancient but not young either..

 

      Clear Skies,

        Matt.



#31 REC

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Posted 14 November 2020 - 01:30 PM

Last night around 2:30 while checking out various DSOs while my imaging rig was running, I happened to notice that both Uranus and Neptune were in pretty decent placement to view as I was swiping around Stellarium on my phone. I did a pretty long star hop starting in Pegasus and ending up at Neptune, just outside Aquarius. 

 

Wow! While still very small in my 8" dob with ES82 11mm, the color of Neptune was incredible. Brilliant deep blue. Much richer color than any blue stars I've seen. I could faintly make out the disc of the planet, and could confidently say it was not just a point source of light. My skies had a little bit of high level haze, so I didn't try for a high magnification. 

 

Next I turned east to track down Uranus. After another star hop, I had it clearly in my eyepiece. Wow again! This one had a much more distinct disc to it, clearly not a star. The other dead giveaway was the pale green hue. Reminded me of some planetary nebulae. Absolutely beautiful staring deeply at Uranus. I'm fairly certain I could pick up Ariel as well. I'm sure I could have seen other moons if I had realized where they were and tried identifying. 

 

I think it's so amazing that with fairly amateur gear, I can directly observe these incredibly distant planets from my backyard. And of course my wife and I had some fun laughs about me looking at Uranus last night. 

 

Clear skies all!

I need to observe them, it's been a while since I have. Good placement now.



#32 mikemarotta

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Posted 17 November 2020 - 11:50 AM

Next I turned east to track down Uranus. After another star hop, I had it clearly in my eyepiece. Wow again! 

I'm having a bit of trouble there... After two nights of sweeping under alpha and beta Aries, I still have not picked it out. Last night I tried measuring fingers east and fist north from Mars and sweeping there, but no joy. Aside from dogged and relentless pursuit, do you have any tips?

 

Thanks!

Mike M.




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