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Denk Binoviewer did not resolve Jupiters Moons

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#1 teckpro


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Posted 18 November 2012 - 09:20 PM

I was viewing jupiter last night through Denk's ,d21 and power switch, attached to lx200 12" gps,f10.

Jupiter looked sharp with detail ,contrast and color , from my 200+ actual hours of viewing jupiter with different scopes and EP. I would rate this a 9.
However Jupiters moons did not look Like disks, they were unresolved I dont remember seeing this before. About the same with power switch at all 3 of the settings.

Any Ideas???

#2 George9


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Posted 18 November 2012 - 10:41 PM

I don't have an answer, but we were viewing Jupiter last night in a 24 inch dob with a Denk II binoviewer and Denk 21's using the power switch at all three powers. Jupiter was well resolved within the limits of seeing and the moons were definitely extended disks (easily seen by A-B'ing with nearby stars). We did not see actual details on the moon's surfaces.


#3 Eddgie



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Posted 19 November 2012 - 09:49 AM

Seeing or thermal issues with the scope.

Jupiter's moons will resolve as sharp disks even in a 6" scope if seeing is good and the scope is cooled.

And even a 6" scope will hint at detail on the disk, but only when conditions are very good.

So, if you don't seem the as fairly sharp disks, look to seeing or thermal issues in the scope.

Remember, the disks of these moons are smaller than many features that are present on the disk of Jupiter itself. Much of the detail on Jupiter is quite large, but has very low contrast. The bigger the scope, the more of these larger, fainter structures will come out, even if seeing is not great because the big low contrast details are not limited nearly as much because of seeing as the very smallest details.

I have been saying this for years... When seeing is not perfect, a big scope can still show the larger available detail with better contrast than smaller scopes will show. You see all available detail as showing with more contrast than you will see it in a smaller scope, even though the larger scope is not resolving to its limit.

And Ganymede for example is about the size of many small ovals that appear on Jupiter, so they are not at all large targets. It takes very good seeing to show them as fairly sharp disks, and better seeing than that to resolve any features on the surface.

And this is why shadow transits will often be easy while pale ovals of the same angular size will be impossible to see. The shadow stars with near 100% contrast, so even if your scope looses 50% or 60% of the contrast, it will still look black to you because it is against a very pale background, and even when seeing is poor, you can still see it.

But these ovals start with only about 20% contrast, and the scope will drop that to between 5% and 10% (in your size scope, higher in bigger scopes, lower in smaller scopes) even when seeing is perfect.

If seeing is less than very good, even a big scope will no longer show them though they will still easily show the shadow transit.

But the moons start with high contrast, and when seeing is less than excellent, the light from the limb of the moons is scattered randomly to a point just off of the limb of the moons (also happens to ovals of course) and the result is that the limb becomes indistinct.

Patience though will often allow you glimpses. Most of the time when I resolve detail on Ganymede, it is only because I sat there for 30 minutes waiting for moments of good seeing. I don't think I have ever had conditions so perfect that I could just point the scope at Ganymede and see detail. I have always had to wait for it.

Binoviewers make that easier for sure.

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