Last night I saw the effects of seeing on Jupiter
Posted 15 December 2013 - 11:21 AM
At 89x, there was no glare on Jupiter, even though that is at least a 6mm exit pupil. I could see several main and minor cloud bands. Also, the moon did not have much of a halo around it, which told me there was not much water vapor in the air.
An hour later, he had it on 288x, this time zoomed in close enough I could not see all the moons. I could barely make out the two main bands. We figured there must have been some slight invisible clouds that moved in. Two hours before I first arrived, the sky had been completely clouded over. He dropped down to 150x to see if that would make it look sharper. It helped with seeing all the moons in frame, but I still could only faintly see the main cloud bands.
So, my conclusion is that the quality of the atmosphere is the most important factor when looking at the stripes of Jupiter. Too bad I did not have a 60mm telescope there at the time at 90x to compare the resolution side by side.
I also looked at the moon on the scope later (practically full moon) and almost got blinded. I saw an after image for 5 minutes later, and it hurt. That was at 188x, which is just over a 3mm exit pupil. Strange because the moon never hurt my eyes before.
Posted 15 December 2013 - 12:14 PM
Posted 15 December 2013 - 05:59 PM
Posted 15 December 2013 - 06:45 PM
Without a doubt, to my eye, the atmosphere is a huge - the biggest - factor. Sure. It's amazing the difference between Ant I and Ant II, for example. Much finer detail rolls into view in Ant I and even in Ant II you have to be much more patient. Ant III, and things get very difficult. Elevation helps, even 40 degrees is can roll and blur. The zenith, of course, is the best place to really get into the belts and zones.
So, my conclusion is that the quality of the atmosphere is the most important factor when looking at the stripes of Jupiter.
As you elude to, it's unfortunate aperture is also affected by seeing and 24" is pretty big. Depending on the seeing, you can still get some finer detail during moments of steady seeing, if those moments come around. But, you have to wait for them, as you know. If the 24" view is comparable to a 60mm, I'm betting the seeing was horrid. Any idea what scale it might have been?
This makes sense, Mike. Jupiter might look like a disc with a lot of dark features, but it's really a lot of low contrast stuff. That low contrast stuff, most of it's detail, seems to be susceptible to seeing. The main belts probably have the highest contrast and survive bad seeing pretty well.
...there's just more subtle detail there than on any other target.
Posted 15 December 2013 - 08:43 PM
And what do you mean by scale? Magnification? I got my best view of Jupiter with a 60mm at 117x, after it was stopped down to 45mm. For some reason it was more washed out at a full 60mm.
I got a much better view at 89x with good seeing in the big scope, and then at 288x later in bad seeing it was only as good as the 60mm again. The 60mm was better because at the lower magnification I could see all the moons.
Posted 16 December 2013 - 02:03 AM
You may have some response given in the Ruten and Van Verooj book about scope design and influence of seeing where above some seeing levels the result given in a great aperture can be lowered. This means that the effective equivalent aperture given by the seeing filter is on a regression. In other words you may see more details on the same night with for instance a 8-10".
The Rutten book curve gives a good representation of this kind of efficiency. The Danjon and Couder book about telescopes too.
You may have also the FTM curves modifications involved by the seeing for 6, 12 and 24" for different seeing levels in terms of FWHM. The graph is given on this link:
I collected the graph on the web.
At final to say that there is no benefit for using great apertures in some seeing levels of average sites, you collect rather a lost of efficiency.
Until a certain amount of seeing there no "linearity" between the aperture and perceptibility of contrast, when this is you may see that stars are planets look like, diffraction pattern of a star is no longer accessible.