Planetary sharpness

[markse68]

It struck me probably the first time I saw them that it was odd that Saturn often appears sharper than Jupiter under similar magnification. How could that be when Saturn is twice as far away? Is it just that it is smaller in the ep or perhaps the strangeness of the geometric rings kidding the eye that it’s sharper when it isn’t really? But then what about Mars? It’s much much closer yet rarely does it seem as sharp as Jupiter, it’s features less defined. Is that just because it has a fuzzying atmosphere? Similarly Venus?

Or could this be something to do with the distances- the furthest Saturn must be casting closer to a collimated beam of an image than the closer planets and those closer planets more of a divergent image. Could that collimated image beam somehow interact less with atmospheric disturbance? Just wondering out loud- likely nonsense

[vlaiv]

Not sure it is nonsense - I also think it could be distance related.

It could be related to spectrum as well? Maybe Saturn has the least light in blue part of spectrum - one that is the most affected by atmosphere?

If it is distance related - here is what I think is going on - place where closer objects get to focus is too close to where atmosphere gets into focus. I've noticed that when searching for that perfect focus on planets - there is a spot, usually "closer" (or focuser racked out further) where seeing is in focus. I'm not sure I can explain this, but it almost seems that I'm focusing on the blur rather than the planet that is behind the blur.

It's a bit like this image:

Front car is in focus - but so are distortions by heat rising from the road. Look at the second car - it is not in sharp focus - yet one sees same ripples - as if ripples themselves are in focus.

Similar effect happens when observing and it makes sense - one can focus on jet stream distance and any disturbance in jet stream will be in sharp focus and planet behind this will be out of focus.

When we know this - then it sort of makes sense - further away from these seeing disturbances you focus - less of an immediate effect on picture quality they'll have and hence planets further away may present better quality of view?

Or it could be due to different reasons - position in the sky or similar, but yes, I've noticed that too - Saturn seems to be "calmest" planet in terms of seeing influence for some reason.

[Astrid]

I might be wrong, but I think that this illusion happens because of the fact that Jupiter has a higher magnitude, so when you are looking thru an inferior to average quality scope, the light Jupiter reflects dims its bands, while for Saturn the ring is just easier to see since it is a separate structure, rather than a colored 'band' on the surface. 

I noticed this with the moon and a cheap scope too. When there is an almost/full moon I can see less details around the terminator/craters. 

[Stu]

I think everything appears as parallel light rays from that distance so doubt there is any difference there. Jupiter has quite low contrast features and significant limb darkened so that may explain its softer appearance. In comparison Saturn is higher contrast, certainly the brighter parts of the rings and there is much less surface detail to look at so that probably helps the perception of sharpness. Again, many of Mars’ features are low contrast and I think the effect of dust can reduce the visibility of these features at times, even when there is not a planet wide dust storm. The polar caps can be quite striking when larger, appearing quite bright white. All fascinating planets to observe!

[vlaiv]

1 minute ago, Stu said:

I think everything appears as parallel light rays from that distance so doubt there is any difference there. Jupiter has quite low contrast features and significant limb darkened so that may explain its softer appearance. In comparison Saturn is higher contrast, certainly the brighter parts of the rings and there is much less surface detail to look at so that probably helps the perception of sharpness. Again, many of Mars’ features are low contrast and I think the effect of dust can reduce the visibility of these features at times, even when there is not a planet wide dust storm. The polar caps can be quite striking when larger, appearing quite bright white. All fascinating planets to observe!

I guess then planetary filters can be used to improve perceived contrast?

Red / Light blue filters for the Mars for example will make it better contrast target (different features), so it could look sharper?

[mikeDnight]

This is an interesting question Mark. I've mulled this over in my mind many times as different planets do handle magnification differently. For me, Jupiter is the most complex planet to observe. It's belt and zone detail is so complex and often so subtle, that over magnifying kills definition and contrast. So Jupiter will often give its best at around 180X no matter what scope is used. Saturn is full of fine linear detail, which appears sharp at higher powers, so 200X to 250X.  Mars and Venus display more subtle albedo markings, and because the disk's are bright, the markings can be difficult to discern. It may seem counter intuitive,  but smaller apertures can often give detailed views of Mars while in larger apertures the brightness can blind out detail. Personally i find Mars and Venus the most rewarding planets to observe, possibly because they are a challenge. Jupiter, although dynamic and highly detailed doesnt excite me too much, and Saturn sits between the two extremes as far as being an interesting target. 

When it comes to the planet's, a sharp image is essential, or at least as sharp as possible, so scopes that take an age to cool or that are sensitive to seeing can fare less well despite having larger apertures. In the end its all about how well a scope defines an image, so magnification needs to be tailored to suit both the scope and the local seeing, as without definition aperture is of no value.

Edited September 6, 2020 by mikeDnight

[markse68]

Lots of interesting points and I'm sure the answer is in there somewhere or maybe a mix of them all!

Thinking about it a bit more the obvious reason for some of the difference with Jupiter and Mars at least is that although Jupiter is about 9x further away from us it is about 20x larger diameter than Mars so it's about twice the angular size if I got my maths right (?), and of course the features we see like the GRS are themselves unimaginably huge- it could swallow 3 or 4 Mars sideways!

And to be fair to Mars like Stu rightly pointed out the ice caps have been well defined in good seeing and other than those its features are low contrast and it does have an obscuring atmosphere particularly when there's a dust storm

Thanks for all the input

Mark

[Alkaid]

I think that the reason is contrast.  Saturn's rigs appear sharp(ish) as they are contrasted against the dark sky.   Cassini division is also contrasted well in a large scope.   But try looking for banding on Saturn's globe - far, far harder.....differing shades of yellow next to one another....hard to distinguish.

Same thing happens with Jupiter, reds and browns mix well and are hard to distinguish.

Altitude of the planet also contributes.....Jupiter and Saturn are low right now.    Mars is much higher up, less atmosphere for the scope to penetrate.....sharper views.

[SiriusB]

Just to throw in a curve ball.....

generally i'd agree with OP's observation regarding Jupiter & Saturn~ only the last few weeks i've been getting remarkably good views of Jupiter (considering the low elevation) whilst Saturn has been pants @ same mag! Why?