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# Size of features on Jupiter

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### #1 Marcus Roman

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Posted 31 August 2019 - 12:12 PM

Hi, some nights ago I was looking at Jupiter with my Vixen FL80, a 3" apochromat and, maybe for the first time in a 3", I think I saw in rare moments of steady seeing, some ovals....now, is this reachable by a mere 3" refractor?

I understand the ovals are less than 1 arc seconds?

Anyone can help me about size of ovals in arc seconds as well as of GRS?

How can we measure size of features on Jupiter?

Thanks and clear skies!

Edited by Marcus Roman, 31 August 2019 - 12:21 PM.

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### #2 nicoledoula

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Posted 31 August 2019 - 12:45 PM

Indeed. How can we measure size of features on Jupiter?

### #3 Marcus Roman

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Posted 31 August 2019 - 02:22 PM

Fred Priceâ€™s book gives some hints but it is difficult to me to calculate based upon meridian passage timing...

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Posted 31 August 2019 - 05:05 PM

I've attempted to do a calculation like this. You calculate angular size by using distance and the physical size of the object. For example Jupiter is roughly 600,000,000 km from Earth at opposition and the best measurement I can find for the GRS currently is 16,350 km in width. So that comes out to 5.6 arcseconds wide.

Another calculation would be to determine the smallest feature a telescope can see. If we use the Rayleigh limit for an 80mm (1.73 arcseconds), the smallest feature it can resolve at a distance of 600,000,000 km is about 5,000 km in diameter.

As for measuring sizes on your own, I don't know but I would guess that imaging would make this a lot easier. You could count the pixels, make a ratio of the feature's size to Jupiter's size in your image, then derive the angular size using the currently known angular size of Jupiter which most astronomy apps will give you.

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### #5 Marcus Roman

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Posted 31 August 2019 - 05:34 PM

I've attempted to do a calculation like this. You calculate angular size by using distance and the physical size of the object. For example Jupiter is roughly 600,000,000 km from Earth at opposition and the best measurement I can find for the GRS currently is 16,350 km in width. So that comes out to 5.6 arcseconds wide.

Another calculation would be to determine the smallest feature a telescope can see. If we use the Rayleigh limit for an 80mm (1.73 arcseconds), the smallest feature it can resolve at a distance of 600,000,000 km is about 5,000 km in diameter.

As for measuring sizes on your own, I don't know but I would guess that imaging would make this a lot easier. You could count the pixels, make a ratio of the feature's size to Jupiter's size in your image, then derive the angular size using the currently known angular size of Jupiter which most astronomy apps will give you.

Thanks! Good point about angular size calculation considering Jupiter distance and GRS' estimated diameter in km...I doubt I really saw ovals, they must be no more than 1/10 of GRS' diameter so below Rayleigh limit of a 80mm...

Edited by Marcus Roman, 31 August 2019 - 05:39 PM.

### #6 Asbytec

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Posted 31 August 2019 - 07:23 PM

One way I've done it is to count pixels using a current image and noting the date it was taken as it's angular diameter changes during its apparition. GIMP works. The imaging forum is a good place to find nice current images to work with. You do not have to be completely accurate with measurements, just enough significant digits to get a good approximation.

https://www.cloudyni...w-capture-clip/

We know the equatorial diameter of Jupiter is 143,000km and on 25 July 2019, the date of the image above, Jupiter was 4.54 AU (679,000,000 KM). I chose to use the equatorial diameter knowing the longitudinal diameter varies and I am measuring a feature along its latitude.

https://theskylive.c...-far-is-jupiter

Now we have all the information we need to use the small angle approximation. Plugging in those numbers, we find that Jupiter was 43.4" arc in diameter. The link below converts AU as well as KM, miles, etc.

https://www.vcalc.co...mula Collection

Now, it's simply a matter of ratios. In pixels, I find Jove to be about 740+/- pixels across it's equator and an average white oval is about 20 pixels to the accuracy I care to shoot for. Close enough for a good approximation. Doing the math, comparing Jupiter's known angular dimension to the feature we wish to measure, that particular white oval is ~ 1.2" arc. The GRS is about 90 pixels across its width which gives ~ 5.28" arc across its latitudinal dimension.

Rayleigh and Dawes really do not apply to extended object resolution. It's all about contrast and seeing. Is it possible to resolve extended features less than the Dawes limit. Can a 3" aperture to see a low contrast feature on Jove 1" arc across? I dunno, you tell me.

Edit: I understand the maximum spatial frequency is k = 1 at Lambda/D or 113/Dmm and 1.4" arc with an 80mm aperture. An average white oval of 1.2" arc is smaller than the maximum spatial frequency, so it may not resolve it (though an obstructed aperture might). Even if it could, the white oval would need to be much higher contrast on this tiny scale to survive diffraction effects even in a perfect aperture. An 80mm may well resolve some of the larger ones, like Mickey's Head a few years back. I've seen about 5 of the 9 or so average white ovals in a 6", and surely a 4" can grab a few. At smaller apertures, it becomes less likely to see them. In any case, doing so will require some extraordinary seeing.

Edited by Asbytec, 31 August 2019 - 08:35 PM.

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### #7 Marcus Roman

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Posted 01 September 2019 - 02:40 AM

One way I've done it is to count pixels using a current image and noting the date it was taken as it's angular diameter changes during its apparition. GIMP works. The imaging forum is a good place to find nice current images to work with. You do not have to be completely accurate with measurements, just enough significant digits to get a good approximation.

https://www.cloudyni...w-capture-clip/

We know the equatorial diameter of Jupiter is 143,000km and on 25 July 2019, the date of the image above, Jupiter was 4.54 AU (679,000,000 KM). I chose to use the equatorial diameter knowing the longitudinal diameter varies and I am measuring a feature along its latitude.

https://theskylive.c...-far-is-jupiter

Now we have all the information we need to use the small angle approximation. Plugging in those numbers, we find that Jupiter was 43.4" arc in diameter. The link below converts AU as well as KM, miles, etc.

https://www.vcalc.co...mula Collection

Now, it's simply a matter of ratios. In pixels, I find Jove to be about 740+/- pixels across it's equator and an average white oval is about 20 pixels to the accuracy I care to shoot for. Close enough for a good approximation. Doing the math, comparing Jupiter's known angular dimension to the feature we wish to measure, that particular white oval is ~ 1.2" arc. The GRS is about 90 pixels across its width which gives ~ 5.28" arc across its latitudinal dimension.

Rayleigh and Dawes really do not apply to extended object resolution. It's all about contrast and seeing. Is it possible to resolve extended features less than the Dawes limit. Can a 3" aperture to see a low contrast feature on Jove 1" arc across? I dunno, you tell me.

Edit: I understand the maximum spatial frequency is k = 1 at Lambda/D or 113/Dmm and 1.4" arc with an 80mm aperture. An average white oval of 1.2" arc is smaller than the maximum spatial frequency, so it may not resolve it (though an obstructed aperture might). Even if it could, the white oval would need to be much higher contrast on this tiny scale to survive diffraction effects even in a perfect aperture. An 80mm may well resolve some of the larger ones, like Mickey's Head a few years back. I've seen about 5 of the 9 or so average white ovals in a 6", and surely a 4" can grab a few. At smaller apertures, it becomes less likely to see them. In any case, doing so will require some extraordinary seeing.

Thanks Asbytec, that's great information...I will use those sources and method. Yes, must have been more the desire to see them than actual seeing the ovals, though that night was very steady night and rest of structures of Jupiter were popping up. Thanks!

### #8 Asbytec

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Posted 01 September 2019 - 05:37 AM

Something stimulated you to ask the question. Maybe you did see one or more larger ones. Extended resolution is a difficult area involving the scope and observer acuity. With smaller aperture it becomes less likely until it becomes not likely. Really, only you would know. You'll know them when you see them.
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### #9 Marcus Roman

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Posted 01 September 2019 - 05:51 AM

Something stimulated you to ask the question. Maybe you did see one or more larger ones. Extended resolution is a difficult area involving the scope and observer acuity. With smaller aperture it becomes less likely until it becomes not likely. Really, only you would know. You'll know them when you see them.

Yes, you are right! Well, Vixen Fluorite is so good it works close to its theoretical diffraction limitedness and that night I was using a Baader Neodymium filter.

Maybe I just got a glance of the larger ovals...it was a moment...thank you again!

### #10 Asbytec

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Posted 01 September 2019 - 06:54 AM

Marcus, I'm sure it is and does. It is possible to resolve features below the Dawes limit. Dawes is a special case of "diffraction limited" resolution involving the diffraction of two high contrast point sources set against the black of space. In other words, two stars. The Dawes limit for your scope is 113.4/80mm ~ 1.4" arc. It's possible to resolve slightly below that "special case" on extended objects so long as the object is of high enough contrast, the scope images it very well including excellent seeing, and you (as the observer) are able to detect it in the telescopic image. With any amount of seeing, even in good seeing, the tiny images will be fleeting.

The oval I measured is 1.2" arc, and there are some larger ones. Here's a fairly current map of Jupiter showing the white ovals in the southern hemisphere (and Oval BA, a larger and low contrast white oval near the center of the map.) You may have seen that one. It's fairly large. The problem with the smaller one's is they are not very high contrast. The lower the contrast, the larger the limit of detection. So, Oval BA is very likely, I suppose, if it was visible the night you observed Jupiter. Wagging a guess, it looks to be between 5" arc of the GRS and 1" arc for the smaller ones, so maybe about 3" arc.

http://alpo-j.asahik...19/j190820r.htm

One way to tell is to note the date and time you saw Jupiter and find the central meridian (the part of Jupiter facing Earth) at the time of your observation. The compare it with a current Map or images taken at the same time. It gives results in system I and system II, which are the equatorial and polar regions, respectively. I believe the ovals are in System II more than 10 degrees from the equator with a different rotation rate. Because of this, Oval BA changes it's position relative to the GRS fairly rapidly over time, so you need to know its system II central meridian for any given night.

http://arksky.org/newcmcalc.htm

Edited by Asbytec, 01 September 2019 - 06:58 AM.

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### #11 Marcus Roman

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Posted 01 September 2019 - 12:34 PM

Marcus, I'm sure it is and does. It is possible to resolve features below the Dawes limit. Dawes is a special case of "diffraction limited" resolution involving the diffraction of two high contrast point sources set against the black of space. In other words, two stars. The Dawes limit for your scope is 113.4/80mm ~ 1.4" arc. It's possible to resolve slightly below that "special case" on extended objects so long as the object is of high enough contrast, the scope images it very well including excellent seeing, and you (as the observer) are able to detect it in the telescopic image. With any amount of seeing, even in good seeing, the tiny images will be fleeting.

The oval I measured is 1.2" arc, and there are some larger ones. Here's a fairly current map of Jupiter showing the white ovals in the southern hemisphere (and Oval BA, a larger and low contrast white oval near the center of the map.) You may have seen that one. It's fairly large. The problem with the smaller one's is they are not very high contrast. The lower the contrast, the larger the limit of detection. So, Oval BA is very likely, I suppose, if it was visible the night you observed Jupiter. Wagging a guess, it looks to be between 5" arc of the GRS and 1" arc for the smaller ones, so maybe about 3" arc.

http://alpo-j.asahik...19/j190820r.htm

One way to tell is to note the date and time you saw Jupiter and find the central meridian (the part of Jupiter facing Earth) at the time of your observation. The compare it with a current Map or images taken at the same time. It gives results in system I and system II, which are the equatorial and polar regions, respectively. I believe the ovals are in System II more than 10 degrees from the equator with a different rotation rate. Because of this, Oval BA changes it's position relative to the GRS fairly rapidly over time, so you need to know its system II central meridian for any given night.

http://arksky.org/newcmcalc.htm

That's a lot of good information! Thank you!!!!

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### #12 Redbetter

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Posted 02 September 2019 - 10:53 PM

The individual white ovals vary somewhat in size relative to one another, and in apparent size depending on where Jupiter is relative to our orbit.  Also their apparent size is greatest when on the meridian and become harder to see as they move closer to the limb.  Presently Jupiter is under 39".  I come up with a size of ~1.2 to 1.4" for the largest in that image based on the present angular size of Jupiter, but they are narrower in the vertical dimension.

As Norme says, contrast is a big part of this.  The ovals are low contrast, although the planetary image is bright.  I have not succeeded in resolving them with my 80ED.  For larger scopes they are primarily a test of the seeing.  When the seeing changes from mediocre to good, the larger ovals begin to pop into view.  In better seeing the larger ones are visible in the 110ED and 127 Mak.

If you want a more vivid and quantifiable demonstration of contrast with known angular diameters, watch moon and shadow transits.  The moons have different diameters and colors and can present varying levels of contrast with the portion of Jupiter they are passing over.  This is most apparent over the limbs where the moons are brighter than the limb.  Even a 50mm scope can reveal the bright discs of small moons while they are still within the perimeter of the planet's limb darkened disc

The shadows present the largest contrast possible and can be tracked all the way across even in 50mm of aperture.  Seeing can wreck this of course...

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### #13 Asbytec

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Posted 03 September 2019 - 11:09 AM

"Also their apparent size is greatest when on the meridian and become harder to see as they move closer to the limb."

Yes, Red expresses an important point. That's a factor to be aware of, and you will become aware of it as you gain experience with Jove.