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I had this image, a picture of Jupiter:

enter image description here

I wanted to measure the pixel diameter of Jupiter. As evident, the image is quite blurry, and thus, many value of equally justified pixel diameters can be obtained. To eliminate this I posterised the image (which is technically, quote from Gimp, "designed to intelligently weigh the pixel colors of the selection or active layer and reduce the number of colors while maintaining a semblance of the original image characteristics"):

enter image description here

With the above result, I obtained three pixel diameters of Jupiter (one from each of the coloured circles), and with a little bit of calculation, realised that the yellow circle gave rise to the most accurate value of Jupiter's diameter (which was surprisingly off by less than one percent!).

My questions are:

1) Why is my original image like that? Is it due to some form of light scattering?

2) Is there a better method of obtaining an accurate pixel diameter than posterising the image

3) Is the posterised result expected? Are there names that can be given to the three distinct spherical objects (which seem to resemble shadows, like the umbra, prenunbra, antumbra, etc.)? Is there a reason why the middle posterised object gives the most accurate value?

StopReadingThisUsername
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  • You have used an ad-hoc method and you have gotten an ad-hoc result (that happens to match your expectation). For one thing, your image looks totally overexposed, which means that the estimate should be too large. Measure the point-spread function of your telescope with a nearby star. Convolute that point spread function with a variable size model of Jupiter and minimize the error between the convoluted image and the measured one. This will give you a more rational estimate that removes your instrument response somewhat. – CuriousOne Jun 15 '16 at 07:16
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    When I perform level-2 posterisation on that same input image I get a similar but different output image; the red and yellow bands are thinner than in yours. So it looks like @CuriousOne is right that you got lucky. – lemon Jun 15 '16 at 08:50

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