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full image

zoomed in image on the moon

I do some amateur astrophotography and love the photos nasa produces and while looking at this one of Jupiter I noticed that, around its moon, the is a rectangle with a slightly blue tint and a bit more noise than the rest of the image.

So my question is:

What is the cause of this?

Would they be upping the brightness a bit just on the moon to make it pop more?

Would the brightness of the moon just cause this to happen around it? I don’t think this is the case considering it’s rectangular but I’m not sure.

This isn’t about nasa deniers by the way, I’m just genuinely curious and can’t find much on what exactly nasa does to their photos much less this specific one.

Thanks for the help guys, clear skys. enter image description here

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    \$\begingroup\$ What compression format was that image when you found it? JPEG, PNG, or something else? \$\endgroup\$
    – Zeiss Ikon
    Jul 7, 2021 at 17:18
  • \$\begingroup\$ @ ZeissIkon it was a png but there are much higher quality photos of it out there \$\endgroup\$
    – Beans
    Jul 7, 2021 at 17:27
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    \$\begingroup\$ I'd bet the higher quality image files don't have this artifact. \$\endgroup\$
    – Zeiss Ikon
    Jul 7, 2021 at 17:38

1 Answer 1

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This is clearly an artifact from the image file format they are using. Im assuming you found this image on the web. To use images online efficiently you need to make them lightweight in size so they have shorter loading times. I guess the most common format is JPG which compresses images in clusters such that these box shaped artifacts occur. Look at this image from the Wikipedia article on the JPEG compression

enter image description here

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  • \$\begingroup\$ The OP has confirmed this image was in PNG, but compression artifacts exist in any compressed image, especially in lossy compression. PNG has a lossless option, but probably higher compression if lossy is allowed. \$\endgroup\$
    – Zeiss Ikon
    Jul 7, 2021 at 17:39
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    \$\begingroup\$ Png is not lossy for any setting and compression artefacts can only occur for lossy compression \$\endgroup\$ Jul 8, 2021 at 2:03
  • \$\begingroup\$ @planetmaker Does that mean all compression options when saving as PNG result in non-lossy results? \$\endgroup\$ Jul 8, 2021 at 4:34
  • \$\begingroup\$ @Planetmaker We only see the final result, we don't know what the image has been through. The web is full of PNGs that had a former life as JPEG. \$\endgroup\$
    – xenoid
    Jul 8, 2021 at 8:39

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