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How did they take photos of Jupiter - I mean Jupiter is illuminated and that's a lot of light to produce. Am I missing something, and there was some sort of dark photo technology used, or was there simply enough light from Sun to begin with? Or is this photo a fake?

enter image description here

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The most direct source of information for this image is at ciclops.org/view/97/Io-Transiting-Jupiter CICLOPS is where all Cassini images are released to the public, simultaneous with NASA. You won't get any closer to the horse's mouth than that! –  DarenW Aug 6 at 1:12

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You can see Jupiter in the night sky with your naked eyes due to its reflected sunlight (although I believe that in July and August of 2014 Jupiter is very close to the Sun in the sky and is visible only for a little while near twilight). You can take a picture of Jupiter in the sky with any old camera.

If you want a high-quality picture, your camera needs to have a lens arrangement that will make the image of Jupiter on the camera's CCD larger than the image of Jupiter on your retina. The thing to look for is a lens with a long focal length. If the focal length of the lens1 is long enough, it will need to stand some distance away from the camera's CCD on a rigid mount; this is usually called a telescope. You can replace the camera with your eye and see Jupiter's cloud bands directly.

1Actually most telescopes use a curved mirror rather than a lens, for several technical reasons.

Images as nice as that one usually come (possibly) from professional astronomical observatories on the ground, or from the Hubble Telescope, probably NASA's most successful instrument ever (after a rocky start). Your particular image seems to have been taken by the robotic spacecraft Cassini when it passed near Jupiter en route to Saturn, where it has been orbiting and collecting data for the last ten years. In that case the camera had the advantage of being much closer to Jupiter than I'll ever be :-(

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At 500 nm, Hubble's ideal resolution would amount to less than 500 pixels across the width of Jupiter when it's closest to Earth. So yes, actually going to the planet is really the best way to photograph it. –  Chris White Jul 30 at 4:51
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@ChrisWhite I don't know about details of resolution vs. wavelength vs. CCD pixel size, but there are plenty of impressive Hubble photos of Jupiter. –  rob Jul 30 at 12:57

This was just going to be a comment, but it got too long.


Technically the picture is a fake. It's actually a composite image of what we would see with our eyes (visible light) blended with light seen in 3 different filters (see this NASA article on the different filters on-board Cassini, the probe that took the images).

The image below is taken from the first link. The visible light image is on the left; this is what we would see if we were located where Cassini was when it took the image. The image on the right is the composite image (consisting of 2 Methane filters (around 750 nm and 890 nm) & 1 Red continuum filter (likely around 645 nm), source)

enter image description here

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And this is the great "dirty" little secret of almost all of the incredibly amazing, beautiful astronomy images we see. The scientists all know it's false color, but man does it look awesome. –  tpg2114 Jul 30 at 2:47
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Technically, it's not a fake. A fake is something presented as something it isn't. –  David Richerby Jul 30 at 10:23
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@DavidRicherby: If you went to Jupiter, you wouldn't see the image on the right that is often presented as "what Jupiter looks like," you would see the image on the left. So yes, it could be considered a fake. Note also that I added the comment about its fakeness for effect, not because it is completely fake. –  Kyle Kanos Jul 30 at 10:28
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True! Although by this standard it's probably less fake than the cover photos of all the magazines at my supermarket's checkout lane. –  rob Jul 30 at 12:35
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How human-centric to call it fake because it's not what you would see. :) –  Kevin Krumwiede Jul 30 at 22:26

The light that shines on Jupiter is of course the Sun's. It is indeed fainter than on Earth, by a factor of about 25, but that is still plenty. Using appropriate cameras, and long enough exposures, one can photograph much fainter targets.

You can also note that there is a moon transiting in that picture, and its shadow is clearly visible. This should help you understand that the light source that's creating that shadow must be very far away. The only thing that could possibly do that is the Sun.

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The location of the shadow should also tell you something about the angle between the Sun, Jupiter, and the camera. Had the picture been taken from Earth, I think that angle would have had to be smaller, than it is in this picture. –  kasperd Jul 30 at 7:32
    
@kasperd The separation between Jupiter's moons and the shadows they cast on the planet can be much larger than in the OPs picture. ex in this amateur picture the shadows are on the left edge of the planet while the moons are to the its right. Both moons are in the segment of their orbits that is mostly between earth and Jupiter but slightly to the right. (IO orbits at ~ 5 R_j; the other major moons are several times farther out.) –  Dan Neely Jul 30 at 13:48
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@DanNeely That's a slightly tricky perspective. The angle between the Earth, Jupiter and the Sun is never more than 15° if my trig is right. The large separation in that picture is partly because we cannot resolve the depth of the picture, and the moons are a lot closer to us than they look at a first glance. –  Emilio Pisanty Jul 30 at 14:04
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@DanNeely, the angle defined by the wedge Earth-Sun-Jupiter was actually first measured by determining when Jupiter's shadow fell on its moons. That allowed the first geometric determination of the distance of Jupiter to the Sun (in AU). It's a very important measurement in the history of modern heliocentrism. I just thought I'd mention that, since you guys are already discussing the Earth-Jupiter-Sun angle. –  Thucydides411 Jul 30 at 19:25

You are correct: Jupiter is rather dim photography-wise. We can't use flash photography, as Jupiter is way too big for that to make any sense (even from a probe), so all the light in this image is indeed from the Sun, or from photo enhancement applied to the image after it was recorded (in which case it's still originally from the Sun).

There are ways to take images of faintly lit objects with cameras that work: we can use digital equipment that is highly light-sensitive, or use longer exposure times to pull in more light. Some of the approaches get quite complicated if we need to block out any light from behind the object (this doesn't matter for Jupiter, but for extrasolar objects like exoplanets it can be a huge obstacle).

We've taken images like this remotely from the Hubble, and close-up from probes. It's harder to do this remotely, for obvious reasons.

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Jupiter is not that dim. I took a number of pictures of Jupiter with a digital camera through a telescope and the exposure required is pretty short. Just a small fraction of a second. You need long exposures for dim objects like galaxies and nebulae, but not Jupiter. –  mpv Jul 30 at 12:41
    
That's pretty cool, actually! Can you tell from the image above if it were taken from Hubble vs. a probe, or is it impossible to tell? –  Trixie Wolf Jul 30 at 13:06
    
Er, n/m. Super-long-answer has sated my curiosity. :) –  Trixie Wolf Jul 30 at 13:14
    
I'd add that most amateur images of the planets are made using a video camera and the combining the handful of best frames (from when the air was exceptionally steady) from several minutes of recording (longer than that and motion blur becomes a problem due to rotation). –  Dan Neely Jul 30 at 13:53

protected by Qmechanic Jul 30 at 9:58

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