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While reading reports about the New Horizons misson, I noticed an odd vertical, black stripe in the images of Pluto.

Here is an example: Hubble Discovers a Fifth Moon Orbiting Pluto Source: Hubble Discovers a Fifth Moon Orbiting Pluto (07.11.12). Credit: NASA; ESA; M. Showalter, SETI Institute

Why is that black stripe there?

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    $\begingroup$ I could be mistaken, but I believe Pluto has spent the last 8 years working to clear its neighborhood so it can reclaim its status as a planet. It's slow progress, but it's trying hard! $\endgroup$ – corsiKa Dec 8 '14 at 21:20
  • $\begingroup$ The government has been working with extraterrestrials to set up a series of spy satellites to watch us from Pluto. Since no one on Earth but the US government has any hope of getting there, it is the most logical place to put them. Obviously they can't be seen showing up in NASA photos $\endgroup$ – gillonba Dec 8 '14 at 21:42
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    $\begingroup$ I've taken the liberty of attaching the photograph in question to your question. NASA media usage guidelines allow unrestricted use for educational or informational purposes. $\endgroup$ – Lilienthal Dec 9 '14 at 0:07
  • $\begingroup$ @Lilienthal: Thanks, that's in fact the image I was referring to in the first place. $\endgroup$ – Aaron Digulla Dec 9 '14 at 9:18
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    $\begingroup$ @NewWorld Aaron is referring to the black bar that seems to overlap the center of the image where Pluto and Charon are. You can see the separation between the two images by the sudden lack of smaller light points that are visible in the other part of the image. $\endgroup$ – Lilienthal Dec 9 '14 at 12:43
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That photograph is a composite of two images taken with different exposure times.

To be correct we'd have to say that the exposure of the two photographs is different, i.e. the outer photo was created by absorbing more light. In this case we can assume that the focal ratio (derived from Hubble's lens aperture) and the luminance of the scene (how much light is travelling in the lens' direction) are identical for both photographs, which leaves only the exposure time as a free variable when it comes to determining exposure.

This is necessary because we're photographing objects with very different brightness. For Pluto to show up a relatively short exposure time is required, but its moons reflect much less light and would need a longer exposure time to visible. As long as the sensor is exposed, Pluto would continue to increase in brightness to the point that it becomes washed out. Objects that are significantly brighter become over-exposed resulting in a loss of detail and fidelity, known as blown-out highlights in photography. In our case Pluto would turn into a solid white dot compared to the more detailed picture that is now possible. You can draw a parallel with false color images rendered from infrared: this composite is not what the human eye would see if it was capable of picking up this level of light and detail.

In another Hubble image, NASA explained the reason why composite imaging is used:

This is a composite image because a single exposure of the stellar background, comet Siding Spring, and Mars would be problematic. Mars actually is 10,000 times brighter than the comet, so it could not be properly exposed to show detail in the Red Planet. The comet and Mars also were moving with respect to each other and could not be imaged simultaneously in one exposure without one of the objects being motion blurred. Hubble had to be programmed to track on the comet and Mars separately in two different observations.

Source: Hubble Sees Comet Next to Mars

Very long exposure times are often necessary since relatively little light is coming our way from distant planets and stars. As the Hubble website explains for its Deep Fields images:

Hubble has made a series of very deep observations taken in very dark parts of the sky. Like using a long exposure on a digital camera, these long exposure shots (up to several weeks) reveal very faint details that are not normally visible in shorter exposures.

Source: "What are the Hubble Deep Fields?", Spacetelescope.org FAQ.

Wikipedia summarises a paper by Robert E. Williams and the HDF team, "The Hubble Deep Field: Observations, Data Reduction, and Galaxy Photometry" as follows:

Between December 18 and December 28, 1995—during which time Hubble orbited the Earth about 150 times—342 images of the target area in the chosen filters were taken. The total exposure times at each wavelength were 42.7 hours (300 nm), 33.5 hours (450 nm), 30.3 hours (606 nm) and 34.3 hours (814 nm), divided into 342 individual exposures to prevent significant damage to individual images by cosmic rays, which cause bright streaks to appear when they strike CCD detectors. A further 10 Hubble orbits were used to make short exposures of flanking fields to aid follow-up observations by other instruments.

Source: Hubble Deep Field, Wikipedia, retrieved 2014-12-09

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Pluto itself so bright that Charon would not be visible in the image if it were exposed in a way to show the remaining moons. Likewise, the remaining moons as so faint as to not be visible in an image that resolves Charon. Thus, the photo that you see is a composite of two image processing techniques: one designed to showcase Charon and one designed to showcase the remaining moons.

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I'm pretty sure that the stripe is there because the two sections of the image have different contrasts. Pluto and Charon are so bright relative to Nix, Hydra, P4 and P5 that to have one contrast, you'd either (a) not be able to see the smaller four, or (b) totally wash out the image with the light from the brighter two. So we simply superimpose two images at different brightnesses (or gains, I guess).

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Without knowledge of this particular image, I'm pretty sure it is because of the use of a Coronagraph. One of a set of different methods to block or divert or phase shift or in some other way get rid of unwanted light reflected from Pluto and Charon which would blind the light detector from the much weaker light reflected by its tiny moons.

This link about the coronagraph on Hubble is way beyond me, but maybe it is helpful to someone:

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    $\begingroup$ "Coronagraphs on the Hubble Space Telescope" by John Krist has some specifics on the Coronagraphs on the Hubble and seems much more readable. As the article states a Coronagraph might not have been required for this composite as light diffraction and scattering is much less problematic in LEO. $\endgroup$ – Lilienthal Dec 9 '14 at 0:13

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