Physics Question #9254

Barry, a 60 year old male from Vancouver, BC asks on July 16, 2015,

Like everyone else, I’ve been enjoying the images of the planet pluto from New Horizons. But I have a question: if it is billions more miles away from the sun than we are, how is there enough light to see features? Wouldn’t the light from the sun be heavily attenuated at that distance? I know we now have excellent low light cameras but this has got to be the lowest light in the solar system, so how do they get the images?

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The answer

Donald J. Barry answered on July 17, 2015

Pluto is currently at 32.9 AU (Astronomical Units -- roughly the distance from the Earth to the Sun) so by the inverse square law the brightness at its surface is 1/32.9^2 = 1/1082 the average at Earth's. But daytime brightness at Earth is *really* bright: even slow (ASA 100) photographic film, which responds to a fraction of a percent of the light which strikes it, can take daytime images at f/8 with exposure times of 1/100 second. The fastest photographic films (around ASA 400) respond to only 1% of the light which strike them: they can take well-balanced daytime images (in the narrow response range which film has compared to digital detectors) at f/8 with exposure times of 1/400s.

So the same photographic camera which recorded that image could take the same type of photo of Pluto with an exposure time of 2.5s. While most high-end astronomical CCDs have responses to light of as high as 95%, the robust ones our probes are outfitted with are closer to 40% efficiency. That's still 40x times better than film, and would allow exposures of 0.06s at f/8. The actual LORRI camera (the long-range monochromatic one) on New Horizons has an optical system with aperture of 21cm and focal length of 263cm, for an f/12.6 optical speed. By scaling the old photographic guidelines above, we'd have to up the exposure time by (12.6/8)^2 or 2.5 times, for an ideal exposure of 0.15s.

You can look at the LORRI instrument page of exposures and see that the typical chosen exposure times of Pluto imagery with LORRI have been around 0.1s. And the LORRI camera's sensors have much greater dynamic range: one doesn't have to worry about "misexposure" nearly as much. Exposures of Charon are slightly longer, because it's known to be darker.

The multi-color Ralph camera on New Horizons doesn't have this nice page which gives specs on exposures, but one could go through the same exercise. As a wider-field camera, it's probably got a faster optical system and will require shorter exposures: a good thing when one is sailing by Pluto and longer exposures produce blur.

Another thing to recall is that Neptune right now is only slightly closer than Pluto because Pluto spends some 30 years of its orbit inside of Neptune's from its high eccentricity. When Voyager 2 went by Neptune in 1989, it did not have the same high quality image capturing sensors as New Horizons. It used slow-scan vidicon television technology, which had maximum response to light of only a few percent -- better than film but not close to the newer sensors that were available 9 years ago when New Horizons launched, but with its own significant problems. Exposure times at Uranus and Neptune were several seconds, during which the image would blur during close approach: they developed an entirely new way of slowly moving the scan platform during exposures to pan along the expected blur and stabilize the image to produce good quality images, all of this tech developed as software patches while the spacecraft was in transit.

More on this old school technology here: Voyager camera system

Here's another significant problem that had to be corrected: Geometric distortion

People take night-time long-exposure images on Earth all the time. It's a way to get a feeling of how bright the light is on Pluto. Consider: the brightness of the full moon is -12.7, that of the daytime Sun is -26.7. Every 2.5 magnitudes is a factor of 10. So the brightness difference is (-26.7)-(-12.7) or 14 magnitudes. That's 14/2.5 orders of magnitude or 5.6. 10^5.6 = 400,000, so that's how much fainter it is at night with a full moon. That's 400 times fainter than the surface illumination at Pluto.

So from that point of view, it isn't that hard to capture images at Pluto! It's amazing that our eyes have the incredible dynamic range to let us see in the daytime *and* at night.

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