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Pluto and Charon, photo taken by New Horizons on July 8, 2015 from a distance of 6 gigameters.

Photo of Pluto and Charon

It's hard for me to believe there were no stars behind the twin dwarf planets in the field of vision. I also would not expect the planets to be illuminated so brightly (how many zillion of kilometers are they away from the sun?) that it washes out the stars.

Is it because of a time exposure and movement of the spacecraft? But then why is the detail on Pluto not smeared away?

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    $\begingroup$ You can look up the camera settings behind these images. In case of New Horizons exposure times of a fraction of a second are sufficient to get a perfect image of Pluto. Now you can get your digital camera, which is very similar to what New Horizons has, go outside and take a picture of the sky at a fraction of a second exposure time. How many stars do you expect to see on that image? I did, by the way, take a couple pictures of the full Moon the other night. The ideal setting was ISO100, f/6.5 and 1/200th second. No stars... for those I have to use ISO1600, f/4.5 and 30 seconds. $\endgroup$
    – CuriousOne
    Commented Sep 9, 2015 at 13:22
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    $\begingroup$ Here is the link to one of the raw images: pluto.jhuapl.edu/soc/Pluto-Encounter/view_obs.php?image=data/…. The exposure time of the camera was 100ms. That's far too short to image stars as nice bright spots. If there was a bright star in the image (which is very unlikely) it might show up with a few photoelectrons in the data, if you know where to look. $\endgroup$
    – CuriousOne
    Commented Sep 9, 2015 at 13:31
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    $\begingroup$ This question reminds of the argument of some hollywood-moon-landing-conspiracy theorists that NASA "forgot" to add stars to their "fake" photos, whereas the lack of stars is easily explained by exposition considerations $\endgroup$
    – Hagen von Eitzen
    Commented Sep 9, 2015 at 17:25
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    $\begingroup$ @CuriousOne your comments here are arguably a better explanation (in terms of people understanding what's going on) than the accepted answer, and probably should be converted to an answer so that people will see them. $\endgroup$
    – msouth
    Commented Sep 9, 2015 at 19:04
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    $\begingroup$ @HagenvonEitzen: Every professional photographer knows that the only way to get the Moon and the stars into one image with the "right" visual brightness is a collage of two images, one taken of the Moon with short exposure and another one of the stars with a much longer exposure time... so the very thing that the conspiracy theorists are looking for would be a guaranteed photographic fake and everybody who ever had a camera and knew how to use it would know. $\endgroup$
    – CuriousOne
    Commented Sep 9, 2015 at 20:13

3 Answers 3

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The sun is still surprisingly bright at Pluto. While it is approximately 1500 times less bright than at Earth, this is still approximately 250 times brighter than a full moon.

If you consider the effect the difference between a full and new moon has on star viewing I expect that the reflected sun at Pluto is still bright enough to make it difficult to see any stars, without saturating the view of the planets.

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    $\begingroup$ even though the sun, as seen from Pluto, is 250 times brighter than the full moon (i presume as seen from Earth), this doesn't mean that the sunlight, reflected from Pluto, as seen from 6 billion meters away, as bright as that. but it's evidently still so much brighter than any of the stars in the field of the photo that they are washed out. i guess that is the answer. $\endgroup$
    – robert bristow-johnson
    Commented Sep 9, 2015 at 15:07
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    $\begingroup$ The light reflected from Pluto would not have had any effect on the camera's ability to image stars. The problem is that the camera has limited dynamic range. The stars simply aren't bright enough to appear in the image when the camera settings allow it to capture details of Pluto's surface. If the same camera were configured to let you see stars, then Pluto and Charon probably would appear as white, featureless blobs. $\endgroup$
    – Solomon Slow
    Commented Sep 9, 2015 at 22:54
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You can look up the camera settings behind these images. Here is the link to one of the raw images. The exposure time of the camera was 100ms. New Horizon's LORRI camera is a Ritchey-Chrétien telescope with a 20.8 cm diameter primary mirror and a focal length of 263 cm. That gives us approx. f12.6, which is a rather long i.e. fairly slow optical system (but typical for an astronomical instrument). The CCD in this camera probably makes somewhat up for that by being a little more sensitive than the average digital camera chip (that's maybe worth 1-2 f-stops). It's also a panchromatic camera (science-speak for black-and-white), which buys us another couple of f-stops in sensitivity over a CCD with built in RGB-filter, so that we are roughly dealing with the equivalent of an f8.0 or so consumer camera.

Now you can get your digital camera, set it to some reasonable gain (i.e. ISO 400) where the readout noise isn't dominant, yet, and take a picture of the night sky at f8.0 and 1/10th of a second exposure. Are you going to see stars? Probably not.

I did take a couple pictures of the full Moon the other night. The ideal setting was ISO100, f/6.5 and 1/200th second. No stars... for those I have to use ISO1600, f/4.5 and 30 seconds, i.e. conditions which gather a thousand times more light and which amplify the signal electronically to get the dimmer stars to show up somewhat. If I wanted a (fake) image with Moon and stars I would have to photoshop the two together. That is exactly how all the pretty Hollywood movies do their astro-shots: it's image manipulation and cgi.

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    $\begingroup$ Curious, on this page the exposure time is given as 40 s … ;-) $\endgroup$
    – chirlu
    Commented Sep 10, 2015 at 14:55
  • $\begingroup$ I would venture to guess that they meant "40ms" and/or that somebody made a mistake in editing the page. The LORRI camera would give you a very nice star background and a completely overexposed Pluto/Charon at 40s exposure time. They have those images, too, they are being used for navigation and to calibrate the relative spacecraft inertial guidance/camera angles etc.. $\endgroup$
    – CuriousOne
    Commented Sep 10, 2015 at 14:57
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    $\begingroup$ @chirlu: Bwahhhaahhhahhhaaaahhh! You hacked the website! Wow. Somebody needs to tell John-Hopkins that their server is not safe. One wonders what else can happen with malicious requests. :-) $\endgroup$
    – CuriousOne
    Commented Sep 10, 2015 at 15:39
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    $\begingroup$ You can put any desired text into that link and it will display (another example) They do filter out HTML tags, though, thankfully. – Just wanted to show that the direct link to this image page doesn’t prove anything about the exposure time (but the overview page does). $\endgroup$
    – chirlu
    Commented Sep 10, 2015 at 15:42
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    $\begingroup$ @chirlu: That's one poor design... seriously, who does that? You are, of course, absolutely right. Data taken from a website is not a reliable science product. To be very honest (and I have pointed this out multiple times on other posts), the only way to get reliable scientific data is to personally contact the folks who took it and to have a long discussion about how it was really taken. You almost got me with this one. Where do I send the beer? :-) $\endgroup$
    – CuriousOne
    Commented Sep 10, 2015 at 15:45
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Just to add up on the previous answers, you can indeed see a few stars in the images but they are faint

By adjusting the levels of the raw image, I obtain the following image where you can spot a few

enter image description here

The adjustment I used is the level adjustment from gimp/photoshop/imageJ and I pulled the max level down.

This effectively multiplies all pixel values by a constant higher than one, effectively increasing the gain of the camera ie the sensitivity / iso number.

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    $\begingroup$ It's great that someone finally did this. Could you maybe add a link to the raw data you used, and a couple words on what adjustment you did? BTW welcome to the site! $\endgroup$
    – user10851
    Commented Sep 15, 2015 at 0:04

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