Why no Stars visible from space video footage?

Discussion in 'Astronomy, Exobiology, & Cosmology' started by Starsky, Sep 1, 2004.

  1. Starsky Registered Member

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    Has anyone noticed that the stars aren't visible in video footages from the Space shuttle or space station? The background in these footages is always pitch black with no stars. Even shots from the Moon landings didn't show any stars while here on earth we can always see stars at night. Whats the reason for this?

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  3. (Q) Encephaloid Martini Valued Senior Member

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    Cameras are not sensitive enough to capture faint stars, especially through the glare from the reflected surfaces of the Earth and Moon.
     
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  5. slotty Colostomy-its not my bag Registered Senior Member

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    I think it has something to do with the earths orbeado ( something like that, its a planets reflective index) the earth is 5 times as bright as a full moon so orbiting that close to earth, i would imagine you have no chance of seeing a faint star. It would be like trying to look at a candle with a car headlight shining in your eyes.
     
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  7. Closet Philosopher Off to Laurentian University Registered Senior Member

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    It's partially the fact that most cameras used in space aren't sensitive enough to capture the light of the stars. It's like using a normal camera to capture the stars (or as many people attempt to do in my area, capture the Northern Lights). While the stars may seem bright to humans, who have ultra-sensitive eyes, the film in a camera will have to have a high ISO and be exposed for a long time to have beautiful clear images of stars and faint light in the sky. I took some shots of the stars for Photography class once, it's hard to get the right exposure.

    The light reflected off the earth and moon make it even worse. The atmosphere of the earth will also disrupt the view of the stars.
     
  8. James R Just this guy, you know? Staff Member

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    It's the same reason that you can't see stars when you look up at the night sky from a well-lit city street.
     
  9. invert_nexus Ze do caixao Valued Senior Member

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    I seem to recall part of the "moon landing hoax" thing being something about people at nasa painting the skies black. Eradicating the stars. This was, of course, used as 'evidence' for the hoax, and the debunkers didn't deny that this occurred, merely said that it was standard practice.

    I think I read it in a thread here, as a matter of fact.
     
  10. Now here's a question. Why remove stars from the film? First off, that would be an immensely complicated procedure, particularly back in those days. Secondly, why remove evidence that would validate the footage? Well, I'll grant that I don't think anyone involved actually expected to be accused of being hoaxsters, but still. If stars actually showed up, the pictures would have been even more dramatic.

    Remember that stars are immensely tiny. Despite our ability to perceive them, any given star's actual apparent diameter is incredibly small, smaller than I'm sure most cameras on low exposure could hope to pick up. Only with a long exposure, burning the light into the film, would they resolve. They human eye is a wonderful thing, aye? :m:
     
  11. chunkylover58 Make it a ... CHEEEESEburger Registered Senior Member

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    The human eye can perceive up to 40 levels of contrast in a scene. Print film can render 10 levels. Color transparency film (slide film) can render only 5. Bascially, with slide film, there are 5 "positions."

    0=proper exposure. Medium gray is medium gray ("medium gray" is defined as 18% refelctivity and is the standard by which photographic meters are calibrated).

    +1 (over exposure) would make medium gray very light gray, with some detail.

    +2 or more would be detailless white.

    -1, dark gray.

    -2, detailless black.

    So, if the foreground is more than 2 positions ("stops") different from the background, there would be no possible way to retain detail in both foreground and background. With a dark background/light foreground situation, such as the lunar photos, if an exposure is made for the foreground, the background can be rendered no other way than detailless black. (If you took a meter reading of the light being reflected off of, say, a lunar rover, your exposure would probably be in the range of 1/125 second shutter speed with an aperture of F/22. The reading on the dark background (the vast blackness of space) most likely could not even be measured by a photgraphic light meter. The shutter speed required to get any detail at all would be several seconds. So, in that situation, you're probably looking at tens of stops difference in exposure value. If an exposure was made to get detail in the dark background, the foreground would be so washed out and overexposed that everything there would be just white, detailless blobs.

    This is the reason many people who try to take pictures of the full moon and get such suck-ass results. The moon is lit up by the same sun that gives us daylight, so its photographic exposure with 100 speed film would be roughly 1/125 sec @ F/16 (when full). The surrounding night sky would require several seconds of exposure to be able to get any visible stars. So, one of two things can happen:

    1. Nice, clear snap of the moon, if the exposure is made from the moon itself, with a completely black background (no stars).

    2. Big overexposed white, detailless blob of a moon and maybe a few stars, all of which is a big blur due to the Earth's rotation during the several second exposure time required by the black space.

    Most people get number 2 because their camera's meter picks up the most dominating light source which, unless using a super long telephoto lens to get the moon to fill the frame completely, is the black spacey background. Being so many stops difference, most of the full moon photos that I would see would be tiny, white, shiny blurs. (Probably need at least 1000mm lens or more to get the moon to be the dominating light source on your meters eye. When shooting the moon, 100mm of lens focal length will render a moon 1mm across on a piece of 35mm film. A 35mm film frame is 24mm x 36mm. So, 1000mm focal length would get you barely half the vertical space of the frame. Very few point and shoot cameras have more than 120mm lenses and most amateur SLR shooters have no more than 300mm.)

    Any photos you may see with both fully lit foreground and well exposed background in a situation that would normally be of a wide contrast range is a composite of two images: One exposed for the foreground, one for the background.
     
  12. Pete It's not rocket surgery Moderator

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    Or using a slow synch flash... but that's a bit hard to do for the moon!
    ("Yeah... NASA? I'm doing some lunar snaps, so if I could just get you to switch off the Sun, then give it a 1/125s flash when I say "now", OK? Hello? Hello?")
     
  13. vslayer Registered Senior Member

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    quite simply, with all the other light sources so close to it, it is like thrying to look at stars in the daytime. if they took a picture from the dark side of the moon or painted the spacestation black then there would be no interfering light and you could see the stars
     
  14. Blindman Valued Senior Member

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    Digital cameras will go to the moon next time. We will see the stars then.

    Some digital cameras have 16Bits per channel. Using such a camera helps to retain detail in the image. Image processing software could easily display the hidden stars. There are also the HDR image formats that store pixel componants as floating point numbers. It would be easy to reprogram a standard digital to create HDR images, you can then adjust the exposure setting on the computer to extract the stars.
     
  15. chunkylover58 Make it a ... CHEEEESEburger Registered Senior Member

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    Right. And since the exposure time to get the stars would be so long, you'd have a big blur in the background, anyway (albeit a properly exposed blur).
     
  16. Pete It's not rocket surgery Moderator

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    10,166
    I don't think so!

    A) the moon is darker than the stars for most of our exposure (because the Sun is switched off)
    B) the moon moves slow enough against the background of the stars that it wouldn't matter anyway (assuming we're looking for reasonably bright stars and not any nebulosity)

    ...but this is silly - I can't believe I'm arguing the merits of a switched Sun for Lunar photography! :bugeye:
     

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