Where are the stars?

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seanthompson
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Where are the stars?

Post by seanthompson » Tue Feb 16, 2010 1:55 pm

Here's one that I have never understood. If you look at the main page today of Endeavor on its way to the space station, you see the earth with its layers of atmosphere. Why can't we see any stars in the black space? When we look up at night outside of the city, we can see so many stars, they almost seem to overlap. Why is it that I don't see stars in photos taken from the lunar surface or from orbit?

Curious,

Sean

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Chris Peterson
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Re: Where are the stars?

Post by Chris Peterson » Tue Feb 16, 2010 3:16 pm

seanthompson wrote:Here's one that I have never understood. If you look at the main page today of Endeavor on its way to the space station, you see the earth with its layers of atmosphere. Why can't we see any stars in the black space? When we look up at night outside of the city, we can see so many stars, they almost seem to overlap. Why is it that I don't see stars in photos taken from the lunar surface or from orbit?
You don't see stars because the images are exposed for objects that are sunlit. That is, these pictures are made with the camera settings just the same as you would use when making an outdoor image on a sunny day. With such settings, stars are not bright enough to be recorded (a few of the brightest stars or planets may just be visible, and are sometimes seen on space images). If you doubt this, set your camera on a tripod and take a daytime shot, and then at night, when the stars are out, take another shot with the same settings. You won't see any stars. To get the stars, you need an exposure of several seconds- a setting that would totally overexpose anything lit by the Sun.
Chris

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geckzilla
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Re: Where are the stars?

Post by geckzilla » Tue Feb 16, 2010 9:31 pm

And now you know when you see those artist renderings that they are taking the liberty of adding the stars to make the image more appealing. You'll never see the sun or any sufficiently bright object surrounded by tiny star specks with your eyes or with a camera. ...it really bugs me.
Just call me "geck" because "zilla" is like a last name.

seanthompson
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Re: Where are the stars?

Post by seanthompson » Wed Feb 17, 2010 3:43 pm

Chris Peterson wrote:
seanthompson wrote:Here's one that I have never understood. If you look at the main page today of Endeavor on its way to the space station, you see the earth with its layers of atmosphere. Why can't we see any stars in the black space? When we look up at night outside of the city, we can see so many stars, they almost seem to overlap. Why is it that I don't see stars in photos taken from the lunar surface or from orbit?
You don't see stars because the images are exposed for objects that are sunlit. That is, these pictures are made with the camera settings just the same as you would use when making an outdoor image on a sunny day. With such settings, stars are not bright enough to be recorded (a few of the brightest stars or planets may just be visible, and are sometimes seen on space images). If you doubt this, set your camera on a tripod and take a daytime shot, and then at night, when the stars are out, take another shot with the same settings. You won't see any stars. To get the stars, you need an exposure of several seconds- a setting that would totally overexpose anything lit by the Sun.
Thanks, Chris!!!

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rstevenson
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Re: Where are the stars?

Post by rstevenson » Wed Feb 17, 2010 6:48 pm

geckzilla wrote:And now you know when you see those artist renderings that they are taking the liberty of adding the stars to make the image more appealing. You'll never see the sun or any sufficiently bright object surrounded by tiny star specks with your eyes or with a camera. ...it really bugs me.
Having spent too many years messing around in darkrooms and lugging old cameras to unlikely places, I can imagine a few ways to actually take a picture which shows both background stars and detail in planets. I'd start by using techniques similar to those used to photograph the Sun's corona, and with a little experimentation, I'm sure I could do it. Even easier with today's digital cameras, I'd think. The trick, as with all from-space photography, is getting somewhere to set up the tripod!

Rob