by Chris Peterson » Mon Jul 23, 2018 1:31 pm
LMMdT wrote: ↑Mon Jul 23, 2018 7:38 am
bystander wrote: ↑Sat Jul 21, 2018 8:02 pm
ksp wrote: ↑Sat Jul 21, 2018 7:44 pm
Apologies if this is a stupid question, but I've always been curious why there are no stars visible in the background in pictures such as this from the moon.
The same reason you can't see stars during the daytime on Earth, the foreground is too bright. The reason the sky is black is there is no atmosphere to scatter the light and turn the sky blue.
I thought that the scattered light was precisely what prevented us from seeing the stars.
There is "seeing" and there is "imaging". They operate in somewhat different domains with respect to dynamic range. Scattering is certainly a factor. If you were on the Moon during the day, and looked at the sky from the bottom of a well, you'd see stars. Not so on Earth. But with images, it really just comes down to dynamic range and object brightness. There's not much difference in brightness between a daytime scene on the Moon and one on the Earth. In either case, with a film camera, you might have an exposure time of a hundredth of a second or less. Set up your camera for a well exposed shot during the day, and now wait for nighttime. Take a shot using the same settings. How many stars do you capture? Probably none, and certainly not a sky full of them. That's what's happening on lunar shots. You have daytime exposure settings. The lack of atmospheric scatter means the sky looks black, but the stars are so much dimmer than the foreground that they don't rise above the noise. You
could see stars in a daytime lunar image, simply by making a several second long shot. Of course, the landscape would be massively overexposed, and there'd still be glare in the optics interfering with the image.
[quote=LMMdT post_id=284333 time=1532331503]
[quote=bystander post_id=284303 time=1532203332 user_id=112005]
[quote=ksp post_id=284302 time=1532202246]
Apologies if this is a stupid question, but I've always been curious why there are no stars visible in the background in pictures such as this from the moon.
[/quote]
The same reason you can't see stars during the daytime on Earth, the foreground is too bright. The reason the sky is black is there is no atmosphere to scatter the light and turn the sky blue.
[/quote]
I thought that the scattered light was precisely what prevented us from seeing the stars.
[/quote]
There is "seeing" and there is "imaging". They operate in somewhat different domains with respect to dynamic range. Scattering is certainly a factor. If you were on the Moon during the day, and looked at the sky from the bottom of a well, you'd see stars. Not so on Earth. But with images, it really just comes down to dynamic range and object brightness. There's not much difference in brightness between a daytime scene on the Moon and one on the Earth. In either case, with a film camera, you might have an exposure time of a hundredth of a second or less. Set up your camera for a well exposed shot during the day, and now wait for nighttime. Take a shot using the same settings. How many stars do you capture? Probably none, and certainly not a sky full of them. That's what's happening on lunar shots. You have daytime exposure settings. The lack of atmospheric scatter means the sky looks black, but the stars are so much dimmer than the foreground that they don't rise above the noise. You [i]could [/i]see stars in a daytime lunar image, simply by making a several second long shot. Of course, the landscape would be massively overexposed, and there'd still be glare in the optics interfering with the image.