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Solon
2011-May-23, 06:37 PM
Looking at this image from the Dawn mission framing camera as it
approaches Vesta, you see a couple of very faint stars in the background.
They then overexpose Vesta to get the stars to stand out a little more.
"a spectacular backgroind of stars" they say. Huh? Spectacular?
http://dawn.jpl.nasa.gov/feature_stories/images/PIA14118_360.jpg
http://dawn.jpl.nasa.gov/feature_stories/first_image_asteroid_vesta.asp
This image taken from the ISS, with a Nikon D1, is more like what I
would call spectacular.
http://earth.jsc.nasa.gov/sseop/images/ESC/small/ISS006/ISS006-E-28028.JPG
http://earth.jsc.nasa.gov/scripts/sseop/photo.pl?mission=ISS006&roll=E&frame=28028
The CCD device in the Dawn camera has a 60,000 dynamic range to begin
with, so the difference in brightness between Vesta and the background
stars must be huge. They can go to 3.5 hours per exposure too, and, if
I am understanding it correctly, the CCD has onboard electron multiplication,
photomultiplication in effect.
Kubrick consulted closely with NASA when producing 2001, so which versions
is correct, the Dawn missions Kubrick-like background, or the D1 version?
Or is the D1 much more sensitive than the camera they sent out on Dawn?

slang
2011-May-23, 08:38 PM
Looking at this image from the Dawn mission framing camera as it
approaches Vesta, you see a couple of very faint stars in the background.
They then overexpose Vesta to get the stars to stand out a little more.
"a spectacular backgroind of stars" they say. Huh? Spectacular?
http://dawn.jpl.nasa.gov/feature_stories/images/PIA14118_360.jpg
http://dawn.jpl.nasa.gov/feature_stories/first_image_asteroid_vesta.asp
This image taken from the ISS, with a Nikon D1, is more like what I
would call spectacular.
http://earth.jsc.nasa.gov/sseop/images/ESC/small/ISS006/ISS006-E-28028.JPG
http://earth.jsc.nasa.gov/scripts/sseop/photo.pl?mission=ISS006&roll=E&frame=28028
The CCD device in the Dawn camera has a 60,000 dynamic range to begin
with, so the difference in brightness between Vesta and the background
stars must be huge. They can go to 3.5 hours per exposure too, and, if
I am understanding it correctly, the CCD has onboard electron multiplication,
photomultiplication in effect.
Kubrick consulted closely with NASA when producing 2001, so which versions
is correct, the Dawn missions Kubrick-like background, or the D1 version?
Or is the D1 much more sensitive than the camera they sent out on Dawn?

I don't understand your question. You disagree with the use of the word spectacular? What do you mean, "correct"? What's Kubrick got to do with anything?

Solon
2011-May-23, 09:31 PM
@slang

You disagree with the use of the word spectacular?
In the Dawn image, absolutely.

What do you mean, "correct"?
Can we see thousands of stars in space, or just a few? Or is this to do with
the type of camera, optics and exposure settings?

What's Kubrick got to do with anything?
Kubrick wanted 2001 to be as accurate a representation of the space environment
as possible. His background stars are few and faint.

StupendousMan
2011-May-23, 09:56 PM
The exposure of Vesta was no more than a few seconds in length, if that. If you want to make a fair comparison, compare the image of Vesta with an image of the Milky Way which has an exposure time of 2 seconds or so.

Oh, and were you sure that Vesta, as seen from the spacecraft, was in the middle of the Milky Way? If not, compare the image of Vesta to an image of some portion of the sky which is NOT the Milky Way and has an exposure time of 2 seconds or so.

slang
2011-May-23, 09:56 PM
In the Dawn image, absolutely.

Well, you personally may not find it spectacular, but it may certainly be so for those usually looking at objects like moons, asteroids etc. Usually their brightness (close up) is so high that background stars are not visible at all.


Can we see thousands of stars in space, or just a few? Or is this to do with
the type of camera, optics and exposure settings?

This question exposes a flaw in your thinking, I think. You put "can we see" versus "can we create images". And you're right, what we see in images is highly dependent on the factors you mention, and some others too.


Kubrick wanted 2001 to be as accurate a representation of the space environment
as possible. His background stars are few and faint.

What a human eye can see in space different from what a camera or telescope can image. It really depends on where you aim your camera, how long the exposure is, how large the field of view is, etc etc. The Hubble Ultra Deep Field has an exposure time of many hours, but not a single star in view. Just galaxies. Because it's a very small field of view of a rare part of the sky without background (in the HUDF case: foreground!) stars.

Romanus
2011-May-23, 10:32 PM
If my estimates are correct, Vesta was photographed against or near the border of Capricornus and Aquarius, a particularly star-poor area of the celestial sphere.

Solon
2011-May-24, 03:26 AM
@stupendousman

Oh, and were you sure that Vesta, as seen from the spacecraft, was in the middle of the Milky Way? If not, compare the image of Vesta to an image of some portion of the sky which is NOT the Milky Way and has an exposure time of 2 seconds or so.
@romanus

If my estimates are correct, Vesta was photographed against or near the border of Capricornus and Aquarius, a particularly star-poor area of the celestial sphere.
Good points!
@slang

This question exposes a flaw in your thinking, I think. You put "can we see" versus "can we create images". And you're right, what we see in images is highly dependent on the factors you mention, and some others too.
I presume they would be using the clear filter, so wouldn't it be visible light, as we would see with our eyes?
@stupendousman

The exposure of Vesta was no more than a few seconds in length...
I have looked high and low for any info on exposure times. Care to share your source?

Hornblower
2011-May-24, 10:32 AM
Looking at this image from the Dawn mission framing camera as it
approaches Vesta, you see a couple of very faint stars in the background.
They then overexpose Vesta to get the stars to stand out a little more.
"a spectacular backgroind of stars" they say. Huh? Spectacular?
http://dawn.jpl.nasa.gov/feature_stories/images/PIA14118_360.jpg
http://dawn.jpl.nasa.gov/feature_stories/first_image_asteroid_vesta.asp
This image taken from the ISS, with a Nikon D1, is more like what I
would call spectacular.
http://earth.jsc.nasa.gov/sseop/images/ESC/small/ISS006/ISS006-E-28028.JPG
http://earth.jsc.nasa.gov/scripts/sseop/photo.pl?mission=ISS006&roll=E&frame=28028
The CCD device in the Dawn camera has a 60,000 dynamic range to begin
with, so the difference in brightness between Vesta and the background
stars must be huge. They can go to 3.5 hours per exposure too, and, if
I am understanding it correctly, the CCD has onboard electron multiplication,
photomultiplication in effect.
Kubrick consulted closely with NASA when producing 2001, so which versions
is correct, the Dawn missions Kubrick-like background, or the D1 version?
Or is the D1 much more sensitive than the camera they sent out on Dawn?

Neither one is "correct", and neither one is "incorrect". Here we have different missions, different needs, different equipment, different fields of view, and different exposure times.

fosborn
2011-May-24, 11:19 AM
Solon wrote;

and, if
I am understanding it correctly, the CCD has onboard electron multiplication,
photomultiplication in effect.

I thought they used a Frame Transfer CCD (same as Messenger Mission), not a L3CCD type.

StupendousMan
2011-May-24, 03:51 PM
I have looked high and low for any info on exposure times. Care to share your source?

Use physics.

The specs on the framing camera can be found in a variety of sources -- for example,

http://www.google.com/url?sa=t&source=web&cd=5&ved=0CD0QFjAE&url=http%3A%2F%2Fwww6.cet.edu%2Fdawn%2Fmultimedia% 2Fdocs%2Fcamera.pdf&rct=j&q=dawn%20framing%20camera%20&ei=LM3bTczxKcTr0gHFlYXNDw&usg=AFQjCNEYsCbjeJwZLW4X4ObT6lYv0Ha5-g&cad=rja

http://www.mps.mpg.de/en/projekte/dawn/

The camera has an aperture of 20 mm and feeds a Thomson 7888A CCD. The field of view is 5.5 degrees, so each of the 1024 pixels is about 19 arcseconds on a side. The full-well capacity is about 320,000 electrons.

Now, when the picture was taken, Dawn was about 1.2 million km from Vesta. At that time, Earth was about 300 million km from Vesta. Since the apparent magnitude of Vesta from Earth was about 7.3, the apparent magnitude from Dawn would be about -5 (yes, yes, phase effects, blah blah blah). Since the framing camera was (probably) using its clear filter for this picture, one can compute the number of photons per second which would have entered the 20 mm aperture: about 120 million. Vesta's disk, as seen from Dawn, was about 5 pixels in diameter, or about 20 pixels in area. That means each pixel should have received around 60 million photons per second. Assuming a 20 percent overall QE for the optics and CCD, that means about 12 million electrons per second would be generated in each pixel.

The chip saturates around 320,000 electrons, so in order to prevent saturation, the image would have to be around 1/40 of a second.

Satisfied?

Solon
2011-May-24, 06:58 PM
@stupendousman

Satisfied?
Gobsmacked!:clap:
I have looked at as much info as I can find on the framing camera, but couldn't have
put all the info together like you did there, to come up with hard numbers. Cheers.
One thing I could not find though, was a detailed description of the optics. I have seen
the diagrams, but they don't tell me what is actually going on in there. Are you as
knowledgeable with optics too?

slang
2011-May-24, 08:10 PM
@slang

I presume they would be using the clear filter, so wouldn't it be visible light, as we would see with our eyes?

No, you keep missing the point. A human eye is very different from a camera. Sure there are some similarities, but we can't "see" with exposure times of 20 minutes! If our brain were somehow capable of collecting light through our eyes for several minutes before it would form the image in mind, it would look more like the dazzling second image in your post. But we can't... all we can do is let our eyes adapt to the dark so we become a little bit more sensitive to light, but nothing like we can do with a camera.

StupendousMan
2011-May-24, 09:33 PM
@stupendousman
One thing I could not find though, was a detailed description of the optics. I have seen
the diagrams, but they don't tell me what is actually going on in there. Are you as
knowledgeable with optics too?

Have you read these documents?

http://www.google.com/url?sa=t&source=web&cd=2&ved=0CB8QFjAB&url=http%3A%2F%2Findico.cern.ch%2FmaterialDisplay. py%3FcontribId%3D19%26materialId%3D6%26confId%3D43 007&rct=j&q=dawn%20framing%20camera%20optics&ei=hCPcTYj-JYXdgQeE6vwL&usg=AFQjCNEwcJAdlsc-qWYHEh9L2qR-r7YpoQ&cad=rja

http://www.google.com/url?sa=t&source=web&cd=3&ved=0CCUQFjAC&url=http%3A%2F%2Fwww6.cet.edu%2Fdawn%2Fmultimedia% 2Fdocs%2Fcamera.pdf&rct=j&q=dawn%20framing%20camera%20optics&ei=hCPcTYj-JYXdgQeE6vwL&usg=AFQjCNEYsCbjeJwZLW4X4ObT6lYv0Ha5-g&cad=rja

What more do you want to know?