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 Question about Hubble Deep Field

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Mars1



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PostSubject: Question about Hubble Deep Field    Tue Aug 23, 2011 4:44 pm

Please explain to me how it works to point Hubble at a patch of space for 11 days. I simply cannot get my mind around WHY the longer it is pointed, the more it can see.

If I were to stare into my telescope for 2 hours, would I see more than if I were to stare for 1 hour?

I know this may seem like a lame question, but I need some help here.
Thanks
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astrolover

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PostSubject: Re: Question about Hubble Deep Field    Tue Aug 23, 2011 4:45 pm

In our experience, very long exposures ( employing photographic plates ) can detect very faint objects normally
hidden from our immediate view. It requires extraordinary, steady focus of the instrument in question .
We can employ charged coupled devices and extremely cold gases to enhance our detectors , but it is superbly enhanced by the telescopes' presence in high orbit where it can view clearly beyond atmospheric interference.
If it was sound, it would be like trying to hear a fly stamp his feet on mars. But if you have a big enough mirror
and patience , you can do extraordinary things. Very Happy
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scopeman

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PostSubject: Re: Question about Hubble Deep Field    Tue Aug 23, 2011 4:45 pm

Mars1 wrote:


If I were to stare into my telescope for 2 hours, would I see more than if I were to stare for 1 hour?

If your eye could sum up all the photons that hit your retina over that period of time, yes it would. But eyes can not do that. However, electronic devices and film can.

It isn't much different from taking a very long exposure with a camera at night, lets say of your street. An exposure of 1/60th of a second, typical for daytime, would show almost nothing at night. Set your exposure for 10 or 20 seconds, and you'll see a lot.
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AstroTurk

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PostSubject: Re: Question about Hubble Deep Field    Tue Aug 23, 2011 4:46 pm

Mars1 wrote:
Please explain to me how it works to point Hubble at a patch of space for 11 days. I simply cannot get my mind around WHY the longer it is pointed, the more it can see.

If I were to stare into my telescope for 2 hours, would I see more than if I were to stare for 1 hour?

I know this may seem like a lame question, but I need some help here.
Thanks

The big issue is the number of photons you see. Let me explain it this way.
On a moonless night far away from any city lights go outside. Can you see much? Your eyes are still receiving some photons just not many.
Now after a while your pupils will fully dilate letting more photons reach your retina but you will still not be able to see much more. This is because the rods and cones don't let you build up an image over time.

If you took a camera and set it up and took a picture you wouldn't see much.
Next let the shutter stay open for a long time, say an hour, then looked at the photo you would see a lot more because that picture got tens of thousands more photons hitting the film/ccd.

The HST essentially is doing the same thing. Just letting enough photons build up to make a picture. But unlike the night time setting you may be in the HST is looking at a part of the sky where it may literally only receive one photon at a time. Then it becomes and issue of like painting a picture 1 dot at a time or like watching a jigsaw puzzle being put together but the pieces are being put down in the right location but a completely random order.
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AstroTurk

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PostSubject: Re: Question about Hubble Deep Field    Tue Aug 23, 2011 4:46 pm

Oh you might then ask why the images aren't really blurry because if you took a time lapsed picture of anything on Earth for days then the image would not be clear. This is because, in astronomical terms, 11 days or even 11 years isn't anything when looking at something like a galaxy. Essentially nothing will change in that time at the resolution you are observing so the image doesn't suffer.
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astro_alan

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PostSubject: Re: Question about Hubble Deep Field    Tue Aug 23, 2011 4:49 pm

Mars1 wrote:

If I were to stare into my telescope for 2 hours, would I see more than if I were to stare for 1 hour?

No, because your eyes do not work the same way as the imaging apparatus on the HST. The ability to see and to photograph are two different things.

Inside your eye are light sensitive cells. Every time a photon of light interacts with one a signal is sent to the brain. However, once that signal is sent the cell returns to its normal state. The change triggered by the photon is transient. Every photon interaction is essentially starting from scratch in terms of its effect on the eye. Your eye retains no record of the light interactions that happened in the past. You see what is entering your eye now.

On a photographic film or on the CCD in the Hubble Space Telescope, the photon triggers a change that is not reversed, causing a chemical change in a molecule on the film coating, or causing a charge to develop in a tiny piece of the CCD. The more photons interact, the more film coating moecules change or the bigger the charge buildup on the CCD. The image is the sum of all the photons that hit the film or CCD over the length of the exposure. The longer the exposure, the more photons cause changes in the detector, and the more detail appears on the image.

A (very) rough analogy would be the way weather reporters measure rainfall. The rain falls at a regular rate over a wide area, but you can't tell how much rain has fallen by looking at the ground, or the windshield of your car, because every raindrop that lands on it runs off, so the car windshield will be just as wet after five minutes as after two hours in the same level of rainfall. That's like your eye, with raindrops instead of photons. To measure rainfall you need to have an open container out in it that collects the raindrops rather than allowing them to run off, and so the contents of the container are the sum of all the raindrops that fell into it over however long it was left out in the rain. If you leave it out for five minutes it might have just half an inch of rain in it. If you left it out in the same rain for half an hour it would have 3 inches in it. After an hour, six inches, after two, twelve inches and so on. That's like a camera.


Quote :
I know this may seem like a lame question, but I need some help here.
Thanks

There's no such thing as a lame question.

There is such a thing as a lame answer, so I hope this wasn't one. Laughing Smile
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