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Capturing light photons with the Hubble

  1. Jan 6, 2010 #1
    Seeing the incredible images from the Hubble today really perplexed me. I am quite sure this is basic Physics, however I cannot find the answer anywhere on the Interweb :(

    My questions all have to do with peering farther back in to time, by collecting light photons in some different or improved manner. So...

    To observe objects we gather individual photons of light which are emitted from a source and build them into an image.

    - How does the massive distance from the point of collection to the object and the slow speed at which we can collect less incoming photons due to the distance affect how the image is recreated:

    - Are the individual photons streaming to our collector in a single line since they are coming from such a small pinpoint in the universe and so infrequently?

    - When we use a bigger collector of photons to peer deeper, are we seeing more photons side by side to build a image, or are we seeing the straight stream of photons over a longer period of time?

    - If the emitting object radiates light photons out in every single direction continuously, how is it that the massive distance doesn't spread out those photons as they get farther away from the object - example: think really long pins in a tennis ball. As the distance increases away from the ball the distance from the head to head of each pin would grow. Is this why a wider collector gather more photons and would somehow see a deeper image???(really confused now :)

    What in the method of the collection of light photons with better equipment affects our ability to look deeper into space - this is the question.

    Forgive me if this is too elementary for this site and if so I would really appreciate a link to the right site!
  2. jcsd
  3. Jan 6, 2010 #2


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    It means you need a bigger mirror, more sensitive detectors and longer exposures

    A bigger aperture allows you to collect more photons side by side. At a very large distance with a source that is much bigger than the telescope the light comes parallel.

    Yes - imagine a source putting out many many photons in every direction.
    Now pick a small lens held upto that source, we only get the fraction going in our direction through that area.
    Whats worse is if you move 2x further away all those photons are now spread over an area of a sphere centred on the source, and when the radius doubles the area goes up by 4x so the number going through your small lens is only 1/4 as much.

    The main win is the larger area, the biggest ground based telescopes have mirrors nearly 10m across, hubble has a mirror about 2.5m, your eye is only 0.005mm across! And rememebr that when you make the mirror twice as wide, it's area goes up by 4x.

    The detectors are very sensitive, they can record around 50-80% of the photons arriving, compared to only 1% for photographic film.
    Then you can wait a long time to build up the image. The hubble deep field image took almost 10days of exposure time - your digital camera takes an image of only 1/1000 second!

    Nothing is too elementary for this site - welcome to PF
  4. Jan 6, 2010 #3
    Thanks - I think I pretty much understand the "wider is better" rule of thumb.
    More specificly, are the new instruments on Hubble (or any collector for that matter) just more sensitive and thus more efficent gathering more of the protons which went previoulsy uncaptured? Or are we capturing them in some different manner to look deeper?
  5. Jan 6, 2010 #4


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    The new detectors are more efficient - the percentage of incoming photons that actually get detected is better. They are also lower noise, so if you have a noise equal to 10photons you need to detect 20photons to be sure you have something, if you replace that with a detector with 5photons of noise, you only need to detect half as photons from the source - so can see fainter.

    The later hubble instruments also added detectors that could work in other wavelengths (especially infrared) this lets us see different kinds of objects and different physical events.
    Also for reasons to do with the expansion of the universe, very distance objects are redder - so we need to look into the infrared.
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