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Stargazing Could a very large telescope see a star as a disk?

  1. Feb 19, 2010 #1
    When I was a kid, the largest telescope was Mt Palomar, a little over 5 meter diameter mirror. In spite of its large size, I always read that stars appeared as points of light through the scope if it were used visually, i.e. stars are too small and too distant to appear as disks.

    However with the advent of very large telescopes(10 to 15 meters diameter)and the possibility of 40 meter telescopes in the future, I have a question:

    How large a telescope is necessary to show a close star like Sirius as a small disk, approximately equal to the size of the planet Neptune as seen through a 4" telescope at medium power?

    For calculation purposes assume this is a space-based telescope so there are no atmospheric interferences and also assume perfect optics
  2. jcsd
  3. Feb 19, 2010 #2
    It would need very large angular resolution. In other words, tag.
  4. Feb 19, 2010 #3


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    A nearby big star like Betelguse would be a disk for a 4m telescope.
    The problem is the atmosphere
    There are three options
    1, Adaptive optics = big complex and expensive
    2, Interferometer, either a number of telescopes linked together or a few tricks you can do with patches of a single telescope
    3, Lucky imaging - you take lots of very short exposure images and add the 'good' ones
  5. Feb 19, 2010 #4
    I'm very very lazy right now, but I'm willing to bet you could calculate this on your own.
  6. Feb 19, 2010 #5


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  7. Feb 19, 2010 #6


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    Sirius is only 10 LY distant and pretty big. It's disc is resolvable with existing scopes, if desired. Betelguese is not resolvable as a disc, it is at least 500 light years distant.
  8. Feb 20, 2010 #7


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    It ain't much to look at, but Betelgeuse is resolvable....and Sirius is not:
    http://zuserver2.star.ucl.ac.uk/~idh/apod/ap960122.html [Broken]

    Sirius is twice the diameter of the sun - Betelgeuse is 1000x the diameter of the sun.
    Last edited by a moderator: May 4, 2017
  9. Feb 20, 2010 #8
    The problem is that they aren't points of light but blurry disks. There are a few in resolving stars. The first problem is diffraction. If you put light through a pinhole is spreads out. This also happens with telescopes and so the image of a star is blurred by the light in the telescope spreading out.

    The other problem is the atmosphere. Basically if you have an image of a star, the atmosphere is going to blur it out.

    We can go through this calculation, but I think it's the wrong problem. Anything larger than about 10 inches is going to be limited by the atmosphere blurring. The smallest thing that you can see with any ground telescope without image processing is 1 arcsecond.

    Also what people have been able to do is to take images from two telescopes that are far apart and electronically combine then so that they create an image from one giant "artificial" telescope. This technique has been used in radio astronomy for a long time, but people are starting to use it in optical astronomy.


    One other thing, is that a lot of the recent advances in astronomy have been because of cheap computers and electronics. One thing about space telescopes is that it turns out to be not that useful for getting precision pictures of things since it turns out that you can do more with ground telescopes and lots of computers. The thing that Hubble *does* get you is that you can take measurements in frequencies that get blocked on the ground (ultraviolet).
    Last edited: Feb 20, 2010
  10. Feb 20, 2010 #9


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    It's nowhere close to that bad. With computer image processing techniques and good seeing, amateurs (such as myself) can get somewhere around .2 arcsec.

    Here's an article on achievable resolution and the Cassini Division in Saturn's rings, which has a width of about .75 arcsec and is resolvable even in even relatively small telescopes. http://www.cloudynights.com/item.php?item_id=1132
    Last edited: Feb 20, 2010
  11. Feb 20, 2010 #10
    The one arcsecond number is for raw unprocessed images. With computer image processing you can get things much, much better, and something that is great about these image processing techniques is that they are very widely available (i.e. you can download the software from the web). One thing that is amazing is how cheap computer power has totally transformed observational astronomy. When I was a young graduate student, people were just starting to experiment with these techniques.

    I had an interesting talk with someone that was not much older than me, and I got one of those "when I was a young graduate student, you kids are spoiled" talks in which he was talking about what people did when they used photographic plates. You had to carefully and slowly develop the photograph plates, and there was none of the fancy, smancy computer processing. When you wanted to measure something you had to get a ruler and record the number on a notepad.
  12. Feb 20, 2010 #11


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    I had to do that as an undergrad, and you had to cut the glass plates in the dark and then lick them to taste which side had the emulsion before loading them in the telescope.

    Damn - I only just turned (officially) middle aged and already I sound like the old guy.
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