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Horizontal and vertical lines - star

  1. Jan 2, 2008 #1
    Hi all,

    I'm wondering why we see those horizontal and vertical stripes of lights when looking at a light source (see e.g. this picture). Why are these lines of light present in the first place and why are they only horizontal and vertical in the second place?

    Thanks,
    Jeroen
     
  2. jcsd
  3. Jan 2, 2008 #2

    dst

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  4. Jan 2, 2008 #3

    mgb_phys

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    They are caused by the spider supporting the secondary mirror.
    They tend to be a aligned horizontally/vertically because:
    The engineer designed the secondary support that way
    The bolt holes for the camera are also horizontal
    The astronomers are used to it and will call the tech out in the middle of the night if they aren't aligned like that
    The only sensible reason I can think of is that CCDs can create horizontal and vertical streaks from very bright objects, by orientating the detector this way you only lose one set of pixels.

    With a camera mounted on an alt-az telescope you have to be able to rotate the camera as the field rotates when the telescope tracks and so you can set the camera up so the spikes are in any orientation - ideally so they don't hit a faint object near a bright one.
     
  5. Jan 2, 2008 #4

    russ_watters

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    Sometimes they are even put in for aesthetics. My telescope doesn't have spider vanes to support the secondary, so my pictures don't normally have diffraction spikes in them. So when I take pictures of certain objects where I want to see them (open clusters and nebulae with bright stars, mostly), I cause the spikes myself by stringing a couple of pieces of thread across the objective of my scope. For example:

    http://www.russsscope.net/images/Beehive-3-30-07.jpg
     
  6. Jan 3, 2008 #5
    I also notice that I sometimes get a similar effect when looking at lights after wiping off my glasses, only there tends to be more like 6 spikes instead of 4. Since my glasses don't have a secondary mirror to support, what causes this? Is it because I've smeared a film of whatever oil and gunk was on my glasses over them? And any guesses on how this diffraction works?
     
  7. Jan 3, 2008 #6

    DaveC426913

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    Yes, it works exactly the same way.


    Probably the easiest way to visualize it is to look at what gentle waves on a lake do to a light source like the sun or moon. They draw it out into a line that is basically perpendicular to the wave crests. If the "waves" are small enough (such as streaks on glass) and are oriented in several different directions (say, by vigorous wiping) then the lines will look like corresponding spikes.
     
  8. Jan 3, 2008 #7

    Ben Niehoff

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    One also gets diffraction spikes in photography (especially night photography), by diffraction off of the aperture blades. Typically there are six blades, and you get 6-pointed stars as in this example:

    http://www.pbase.com/frida_87/image/88315282

    Some nicer lenses have 7, 8, or 9 blades (where odd numbers are especially favorable). For an aperture of N blades, you will get N diffraction spikes if N is even, but 2N spikes if N is odd. Therefore, a 7- or 9-bladed aperture makes very brilliant star shapes.
     
  9. Jan 3, 2008 #8

    DaveC426913

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    Heh. Just yesterday while watching an ad, I made my wife roll her eyes at my geekiness. The background of the ad was a Christmas light display but out-of-focus. All the little lights had been blurred from pinpoints out into perfect 7-sided polygons, showing that the lens iris was 7-bladed.
     
  10. Jan 3, 2008 #9
    My bare eyes also have those spikes, only 4, when I look to some street lights from my window. They are not vertical and horizontal, and not perpendicular either. When I turn my head, they also turn. It may have something to do with thin layer of water on the lens.
     
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