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The shape of what we can observe?

  1. Mar 10, 2015 #1
    On a previous thread I viewed I saw a video about the observable universe:

    Now if you skip to the 3rd minute, you will realize that the shape of the areas we have observed is shaped like "butterfly wings", why did we not map the two triangle-shaped spots yet, why? And why is it particularly that shape (the butterfly winged)? And also, why is the color different at each side (green, then yellow)?
     
  2. jcsd
  3. Mar 10, 2015 #2

    Bandersnatch

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    What you see there is an animated tour made by recording the Sloan Digital Sky Survey (http://www.sdss.org/) data as displayed by the Partview interface of the Digital Universe Atlas.

    The SDSS uses a single telescope in New Mexico, with drift scanning technique (http://nexsci.caltech.edu/workshop/2005/presentations/Rabinowitz.pdf). Basically, the telescope is fixed, and photos are taken as the sky moves in front of the telescope, producing long narrow strips of scanned volume that look like those large triangles in the visualisation, their angular extent corresponding to the angle the sky moved by during the night of operation of the telescope. These are much less than 180 degrees, as naively one could expect them to be, as the lower elevations above the horizon are unsuitable for observations (atmospheric interference etc).
    If you look closely, the bow tie shape is actually composed from a multitude of those narrow triangular strips.

    The regions of the sky suitable for scanning are additionally constrained by the location of the telescope on Earth (can't point it low over the horizon due to poor visibility there, so Northern and Southern areas of the sky are unsuitable), and by the location of the Solar System in the Milky Way (you don't want the disc of our galaxy to obstruct your view).

    The SDSS is ongoing, so expect more sky coverage, and/or greater depth and variety of data. But again, I don't think it's ever going to get full sky coverage due to the aforementioned constraints.

    The colouring scheme is attributed by the Partview interface of the Digital Universe atlas. I don't remember off the top of my head what it was supposed to mean - most likely it differentiated between different data sets (the video is an amalgam of many different sets).

    But if you're interested, head to the site of the DU atlas:
    http://www.amnh.org/our-research/hayden-planetarium/digital-universe
    and download the thing (it's free).
    You'll need to read the guide (available from the same site) to be able to use it correctly, though, as the interface is a far cry from user friendly. The guide should also explain the different colouring schemes.
    It requires a bit of commitment to wrap one's head around, but it's worth it. It's the most up-to-date and extensive planetarium program you can get (for free, at least). You can navigate in 3D through everything you saw on that video you linked to, although you need to run separate instances for separate levels of scale.
     
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