Astronomy Picture of the Day recently featured a nice starfield image made
available at 6000x3000 by The European Southern Observatory (ESO) under a
Creative Commons license.  It appears to my untrained eye work very well as a
spherical image_map applied to a sky_sphere.  If any of you celestial mechanics
would like to point out how to calculate dw_startrot properly (as is done in
sunpos.inc) I'd appreciate it.

#declare dw_latitude=29.53;  // Observer's latitude.
#declare dw_startrot=65;     // Found by "hand calibrating" to a star chart
                             // for 2009-09-12T07:16 CDT at this location.

sky_sphere {
   pigment {
      image_map{
         png "phot-32a-09-fullres.png" // High resolution starfield image.
         //png "constellations.png"    // Image with constellations labeled.
         map_type 1                    // 0=planar, 1=spherical,
                                       // 2=cylindrical, 5=torus
         interpolate 4                 // 0=none, 1=linear, 2=bilinear,
                                       // 4=normalized distance
         once                          // No repetitive tiling.
         }
      scale  <-1,1,1>                  // Reverse projection handedness.
      rotate <118.5,0,-152.8>          // Put Polaris at +y.
      rotate  y*(dw_startrot+360*clock)// Rotate around polaris.
      rotate  x*(90-dw_latitude)       // Adjust for latitude.
      }
   }

Interpolation 0 seems best for most situations, though 4 is good for some.  No
interpolation makes stars behind a partially transparent layer either present or
absent, and seems better for dark skies, while 4 causes them to fade nicely when
behind another partially transparent pigment and seems better for lit skies.
Interpolation 1 does not seem to work, and 2 is in-between 0 1nd 4 but may make
it easier to pick out constellations in some situations.

The ESO's big image I converted to PNG since their TIFF didn't seem to work:
http://www.eso.org/gallery/d/133707-4/phot-32a-09-fullres.tif

I got the constellation-marked version by clicking "See the Constellations!" and
downloading the resulting image from:
http://www.gigagalaxyzoom.org/B.html

For information about "The Milky Way panorama" see:
http://www.eso.org/gallery/v/ESOPIA/Galaxies/phot-32a-09-fullres.tif.html

The Astronomy Picture of the Day that led me to it:
http://apod.nasa.gov/apod/ap090926.html

Serge Brunier did the original, and will reportedly send a very high resolution
version on request:
http://www.sergebrunier.com/gallerie/pleinciel/index-eng.html

Image map code shamelessly ripped from:
http://news.povray.org/web.4a5e5e8141a4106c6a9471340%40news.povray.org

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