Digital Universe Guide - Hayden Planetarium

144 3. THE MILKY WAY ATLAS
3.4 Optimizing the Milky Way Atlas
While technology is quickly catching up with the graphics power our Atlas demands, some people still
find that the Atlas runs sluggishly on their computers. There are a few things you can do to reduce your
computer’s strain.
Threshold the Stars The most common culprits in the Milky Way Atlas are the stars. We distribute
a star catalog with more than 100,000 stars, mostly from Hipparcos and a few from the Gliese Catalog.
Many computers cannot handle all of these stars with their points and textures. Using the Partiview
command thresh, you can remove many of the stars from view, improving the performance.
We have placed many preset threshold statements for the stars in the mw.cf file. Some threshold
distance, others the star’s color. One threshold useful for eliminating most of the stars without altering
the night sky view is a threshold on the stars’ brightness, called the apparent magnitude.
The system of stellar magnitudes has its roots in ancient times. The scale is such that the brighter
the object is, the smaller its apparent magnitude is. The brightest object in the sky, the Sun, has a
magnitude of −26; the moon’s is −12. The brightest star in the sky other than the Sun, Sirius, has a
magnitude of −1. Our eyes can see to about +6.5 magnitude, so thresholding the stars that are dimmer
than this will preserve the night sky while cutting out many of the dimmer stars.
We have set up a threshold statement in the mw.cf file that removes all stars over 7.5 apparent
magnitude. To activate it, type
see m75
at Partiview’s Command Line. Alternatively, you could enter the command by typing
thresh appmag < 7.5. This leaves you with about 25,000 stars and, we hope, improved
performance. If you are still having troubles, though, please read on.
Save Your Changes Once you have the settings configured for your system, open the mw.cf file
in a text editor and alter the file to suit the improved custom configuration.