Optimizing the Extragalactic Atlas
Optimizing the Extragalactic Atlas
There are several ways to improve performance in the Extragalactic Atlas. Chances are that the large galaxy surveys will be the heaviest burden on your system. You can ease this burden by setting thresholds on these data sets, relieving your computer's processor.
Thresholding on Brightness
The data variable that reflects the galaxy's brightness in the 2dF and Sloan surveys is Rmag. For each survey, the range will be different. If you get a report on each in Partiview (using the datavar command), the result will be
datavar 0 Rmag 11.688.. 21.731 mean 17.5139for the 2dF and Sloan surveys, respectively. Of course, the lower the number, the brighter the galaxy, so the brighter galaxies will be under the mean value.
datavar 0 Rmag 14.4315.. 28.3567 mean 18.648
For the 2dF survey, which has more than 229,000 galaxies, try setting a threshold around 17.5 mag using the command
thresh Rmag 11 17.5which displays all data with an Rmag value between 11 and 17.5, the brightest galaxies. This leaves about 96,000 galaxies. If this is too many, set the range to 11-17.0 and you'll be left with only 50,000 galaxies. If you can handle more, set your range to 11-18.
The Sloan galaxy survey has more than 374,000 galaxies. Thresholding on the same data variable, first try the range 14-18. This leaves 66,000 galaxies. Keep adjusting the upper limit on the range until you reach a suitable number of galaxies for your display.
Thresholding on Distance
Rather than selectively display data based on brightness, you can show galaxies within a given distance, thereby cutting down the number of particles to display while retaining a complete data set for those areas shown. For the 2dF and Sloan galaxy surveys, a good cutoff that allows you to see the large-scale structure is around 2 billion light-years. If you execute the following command for both surveys, you will be left with 70% of the galaxies.
thresh distly < 2e9
Lowering this threshold to 1 x 109 shows about 25% of the data, so find a suitable number in between these for your display (1.5 x 109 shows about 50%).
A Random Subset
Partiview will take a random subset of the data using the every command. This command takes an integer argument based on the number of points you want to show. For example, every 1 shows all points, every 2 shows 50% of the data, and so on. The points are chosen at random, and their total brightness is kept constant. So the smaller the sample, the brighter the particles become to compensate. For the Sloan galaxies, every fourth particle tends to be the limit before the structure in the data set disappears. Type
every 4to see the random subset.
We recommend using the every command as a last resort, seeing as it changes the structure of the data. Better to view a complete data set thresholded on some value rather than a data set that is complete nowhere.
With these tricks, you should be able to find a suitable display for your computer. Once you have the settings that you like, open the extragalactic.cf file and add them to the appropriate groups. Remember the eval command, though. For example, these commands in the extragalactic.cf file need to be placed in their appropriate data group and would appear:
eval thresh Rmag 11 17.5
eval thresh distly < 2e9
eval every 4
If you continue to have trouble displaying these data sets, please e-mail us for more ideas.
© 2002-2005 American Museum of Natural History
Last Modified: 2007-12-19 by Brian Abbott