- 2004.09.29
Our planetarium gets installed next week, and when the dust settles
it will be time for us to install our projectors and get to work. We
are using a special 180° fisheye projector for one of our star
projectors, and with it we can project images from programs such as
Stellarium, which I
have discussed before
in this column.
Stellarium is an open-source, free software program developed by
Fabien Chéreau. It shows a very realistic sky from the earth.
Any location or time can be specified, and the user can scroll around
and look in other directions. A zoom feature lets you zoom in on
objects such as the moon, planets, and the Messier objects.
Since I and my students have taken a number of photographs of deep
sky objects, I thought it would be nice to have the zoom feature
present our own images when activated instead of the built-in images.
There are no instructions for how to do this that I could find, but I
figured there might be a resource file somewhere I could replace. You
can do this with Starry Night and The Sky, for example.
It turned out to be easier than I thought.
Most OS X programs are presented to the user as a single icon. Most
users don't know this, but the icon is actually a folder containing the
actual executable code plus separate resource files. Double-clicking
the icon for an OS X app starts the software like any other
program. The advantage of this system over Windows-style installations
is that all support files (with some exceptions, such as Library
support files meant to support several applications and preferences
files) are stored together, so when you delete the program, you just
throw one thing in the Trash. In Windows, you have to run an
uninstaller that gathers all the files that have been scattered all
over the hard drive and deletes them.
I started by control-clicking (right-clicking if you have a 2-button
mouse) on the Stellarium icon and then selected "Show Package
Contents." This opened a window revealing the contents of the
Stellarium "application + resources" folder. If you close this window,
you have to "Show Package Contents" again.
Inside is a single folder marked "Contents." I opened that to reveal
a series of files and folders. The only relevant one to this task is
the one marked "Resources." Opening that reveals a number of folders,
and through trial-and-error clicking I found that "textures" contains
all of the images Stellarium uses when displaying zoomed images. The
image files are in .png format, which GraphicConverter
can translate from anything it opens.
For my test case I selected M13, a globular cluster in Hercules.
Here is M13 as it appears in Stellarium (from a screen shot on the
site): http://stellarium.free.fr/gfx/m13.jpg
Using GraphicConverter, I determined that the picture was exactly
256 x 256 pixels. Several others were as well, so I knew I needed to
convert my picture to this size as well. Here's my version:
Then I simply "saved as" PNG format, replaced the original file with
mine, and presto, Stellarium is customized. Now I can assign my
planetarium students the task of converting and replacing our images on
the planetarium computer. And if they're reading this (and they will),
they should come ask me where the original files are stored on our
server.
We're actually using a version of the program not available on the
website. James Lowery built an OS installer for the latest version
(0.6.0) that shows a 360 degree panorama in a circular display on the
monitor and sent us a copy as a favor. James, you should post your
installer on the Stellarium site - others might want it.
Anyway, we're going to have a very busy week next week, so I'd
better get back to work on that. Until next time....
is a longtime Mac user. He was using digital sensors on Apple II computers in the 1980's and has networked computers in his classroom since before the internet existed. In 2006 he was selected at the California Computer Using Educator's teacher of the year. His students have used NASA space probes and regularly participate in piloting new materials for NASA. He is the author of two books and numerous articles and scientific papers. He currently teaches astronomy and physics in California, where he lives with his twin sons, Jony and Ben.< And there's still a Mac G3 in his classroom which finds occasional use.
