Most Add-ons consist of 1, 2 or 3 sets of files:
1.1:
One or more Catalog files. These describe where the objects are in space.
1.2:
Surface pictures (texture maps) for those objects. These give the objects a
unique appearance.
1.3:
Models of the objects. These give them special shapes.
1.4:
File locations. Each type of file goes into a particular directory.
1.1:
One or more Catalog files describing where the objects are in space.
Although they're all plain text files, different keywords are used in the different types of Catalog files. You can't rename a .SSC file to be a .STC file, for example.
The first part of a Catalog file's name doesn't matter. Celestia tries to load all of the .SSC, STC and DSC files that it finds. The first part of the name is to help you keep track of which file is for what.
Celestia only reads in Catalog files when it starts up. If you modify an old file or add a new Catalog file, you have to exit from Celestia and start it up again so it can find the new file.
Unfortunately, some Add-ons don't include a Catalog file. Their authors leave it up to you to create one.
Celestia includes its own SSC file that defines all of the planets
and many of their moons, along with asteroids, comets and a few
spacecraft. This is the file named solarsys.ssc in
Celestia's data
directory. Some Add-ons provide better surface textures for objects in
our solar system.
If you're careful to make a copy, you can modify
solarsys.ssc to use one or more of these textures
instead of the ones provided with Celestia.
See also "Creating and using .SSC files".
1.2:
Surface pictures (texture maps) for those objects, giving them a
unique appearance.
Lots of people enjoy making replacement surface textures for existing planets or moons. For them, you either have to rename the image files to match the ones specified in the appropriate .SSC file, or you have to edit the .SSC file so it looks for the name of the new surface texture file.
Celestia comes with many surface textures already
in the lores and medres directories,
although it doesn't come with any hires images. It's best
not to mix your images among those that Celestia uses. Use a
separate subdirectory instead. See Section 1.4 below.
See also "Creating Textures for Celestia".
1.3:
Models of the objects, which give them special shapes.
Objects with complicated structures can be created using a 3D design package. Celestia can understand models that are defined in the 3DS file format. This format was originally used by the 3D modelling software named "3D Studio Max". Most 3D design programs can create 3DS files if you use their "Export" command.
Models can be designed either to require particular surface texture
files or to accept a surface texture specified by Celestia. When
surface textures are specified in a model, Celestia loads them from
a textures\medres directory.
Celestia v1.3.0 (and later) lets you organize your Add-on files. Previous versions
of Celestia required that the files be mixed in with the ones that come
with Celestia.
As a result, many older Add-ons put their files into the
directories that Celestia uses itself: extras,
models, and
textures
This causes lots of confusion when you install a
new version of Celestia.
Starting with Celestia v1.3.0, add-on files can go into a separate
Celestia subdirectory (folder) that you
create within Celestia's extras directory.
Now when you install a new version of Celestia, you can simply drag
that whole Add-on directory into the
extras directory of the new version of Celestia.
You no longer have to keep track of which files you added, which ones belong
to which add-on, and which came with Celestia.
Here's one way to organize Add-ons:
extras directory
which has the name addons.
This will let you move all of your Add-ons at once by moving just
this one directory.
addons directory, create a directory for
each add-on when you install it. For example, you might create the
NGC2237 directory.
NGC2237 directory, you must create the directories
for
its models and its surface textures: create the directories
models
and textures.
Within the
textures
directory, create the directories
lores,
medres and
hires.
When you've finished, your directory structure might look something like this:
[-] Celestia130
--- data
[-] extras
| [-] addons
| [+] HIP500000
| [-] NGC2237
| --- models
| [-] textures
| --- hires
| --- lores
| --- medres
--- models
[+] textures
NGC2237 itself.
medres directory. For example, NGC2237.png should
go into the directory NGC2237\textures\medres.
lores, medres or hires
that are in a Celestia subdirectory named textures.
If there is only the one texture file with a unique name, Celestia will look in all three directories while trying to find it.
If there are texture files with the same name in more than one of the
textures
subdirectories, you can toggle among them using the "r" and "R"
keyboard commands.
models.
For example, NGC2237.3ds goes into the directory
NGC2237\models.
When a 3DS model specifies texture maps in the model itself, those
texture files must go into the medres directory. Celestia
does not yet provide a way to select different resolution surface
textures in this case.
If you don't tell me that something's missing, unclear or wrong, I can't improve it.