A (not so) Brief Introduction to Celestia Addons Part 1 (3rd edition)

1: General Overview

Here's a general overview of how Addons are supposed to work:

Most Addons 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.

.SSC (Solar System Catalog) files tell Celestia where to draw planets.
.STC (STar Catalog) files tell Celestia where to draw stars.
.DSC (Deep Space Catalog) files tell Celestia where to draw Nebulas, OpenClusters and Galaxies.
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 Addons 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 Addons 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.

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.

1.4: File locations. Each type of file goes into a particular directory.

Celestia v1.3.0 (and later) lets you organize your Addon files. Previous versions of Celestia required that the files be mixed in with the ones that come with Celestia. As a result, many older Addons 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, Addon 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 Addon 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 Addon, and which came with Celestia.

Here's one way to organize Addons:

Create a directory (folder) within Celestia's extras directory which has the name addons. This will let you move all of your Addons at once by moving just this one directory.
Within the addons directory, create a directory for each Addon when you install it. For example, you might create the NGC2237 directory.
Within the 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:
Catalog files all go into an Addon's main directory. For example, the STC and SSC Catalogs for NGC2237 should go into the directory NGC2237 itself.
An older Addon, one which was created before this new directory layout became possible, usually includes an \extras\ directory containing all of its catalog files. Those files all must be moved into the Addon's main directory and the folder \extras\ that was provided with the Addon should be deleted.
Most Addons come with just one texture resolution. Their pictures all go into the medres directory. For example, NGC2237.png should go into the directory NGC2237\textures\medres.
3DS model files go into a subdirectory named models. For example, NGC2237.3ds goes into the directory NGC2237\models.