A Brief Introduction to Add-Ons (2nd edition)

3: Installing a Nebula Add-On.

I'll assume you're using some version of Windows.

If you've ever downloaded and installed other software, I'm sure this description will be full of things you already know how to do.

These instructions are specifically for the Rosetta Nebula that Rassilon created. You should be able to use a similar procedure for equivalent creations.

Essentially, the steps are:

3.1: Download the Zip archive file containing the add-on you want
3.2: Create the directory tree to contain the Add-on.
3.3: Copy the add-on files from the zip archive to the appropriate Celestia directories
3.4: Start Celestia and view the object.

In detail:

3.1: Download NGC2237

Rassilon's downloads are in the directory http://63.224.48.65/~rassilon/. The Rosetta Nebula Add-on is at http://63.224.48.65/~rassilon/NGC2237.zip or you can browse the Web page to select it.

3.2: Create the directory tree for NGC2237

If you haven't already, create a directory (folder) within Celestia's 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.
Within the addons directory, create a directory for each add-on 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 directory medres. (The lores and hires directories aren't needed for this particular Add-on.)
When you've created the new directories, your directory structure might look something like this:
```
[-] Celestia130
    --- data
    [-] extras
     |  [-] addons
     |      [+] HIP500000
     |      [-] NGC2237
     |          --- models
     |          [-] textures
     |              --- medres
    --- models
    [+] textures
```

3.3: Copy Add-on files

You need to have a ZIP program installed to extract the files. (Windows XP comes with one built in.)

Double click on the Zip archive file NGC2237.ZIP. A new unZip window should open showing the files that are in the archive.
In the Zip archive, locate the file "NGC2237.dsc" This is the "Deep Space Catalog" which defines the Nebula's location, the model to use and its size. Drag it to the Celestia subdirectory extras\addons\NGC2237\
or you can use this copy:
```
######################
# ngc2237.dsc
######################
Nebula "NGC2237" {
        Mesh "ngc2237.3ds"
        RA   6.5383
        Dec  5.05
        Distance  5500
        Radius  75
}
######################
```
Alternatively, here's a link to a copy of this file that you can download:
- ngc2237.dsc
In the zip archive, locate the files NGC2244.stc and NGC2244-2.stc. These are "STar Catalogs". They define a cluster of stars in the middle of the nebula. Drag them to the Celestia subdirectory extras\addons\NGC2237\. (All Catalog files for a specific add-on go into the same "root" directory)
In the zip archive, locate the file NGC2237.3DS This is the 3D shape model for the nebula. Drag it to the Celestia subdirectory extras\addons\NGC2237\models

In the zip archive, locate the file rosetta.png This is a PNG picture that is used to provide surface coloration for the model. It determines the colors and the transparency of the various regions of the nebula. Drag it to the Celestia subdirectory extras\addons\NGC2237\textures\medres\ (All image files used by models must be in the medres directory.)

Now your directory structure should look something like this:

[-] Celestia130
    --- data
    [-] extras
     |  [-] addons
     |      [+] HIP500000
     |      [-] NGC2237
     |           -- NGC2237.dsc
     |           -- NGC2244.stc
     |           -- NGC2244-2.stc
     |          --- models
     |               -- NGC2237.3ds
     |          [-] textures
     |              --- medres
     |                   -- rosetta.png
    --- models
    [+] textures

3.4: Start Celestia.

Check to make sure you're running v1.3.0 or newer: open the "Help" menu and select "About Celestia". A new window will open that has the name "Celestia" and its version number at the top-center. The version number should be 1.3.0 or greater. If it's 1.2.n then the Nebula Add-on can't work. If you've been copying the files to the wrong Celestia directory, you can just drag your new "Add-ons" directory to the right directory.
Test to see if Celestia knows the Nebula's name. To do this, type the following sequence of commands: [return] (Celestia will prompt with "Target name: " ) NGC2237[return]
If the .DSC file is OK, then the name will appear in the upper left corner of Celestia's window along with the distance (5500.000 ly). You have now "selected" NGC2237.
Now, make sure galaxy rendering is enabled:
- Select the "Render" menu
- Select the "View Options" item
- In the new window:
  - check (select) the Show "Galaxies" box
  - also check (select) the Label "Galaxies" box
  - then select the [OK] button (NOT the [x] box at the top right)
Type the letter "c". Celestia will turn toward the selected object. That's NGC2237 in this case. Celestia should pause briefly while it loads the model and its texture. When the viewpoint finally stops turning, you should see a tiny pink blotch in the middle of the screen labeled "NGC2237" in yellow.
If there's no yellow label, then you did not manage to select the "Label Galaxies" box. The "Show Galaxies" box probably isn't selected either. Go back to the "Render" "View Options..." menu and make sure their boxes are checked. You should see the label appear immediately when its box is checked. When you click on [OK], the label will stay while the menu goes away. If you click on [x] at the upper right, the label will go away when the menu does. Celestia will forget the Render changes you just made.
If there is a yellow label but no pink blotch, then either you didn't select "Show Galaxies" or there is a problem related to either the 3DS file or the png file. Check for spelling errors in the names of the files and their directories. If you have a binary file editor like HexEdit, you might try editing the 3DS file to find out exactly what texture it's looking for. You could try changing it to use another texture to find out if the problem is related to the image file.
Then type the letter "g". Celestia will take you to a position near the nebula.
Enjoy!
Here's a snapshot taken by Celestia of this nebula. The constellation of Orion is to the lower right.

NGC 2237 as seen from Io
and here's a Cel://URL to take you to this viewpoint:
NGC 2237 as seen from Io
After you've used this URL to see where the nebula is on the sky, one thing you can do is to hold down the "," (comma) key to increase the magnification of Celestia's "telescope". You can turn it up until the nebula fills the screen!