I am posting this as a separate thread to teach anyone who is interested how to instruct Celestia to load only the add-ons you want it to load, instead of everything that is in your extras folder. This is a valuable technique for several reasons. Primarily, you may have lots of add-ons in your extras folder and if you load them all every time Celestia launches, you could overload the program by forcing it to draw lots of add-ons you don't want to use at the moment. That can slow down the program to a crawl. Also, some add-ons can interfere with each other. For example, there are simple models of the Hubble telescope that come with Celestia, and two complex models of Hubble that exist as add-ons. If you load them all, multiple images of the HST will appear in the same spot at the same time, something obviously to be avoided.
The good news is that you can instruct Celestia to load only certain add-ons at a particular time, at least in Windows operating systems. Step by step, here is how to do it:
1. Drag the extras folder that you currently have in Celestia, out of the directory and place it somewhere convenient (on the desktop, for example)
2. Create new separate extras folders for your different sets of add-ons. For example, instead of just using one 'extras" folder, you can create 3 or 5 or 20 folders named "extras-new", extras-experimental", extras-deep space", extras1, extras2, etc. There is no limit to how many folders you can use, or what to call them. The only rule is to make sure they are one word only with no spaces in the name (e.g. extras-new, rather than extras new). Once created, place them all into the Celestia main directory, side by side. For example, your directory tree might look like this (in Windows):
C:/Program Files/Celestia/extras1
C:/Program Files/Celestia/extras2
C:/Program Files/Celestia/extras-new, etc.
3. Sort all your current add-ons and place/drag them into these individual folders in any order you wish.
4. Copy the default celestia.cfg file which you will find in the main Celestia folder and paste a copy of that file into the same main Celestia directory as the original, but change its name to match your extras folders. For example, you could have the default celestia.cfg file, plus a separate renamed cfg file for each of extras folders you have created. Your Celestia directory tree would then look like this:
C:/Program Files/Celestia/celestia.cfg
C:/Program Files/Celestia/extras1.cfg
C:/Program Files/Celestia/extras2.cfg
C:/Program Files/Celestia/extras-new.cfg
etc.
5 Open each cfg file with a text editor and look for the line that specifies what extras folder the Celestia program will load. It is typically about half-way down the file. The line will look like this:
ExtrasDirectories [ "extras" ]
Change that line in that specific cfg file to point to the specific extras folder you want Celestia to load. For example, if you want it to load only "extras-new", the line would now read,
ExtrasDirectories [ "extras-new" ]
Please note that in this line, you can tell Celestia to load more than one extras folder. For example, the line could read,
ExtrasDirectories [ "extras-new" "extras1" "extras2"]
Also please note that you can customize each of these cfg files to do whatever you wish. For example, you may want your extras1.cfg file to not only load "extras1", but also load a different stars.dat file, or a different star texture. You can edit any line in these files to suit your needs, without changing how Celestia handles itself when using another cfg file.
Save the file and close it. Now, inside the Celestia main directory, you will have multiple extras folders, and multiple cfg files corresponding to each of those files. Each cfg file will tell Celestia to load a different set of add-ons.
6. Create a new folder inside the main Celestia directory named "Extras_shortcuts"
7. Right-click on the main Celestia program (Celestia.exe) and choose "Create Shortcut" from the menu that appears. This creates a Windows shortcut that points to the program. That Windows shortcut is named "shortcut to Celestia.exe". Place that shortcut inside the "Extras_shortcuts" folder that you created in step 6. Now, rename that shortcut with the name of the specific extras folder you wish to load when you start Celestia. For example, the shortcut name would change from "shortcut to Celestia.exe", to become "Shortcut to Celestia-extras-new".
8. Right-click on that shortcut, and choose "Properties". A menu will open. One of the lines will read, "Target "C:\Program Files\Celestia\celestia.exe"
Change that line to read, "C:\Program Files\Celestia\celestia.exe" --conf extras-new.cfg, where the cfg file you want to load is specified. You MUST use the 2 hyphens and the exact punctuation and spacing (i.e. put a space after .exe" ) This edit is called a Command Line. When this shortcut file is clicked on, Celestia will open that specific cfg file (instead of the default), find a line in that file telling it to load only the add-ons in the extras-new folder, and launch Celestia as usual.
The 2nd line in this Properties menu of interest is the "Start in" line. It should continue to read, "C:\Program Files\Celestia"
Click Apply and OK in the menu and close it.
9. Repeat steps 7 and 8 for each extras folder you wish to load separately. For example, if you have five extras folders named extras1, extras2, extras3, extras4 and extras5, you would need to create 5 cfg files, create five Windows shortcuts pointing to the Celestia program, place those five shortcuts into your shortcuts folder, and edit each shortcut so that its target line points to the specific cfg file you want Celestia to look for.
That is IT!
Now, if you want Celestia to load only the add-ons that you have put in your "extras-new" folder or your "extras1" folder, simply open the Windows-Shortcuts file, double-click on the shortcut pertaining to that folder, and Celestia will launch and load only the extras in that particular folder.
Once this is set up the first time, you have to do nothing else but launch Celestia from the shortcuts folders, instead of from an icon on your desktop. In fact, if you wish, you can place all of those shortcuts on your desktop instead of in a separate folder, so you can simply click the shortcut and launch Celestia from anywhere you wish.
Whenever your want to separate out your add-ons further, just create a new extras folder (e.g. - extras11), follow the above steps, and Voila!!
In this way, I have been able to manage over 3 GB of add-ons in the educational activities. I direct Celestia to load only the add-ons I need for a particular educational journey, avoiding any probem of overloading my video card.
Obviously, there are variations on these instructions. For example, you could place your extras folders all together inside another folder instead of place them in the Celestia main directory ... but ... lets not get things confusing.
Hope this has helped!!!
Enjoy
Frank
Wow - That's a great system there . . . ! 
Thanks . . . ! 