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Rod,
They come in various lengths. Most common is 35mm and 50mm, however, you can also get 80mm.
Do you feel that you get almost there? If so, 35mm will be sufficient. But, just to be safe, it may be worthwhile getting a 50mm. Also, they come in two sizes: 1.25" and 2". You'll need to get the appropriate one.
Typically, you don't want to rack the focuser all the way out, and then use a short extension tube. You introduce focuser slop by doing that. It's best to have the focuser as out as little as possible and rely on extension tubes. So, it's typically better to get a longer extension tube than to rack the focuser all the way out and whack in a 35mm.
Hope this helps.
Regards,
Humayun
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