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OK, the focal length of the EP gives you the magnification in your scope if you do the following
Focal length of scope/Focal length of EP = magnification
So a 25mm ep in one of these 8" scopes (FL = 1200mm) gives you power of 1200/25 = 48x. This is sufficient to see most deep sky objects, remember most DSO are not small, just faint! So you dont need much magnification.
The other important thing is the Field of View. Most stock eps are plossls with an Apparent FOV of 50 degrees. Divide this by the magnification and you get the True FOV which is how wide a slice of the sky you are seeing. So for a 25mm plossl you get 50/48 = 1.042 deg. As many of the better DSOs are fairly large you want a wide FOV to see them well. They also stay in view longer.
This is why you will find you will almost always use your 25mm EP the most in a scope of this size. I had an 8" for 6 months before getting the 12" and I think I used the 10mm EP that came with it about 3 times. Can be used on planets and for splitting doubles but thats about it. Things drift out of view really quickly, hard to get decent focus unl;ess the seeing is very good, and makes many objects dimmer.
On the other hand once you get into premium EPs, its a whole new story. I got a 13mm Nagler that gives me 115x with an 82deg AFOV. Superb views and TFOV of about .71deg. Easily my favourite EP but fairly pricey at about $340. Worth every cent I think.
Hope this helps, I think I have the maths right. Best advice I can give is when you get your scope, get along to a Club night, people are always happy to help and show you some abjects, and give you practical hands on advice as to EPs.
Malcolm
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