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The 20mm specifies the focal length of the eyepiece, like the focal length of your scopes lens, mirror etc.
The 66 Deg, specifies the Apperant Field of View (AFOV) which basically means its like having a visual area of 66 deg, but in reality due to the power of your optical system, you actual view may only be 0.5 deg.
Example, i have a 66 deg eyepiece, and im looking at the moon. The moon just fits inside my entire field of view, well the apperant view is i can see from one side to the other side which is 66 deg. but the moon in reality is 0.5 deg wide, so my REAL view is 0.5 deg and not 66 deg.
I hope i made some sense..
Theo
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