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Hi Tony,
The maths to give you an approximation of the true field of view is quite easy.
true fov = eyepiece apparent fov / (telescope focal length / eyepiece focal length)
For example, if you had an 8mm plossl eyepiece with a 52 degree apparent field of view then with your 640mm focal length dob you get:
52 / (640 / 8) = 0.65
Which is just over the 0.5 degree of the full solar disk.
Obviously, an eyepiece with a different apparent field of view will give you a different result, so you'll need to take that into account.
Malc
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