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If you already have a ED80 I wouldn't be bothering with anything around the focal length range of 600mm.
The ED80 is less hassle to use not having to worry about constent collimation and cool down issues that you will have with fast Newts.
The 5D is going to give you a widefield so do you really want an even wider field with a short focal length....maybe you do...???
Some where aroung the 1000mm fl would be nice combined with your ED80 at 600mm this wouldn't demand high pointing accuracy from the mount.
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