Using an F3 380mm Cassegrain primary in a dob

[moriniboy]

Simple question, am I mad, what problems can I realistically expect.

[Peter Drew]

It will depend a lot on what sort of optical specification the original design called for. If it was a classical Cassegrain then the primary would be a paraboloid and used as a Newtonian in conjunction with a Paracorr or similar it should provide decent results. If it was a Dall-Kikham it would have an undercorrected parabola and would not really be satisfactory.