I finally got round to getting a suitable stepper ring for my 10-20mm Sigma lens, so that I could fit my 0.42x semi fisheye converter on it.
Putting the two together, and setting the zoom for a full circle, I get a resultant lens size of about 6.3mm
That's good for about 160º of sky! Time to have a play in the backyard!
And one day I'll buy that new 4.5mm F2.8 Sigma lens that does the full 180º :op.... drooool!