Will change in one of the next releases.
For the time being, the constructor is
ba.OffspecDetector(nphi, phi_min, phi_max, nalpha, alpha_min, alpha_max)
Scattering intensity is computed for nphi azimuthal bins, then naively summed.
Results may vary widely for even vs odd nphi. Check!
Choose the phi range so narrow, and nphi so large that results become stable.