For SAS and GISAS simulations, the two-dimensional area detector is modelled by the Detector2D class.
A Detector2D is a flat rectangular detector.
All pixels have equal size.
In BornAgain, detector images are shown as seen from the back of the detector. In this way we ensure that the $\varphi$ or $q_y$ axis has the same orientation as the real-space $y$ axis.
For plotting and data import and export, BornAgain pretends that the detector bins correspond to bins of constant width in $\varphi$ or $q_y$ and $\alpha$ or $q_z$, even though this is only true in first approximation.
For the actual scattering computation, BornAgain uses true scattering angles, computed as nonlinear functions of the pixel coordinates.
Further distortions arise
if the detector is not perpendicular to the x axis.
This is not unlikely in GISAS
where it makes sense to position the detector perpendicular
to the reflected or transmitted beam.
Corrections for these and other distortions will be implemented
when requested by users.
The constructor has the signature
detector = ba.Detector2D(phi_span, alpha_span, n_phi, n_alpha, phi_center, alpha_center)
The arguments phi_span, alpha_span are the full width of the detector in
directions $\varphi_\text{f},\alpha_\text{f}$.
The arguments n_phi, n_alpha are the number of bins per direction;
their product is the number of pixels of the detector.
The arguments phi_center, alpha_center are the scattering angles
$\varphi_\text{f},\alpha_\text{f}$ at the center of the detector.
When fitting theoretical models to measured diffraction images, it can be helpful to mask part of the detector area. See
For modeling the detector resolution, see Detector resolution.
Until BornAgain 21, we had a RectangularDetector and a SphericalDetector.
In BornAgain 22, they were merged into a new Detector2D class.