## SphericalDetector

For SAS and GISAS simulations, the two-dimensional area detector is modelled by the SphericalDetector class.

### Geometry

A `SphericalDetector` is a 2D detector defined in angular coordinates. 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.

### Constructor

The constructor has the signature

    detector = ba.SphericalDetector(n_phi, phi_min, phi_max, n_alpha, alpha_min, alpha_max)

The arguments `phi_min, phi_max, alpha_min, alpha_max` define the range of the detector in direction $\varphi_\text{f},\alpha_\text{f}$.

The arguments `n_phi, n_alpha` are the number of bins per direction; their product is the number of pixels of the detector.

### Masks

When fitting theoretical models to measured diffraction images, it can be helpful to mask part of the detector area. See

- Fit with masks

### Resolution

For modeling the detector resolution, see Detector resolution.

### History

Until BornAgain 21, we had a `RectangularDetector` and a `SphericalDetector`. In BornAgain 22, they were merged into `SphericalDetector` class.

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