The King function parametrisations for PSFs has been in use in astronomy as an analytical PSF model for many instruments, for example by the Fermi-LAT (see 2013ApJ…765…54A).

The distribution has two parameters GAMMA \(\gamma\) and SIGMA \(\sigma\) and is given by the following formula:

\[dP/d\Omega(r,\sigma,\gamma) = \frac{1}{2\pi\sigma^2} \left(1-\frac{1}{\gamma}\right) \left(1+\frac{r^2}{2\gamma\sigma^2}\right)^{- \gamma}\]

This formula integrates to 1 (see Introduction).


  • ENERG_LO, ENERG_HI – ndim: 1, unit: TeV
    • True energy axis
  • THETA_LO, THETA_HI – ndim: 1, unit: deg
  • GAMMA – ndim: 2, unit: none
    • Model parameter (see formula above)
  • SIGMA – ndim: 2, unit: deg
    • Model parameter (see formula above)

Recommended axis order: ENERGY, THETA

Header keywords:

As explained in HDU classes, the following header keyword should be used to declare the type of HDU:

Example data file: TODO