Point-like IRFs

Point-like IRFs has been classically used within the IACT community. Each IRF component is calculated from the events surviving an energy dependent directional cut around the assumed source position.

The format of each point-like IRF component is analog to the ones already described within the full enclosure IRF specifications (see Full-enclosure IRFs), with certain differences listed in this section.

Any point-like IRF component should contain the header keyword:



In addition to the IRFs, the actual directional cut applied to the data needs to be stored. This cut is allowed to be constant or variable along several axes, with a different format.

In case the angular cut is constant along the energy and FoV, an additional header keyword may be added to the IRF HDU:

Header keyword:

  • RAD_MAX type: float, unit: deg
    • Radius of the directional cut applied to calculate the IRF, in degrees.

If this keyword is present, the science tools should assume the directional cut of this point-like IRF is constant over all axes. In case the angular cut is variable along any axis (reconstructed energy or FoV), an additional HDU is required to store these values. Note any DL3 file with a point-like IRF (with HDUCLAS3 = POINT-LIKE) that has no RAD_MAX keyword within the HDU metadata should have this additional HDU.

In case the directional cut is variable with energy or the FoV, point-like IRFs require an additional binary table. It stores the values of RAD_MAX as a function of the reconstructed energy and FoV following the BINTABLE HDU format.


The RAD_MAX_2D format contains a 2-dimensional array of directional cut values, stored in the BINTABLE HDU format.

Required columns:

  • ENERG_LO, ENERG_HI – ndim: 1, unit: TeV
    • Reconstructed energy axis
  • THETA_LO, THETA_HI – ndim: 1, unit: deg
    • Field of view offset axis
  • RAD_MAX – ndim: 2, unit: deg
    • Radius of the directional cut applied to calculate the IRF, in degrees.

Recommended axis order: ENERGY, THETA, RAD_MAX

Header keywords:

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

Example data file: here.