The instrument response function (IRF) formats currently in use for imaging atmospheric Cherenkov telescopes (IACTs) are stored in FITS binary tables using the “multidimentional array” convention (binary tables with a single row and array columns) described at BINTABLE HDU.

This format has been used for calibration data and IRF of X-ray instruments, as well as for the IRFs that are distributed with the Fermi-LAT science tools.

At the moment (November 2015), this format is used by H.E.S.S. and VERITAS and supported by Gammapy and Gammalib and is being proposed for DL3 IRFs (i.e. the format distributed to end users and used by the science tools for CTA). Note that a different format has been proposed for CTA to serialise multidimensional arrays and axis information: http://adsabs.harvard.edu/abs/2015arXiv150807437W As far as we know, this format is currently not supported by any analysis package.

Here we specify the IRFs in use for IACT data.

IRF axes

Most IRFs are dependent on parameters, and the 1-dim. parameter arrays are stored in columns. The following names are recommended:

  • For energy grids, see here for basic recommendations. Column names should be ENERGY or ENERG_LO, ENERG_HI because that is used (consistently I think) for OGIP and Fermi-LAT. For separate HDUs, the extension names should be ENERGIES or EBOUNDS (used by Fermi-LAT consistently).
  • Sky coordinates should be called RA, DEC, GLON, GLAT, ALT, AZ.
  • Field of view coordinates DETX, DETY or THETA, PHI for offset and azimuth angle in the field of view.
  • Offset wrt. the source position should be called RAD (this is what the OGIP PSF formats use).

The IRF format specs mention a recommended axis format and axis units. But tools should not depend on this and instead:

  • Use the axis order specified by the CREF header keyword (see BINTABLE HDU)
  • Use the axis unit specifiec by the CUNIT header keywords (see BINTABLE HDU)