Dataset Open Access

CTAO Instrument Response Functions - prod5 version v0.1

Cherenkov Telescope Array Observatory; Cherenkov Telescope Array Consortium

Contact person(s)
Zanin, Roberta
Maier, Gernot

CTAO Instrument Response Functions - prod5 version v0.1

The CTA Observatory (CTAO) will provide very wide energy range and excellent angular resolution and sensitivity in comparison to any existing gamma-ray detector. Energies down to 20 GeV will allow CTAO to study the most distant objects. Energies up to 300 TeV will push CTAO beyond the edge of the known electromagnetic spectrum, providing a completely new view of the sky. This data repository provides access to performance evaluation and instrument response functions (IRFs) for CTA.

Citation and Acknowledgements:

In cases for which the CTA instrument response functions are used in a research project, we ask to add the following acknowledgement in any resulting publication:

"This research has made use of the CTA instrument response functions provided by the CTA Consortium and Observatory, see (version prod5 v0.1; [citation]) for more details."

Please use the following BibTex Entry for [citation] in the reference section of your publication:


Monte Carlo Simulations:

The performance values are derived from detailed Monte Carlo (MC) simulations of the CTA instrument based on the CORSIKA air shower code (v7.71, with the hadronic interaction models QGSjet-II-04 and URQMD, [1]) and telescope simulation tool sim_telarray [2]. A power- law gamma-ray spectrum with photon index 2.62 was assumed in the calculations, although none of the instrument response functions (e.g. differential flux sensitivities, effective areas, angular or energy resolutions) depends on the assumed spectral shape of the gamma-ray source. Background cosmic-ray spectra of proton and electron/positron particle types are modelled according to recent measurements from cosmic-ray instruments.

Nominal telescope pointing is assumed, with all telescopes pointing directions parallel to each other (performance estimation for other pointing modes, e.g. divergent pointing will be provided in the future). Performance estimations are available for three zenith angles (20 deg, 40 deg, and 60 deg), and for each zenith angle for two different azimuth angles (corresponding to pointing towards the magnetic North and South). There are significant performance differences found between the two azimuthal pointing directions (especially for the Northern site) as the impact of the geomagnetic field is large enough to influence notably the air shower development. For general studies, the use of the azimuth-averaged instrument response functions is recommended.

Instrument Response Functions (IRFs):

The analysis has been tuned to maximize the performance in terms of flux sensitivity. The optimal analysis cuts depend on the duration of the observation, therefore the IRFs are provided for 3 different observation times, from 0.5 to 50 h. IRFs are provided as binned histogram or FITS tables. It should be stressed, that the full potential of CTA in terms of angular and energy resolution is not revealed by these IRFS, due to the focus on the optimisation for best flux sensitivity.

In general all histograms are binned with a 0.2-binning on the logarithmic energy axis (5 bins per decade); some selected histograms (e.g. effective areas or energy migration matrices) are provided with a finer binning. Effective area and energy migration matrix are available in a double version: one for the case in which there is no a priori knowledge of the true direction of incoming gamma rays (e.g. for the observation of diffuse sources), and another for observations of point-like objects (including among the analysis cuts one on the angle between the true and the reconstructed gamma-ray direction).

IRFs are provided in ROOT format and as FITS tables. The FITS tables can be used directly as input to science analysis tools. The values of the IRFs are identical for the different file format, with one exception: the angular point-spread function is approximated by a Gaussian function for the FITS tables, while the ROOT files contain the full distribution.

Telescope layouts are preliminary and subject to change. The following array layouts (Alpha configuration) have been assumed:

  •  CTA South with 14 MSTs and 37 SSTs (see [figure](figures/CTA-Performance-prod5-v0.1-South-Alpha-Layout.png))
  •  CTA North with 4 LSTs and 9 MSTs (see [figure](figures/CTA-Performance-prod5-v0.1-North-Alpha-Layout.png))

Two zip files are uploaded:

  • full archive with IRFs in FITS and ROOT format:
  • partial archive with IRFs in FITS format only:

File Naming (examples):

  • Prod5-North-40deg-AverageAz-4LSTs09MSTs.18000s-v0.1.root: IRF for CTA Northern site on La Palma, 40 deg zenith angle, azimuth-averaged pointing, optimised for 5 hours of observation time
  • Prod5-South-20deg-AverageAz-14MSTs37SSTs.180000s-v0.1.fits.gz: IRF for CTA Southern site in Paranal, 20 deg zenith angle, azimuth-averaged pointing, optimised for 50 hours of observation time

List of files:

FITS format:

  • fits/CTA-Performance-prod5-v0.1-North-20deg.FITS.tar.gz
  • fits/CTA-Performance-prod5-v0.1-North-40deg.FITS.tar.gz
  • fits/CTA-Performance-prod5-v0.1-North-60deg.FITS.tar.gz
  • fits/CTA-Performance-prod5-v0.1-South-20deg.FITS.tar.gz
  • fits/CTA-Performance-prod5-v0.1-South-40deg.FITS.tar.gz
  • fits/CTA-Performance-prod5-v0.1-South-60deg.FITS.tar.gz

ROOT format:

  • root/CTA-Performance-prod5-v0.1-North-20deg.tar.gz
  • root/CTA-Performance-prod5-v0.1-North-40deg.tar.gz
  • root/CTA-Performance-prod5-v0.1-North-60deg.tar.gz
  • root/CTA-Performance-prod5-v0.1-South-20deg.tar.gz
  • root/CTA-Performance-prod5-v0.1-South-40deg.tar.gz
  • root/CTA-Performance-prod5-v0.1-South-60deg.tar.gz

IRFs for subarrays of e.g., MSTs only are in the files named MSTSubArray (similar for all other telescope types).



We would like to thank the computing centres that provided resources for the generation of the Prod 5 Instrument Response Functions (IRFs):

  • CAMK, Nicolaus Copernicus Astronomical Center, Warsaw, Poland
  • CIEMAT-LCG2, CIEMAT, Madrid, Spain
  • DESY-ZN, Deutsches Elektronen-Synchrotron, Standort Zeuthen, Germany
  • GRIF, Grille de Recherche d’Ile de France, Paris, France
  • IN2P3-CC, Centre de Calcul de l’IN2P3, Villeurbanne, France
  • IN2P3-CPPM, Centre de Physique des Particules de Marseille, Marseille, France
  • IN2P3-LAPP, Laboratoire d Annecy de Physique des Particules, Annecy, France
  • INFN-FRASCATI, INFN Frascati, Frascati, Italy
  • INFN-T1, CNAF INFN, Bologna, Italy
  • INFN-TORINO, INFN Torino, Torino, Italy
  • MPIK, Heidelberg, Germany
  • OBSPM, Observatoire de Paris Meudon, Paris, France
  • PIC, port d’informacio cientifica, Bellaterra, Spain
  • prague_cesnet_lcg2, CESNET, Prague, Czech Republic
  • praguelcg2, FZU Prague, Prague, Czech Republic
  • UKI-NORTHGRID-LANCS-HEP, Lancaster University, United Kingdom
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