Instrument Response Functions (IRFs)

What are IRFs?

Instrument Response Functions (IRFs) define the sensitivity and performance characteristics of a gamma-ray observatory under specific observing conditions. sensipy uses CTAO IRFs to calculate sensitivity curves and simulate observations.

IRFs include information about:

• Effective area: How sensitively the telescope collects photons
• Point spread function (PSF): Angular resolution
• Energy dispersion: Energy resolution
• Background rates: Expected background counts

Supported IRF Versions

sensipy supports the following CTA IRF productions:

• prod5-v0.1 (Alpha Configuration, current recommendation from CTAO)
• prod3b-v2 (Legacy support)

Tip

The most recent Alpha configuration IRFs are prod5-v0.1, which can be found on Zenodo. The fitsonly version is enough for use with sensipy.

Downloading IRFs

Automatically with the built-in IRF downloader

You can use the built-in command to download the latest CTAO IRFs:

uv

uv run sensipy-download-ctao-irfs

pip

pip install sensipy
sensipy-download-ctao-irfs

conda

conda activate my-environment
sensipy-download-ctao-irfs

This will automatically download and organize the latest CTAO IRFs to the ./IRFs/CTAO directory.

Note

For more download options, run sensipy-download-ctao-irfs --help.

Manually

1. Visit the Zenodo repository

   Go to the CTA IRF Zenodo page to access the IRF files.

2. Download the required FITS files

   Download the .tar.gz archives for the sites, zenith angles, and durations you need. For example:

   • CTA-Performance-prod5-v0.1-North-20deg.FITS.tar.gz
   • CTA-Performance-prod5-v0.1-South-20deg.FITS.tar.gz

3. Extract the archives

   Extract the downloaded files into an organized directory structure.

IRF Directory Structure

Organize your IRF files following this structure:

• DirectoryIRFs/CTAO/ (root directory for all IRFs)

  • Directoryprod5-v0.1/

    • Directoryfits/

      • CTA-Performance-prod5-v0.1-North-20deg.FITS.tar.gz
      • CTA-Performance-prod5-v0.1-South-20deg.FITS.tar.gz
      • CTA-Performance-prod5-v0.1-North-40deg.FITS.tar.gz
      • …
    • …
  • Directoryprod3b-v2/

    • Directoryfits/

      • …
    • …

Caution

When initializing the IRFHouse class, you’ll provide the path to the root directory (e.g., IRFs/CTAO/). The package expects this specific directory structure.

Loading IRFs

Once your IRFs are downloaded and organized, you can load them using the IRFHouse class:

from sensipy.ctaoirf import IRFHouse

# Create an IRF house pointing to your IRF directory
house = IRFHouse(base_directory="./IRFs/CTAO")

# Load a specific IRF
irf = house.get_irf(
    site="south",
    configuration="alpha",
    zenith=20,
    duration=1800,
    azimuth="average",
    version="prod5-v0.1",
)

print(irf)