# Effective area format¶

Here we specify the format to store the effective area of a full-enclosure IRF. Effective area is always stored as a function of true energy. (see Effective Area)

## AEFF_2D¶

### Effective Area vs true energy¶

The effective area as a function of the true energy and offset angle is saved as a BINTABLE HDU with required columns listed below.

Columns:

• ENERG_LO, ENERG_HI – ndim: 1, unit: TeV
• True energy axis
• THETA_LO, THETA_HI – ndim: 1, unit: deg
• Field of view offset axis
• EFFAREA – ndim: 2, unit: m^2
• Effective area value as a function of true energy

Recommended axis order: ENERGY, THETA

If the IRFs are only known to be “valid” or “safe” to use within a given energy range, that range can be given via the following two keywords. The keywords are optional, not all telescopes use the concept of a safe range; e.g. in CTA at this time this hasn’t been defined. Note that a proper scheme to declare IRF validity range (e.g. masks or weights, or safe cuts that depend on other parameters such as FOV offset) is not available yet.

• LO_THRES type: float, unit: TeV
• Low energy threshold
• HI_THRES type: float, unit: TeV
• High energy threshold

If the effective area corresponds to a given observation with an OBS_ID, that OBS_ID should be given as a header keyword. Note that this is not always the case, e.g. sometimes IRFs are simulated and produced for instruments that haven’t even been built yet, and then used to simulate different kinds of observations.

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

• HDUDOC = ‘https://github.com/open-gamma-ray-astro/gamma-astro-data-formats
• HDUVERS = ‘0.2’
• HDUCLASS = ‘GADF’
• HDUCLAS1 = ‘RESPONSE’
• HDUCLAS2 = ‘EFF_AREA’
• HDUCLAS3 = ‘FULL-ENCLOSURE’
• HDUCLAS4 = ‘AEFF_2D’

The recommended EXTNAME keyword is “EFFECTIVE AREA”.

Example data file: here.