# The Data Level 3 (DL3) format¶

The data level 3 FITS files consisting of event lists and extra information concerning the observation (pointing direction, time), as well as two index tables that list the observations and declare which response should be used with which event data.

## Data levels in CTA¶

### Instrument response functions¶

For each observation, instrument response functions (namely effective area, point spread function, energy dispersion, background) are distributed. Some details about the origin of these functions are given in IRF Theory. The functions are stored in the form of multidimensional tables giving the IRF value as a function of position in the field-of-view and energy of the incident photon.

The formats used are discussed and described in gadf. This format is still a prototype. In the coming years CTA will develop and define it’s release data format, and Gammapy will adapt to that.

## Application in gammapy¶

The main classes in Gammapy to access the DL3 data library are the DataStore and Observation. They are used to store and retrieve dynamically the datasets relevant to any observation (event list in the form of an EventList, IRFs see irf - Instrument response functions and other relevant informations).

Once some observation selection has been selected, the user can build a list of observations: a Observations object, which will be used for the data reduction process. A convenient way to do this is to use the high level interface, see gammapy.analysis.