The EVENTS extension is a binary FITS table that contains an event list. Each row of the table provides information that characterises one event. The mandatory and optional columns of the table are listed below. In addition, a list of header keywords providing metadata is specified. Also here there are mandatory and optional keywords. The recommended extension name of the binary table is EVENTS.

Mandatory columns

We follow the OGIP event list standard.

  • EVENT_ID type: int64
    • Event identification number at the DL3 level. See notes on EVENT_ID below.
  • TIME type: float64, unit: s
    • Event time (see Time)
  • RA type: float, unit: deg
    • Reconstructed event Right Ascension (see RA / DEC).
  • DEC type: float, unit: deg
    • Reconstructed event Declination (see RA / DEC).
  • ENERGY type: float, unit: TeV
    • Reconstructed event energy.

Optional columns


None of the following columns is required to be part of an EVENTS extension. Any software using these columns should first check whether the columns exist, and warn in case of their absence. Any software ignoring these columns should make sure that their presence does not deteriorate the functioning of the software.

  • EVENT_TYPE type: bit field (in FITS tform=32X)
    • Event quality partition.
  • MULTIP type: int
    • Telescope multiplicity. Number of telescopes that have seen the event.
  • GLON type: float, unit: deg
    • Reconstructed event Galactic longitude (see Galactic).
  • GLAT type: float, unit: deg
    • Reconstructed event Galactic latitude (see Galactic).
  • ALT type: float, unit: deg
  • AZ type: float, unit: deg
  • DETX type: float, unit: deg
  • DETY type: float, unit: deg
  • THETA type: float, unit: deg
  • PHI type: float, unit: deg
    • Reconstructed field of view position angle (see Field of view).
  • GAMMANESS type: float
    • Classification score of a signal / background separation. SHOULD be between 0 and 1, with higher values indicating larger confidence that the event was produced by a gamma ray.
  • DIR_ERR type: float, unit: deg
    • Direction error of reconstruction
  • ENERGY_ERR type: float, unit: TeV
    • Error on reconstructed event energy
  • COREX type: float, unit: m
    • Reconstructed core position X of shower
  • COREY type: float, unit: m
    • Reconstructed core position Y of shower
  • CORE_ERR type: float, unit: m
    • Error on reconstructed core position of shower
  • XMAX type: float, unit: radiation lengths
    • First interaction depth
  • XMAX_ERR type: float, unit: radiation lengths
    • Error on first interaction depth
  • HIL_MSW type: float
    • Hillas mean scaled width
  • HIL_MSW_ERR type: float
    • Hillas mean scaled width error
  • HIL_MSL type: float
    • Hillas mean scaled length
  • HIL_MSL_ERR type: float
    • Hillas mean scaled length error

Mandatory header keywords

The standard FITS reference time header keywords should be used (see Formats). An observatory Earth location should be given as well (see Earth location).

  • HDUCLASS type: string
    • Signal conformance with HEASARC/OGIP conventions (option: ‘OGIP’). See HDU classes.
  • HDUDOC type: string
    • Reference to documentation where data format is documented. See HDU classes.
  • HDUVERS type: string
    • Version of the format (e.g. ‘1.0.0’). See HDU classes.
  • HDUCLAS1 type: string
    • Primary extension class (option: ‘EVENTS’). See HDU classes.
  • OBS_ID type: int
    • Unique observation identifier (Run number)
  • TSTART type: float, unit: s
    • Start time of observation (relative to reference time, see Time)
  • TSTOP type: float, unit: s
    • End time of observation (relative to reference time, see Time)
  • ONTIME type: float, unit: s
    • Total good time (sum of length of all Good Time Intervals). If a Good Time Interval (GTI) table is provided, ONTIME should be calculated as the sum of those intervals. Corrections for instrumental dead time effects are NOT included.
  • LIVETIME type: float, unit: s
    • Total time (in seconds) on source, corrected for the total instrumental dead time effect.
  • DEADC type: float
    • Dead time correction, defined by LIVETIME/ONTIME. Is comprised in [0,1]. Defined to be 0 if ONTIME=0.
  • OBS_MODE type: string
    • Observation mode. See notes on OBS_MODE below.
  • RA_PNT type: float, unit: deg
    • Pointing Right Ascension (see RA / DEC). Not mandatory if OBS_MODE=DRIFT, but average values could optionally be provided.
  • DEC_PNT type: float, unit: deg
    • Pointing declination (see RA / DEC). Not mandatory if OBS_MODE=DRIFT, but average values could optionally be provided.
  • ALT_PNT type: float, unit: deg
    • Pointing Altitude (see Alt / Az). Only mandatory if OBS_MODE=DRIFT
  • AZ_PNT type: float, unit: deg
    • Pointing azimuth (see Alt / Az). Only mandatory if OBS_MODE=DRIFT
  • EQUINOX type: float
    • Equinox in years for the celestial coordinate system in which positions given in either the header or data are expressed (options: 2000.0). See also HFWG Recommendation R3 for the OGIP standard.
  • RADECSYS type: string
    • Stellar reference frame used for the celestial coordinate system in which positions given in either the header or data are expressed. (options: ‘ICRS’, ‘FK5’). See also HFWG Recommendation R3 for the OGIP standard.
  • ORIGIN type: string
    • Organisation that created the FITS file. This can be the same as TELESCOP (e.g. “HESS”), but it could also be different if an organisation has multiple telescopes (e.g. “NASA” or “ESO”).
  • TELESCOP type: string
    • Telescope (e.g. ‘CTA’, ‘HESS’, ‘VERITAS’, ‘MAGIC’, ‘FACT’)
  • INSTRUME type: string
    • Instrument used to aquire the data contained in the file. Each organisation and telescop has to define this. E.g. for CTA it could be ‘North’ and ‘South’, or sub-array configurations, this has not been defined yet.
  • CREATOR type: string
    • Software that created the file. When appropriate, the value of the CREATOR keyword should also reference the specific version of the program that created the FITS file. It is intented that this keyword should refer to the program that originally defined the FITS file structure and wrote the contents. If a FITS file is subsequently copied largely intact into a new FITS by another program, then the value of the CREATOR keyword should still refer to the original program. HISTORY keywords should be used instead to document any further processing that is performed on the file after it is created. For more reading on the OGIP standard, see here.

Optional header keywords

  • OBSERVER type: string
    • Name of observer. This could be for example the PI of a proposal.
  • CREATED type: string
    • Time when file was created in ISO standard date representation “ccyy-mm-ddThh:mm:ss” (UTC)
  • OBJECT type: string
    • Observed object (e.g. ‘Crab’)
  • RA_OBJ type: float, unit: deg
    • Right ascension of OBJECT
  • DEC_OBJ type: float, unit: deg
    • Declination of OBJECT
  • EV_CLASS type: str
  • TELAPSE type: float, unit: s
    • Time interval between start and stop time (TELAPSE=TSTOP-TSTART). Any gaps due to bad weather, or high background counts and/or other anomalies, are included.


Keywords below seem to be pretty low-level and eventually instrument specific. It needs to be discussed whether a recommendation on these keywords should be made, or whether the definition should be left to the respective consortia.

  • HDUCLAS2 type: string
    • Secondary extension class (option: ‘ACCEPTED’). See HDU classes.
  • TELLIST type: string
    • Telescope IDs in observation (e.g. ‘1,2,3,4’)
  • N_TELS type: int
    • Number of observing telescopes
  • TASSIGN type: string
    • Place of time reference (‘Namibia’)
  • DST_VER type: string
    • Version of DST/Data production
  • ANA_VER type: string
    • Reconstruction software version
  • CAL_VER type: string
    • Calibration software version
  • CONV_DEP type: float
    • convergence depth (0 for parallel pointing)
  • CONV_RA type: float, unit: deg
    • Convergence Right Ascension
  • CONV_DEC type: float, unit: deg
    • Convergence Declination
  • TRGRATE type: float, unit: Hz
    • Mean system trigger rate
  • ZTRGRATE type: float, unit: Hz
    • Zenith equivalent mean system trigger rate
  • MUONEFF type: float
    • Mean muon efficiency
  • BROKPIX type: float
    • Fraction of broken pixels (0.15 means 15% broken pixels)
  • AIRTEMP type: float, unit: deg C
    • Mean air temperature at ground during the observation.
  • PRESSURE type: float, unit: hPa
    • Mean air pressure at ground during the observation.
  • RELHUM type: float
    • Relative humidity
  • NSBLEVEL type: float, unit: a.u.
    • Measure for night sky background level


This paragraph contains some explanatory notes on some of the columns and header keys mentioned above.


Most analyses with high-level science tools don’t need EVENT_ID information. But being able to uniquely identify every event is important, e.g. when comparing the high-level reconstructed event parameters (RA, DEC, ENERGY) for different calibrations, reconstructions or gamma-hadron separations.

Assigning a unique EVENT_ID during data taking can be difficult or impossible. E.g. in H.E.S.S. we have two numbers BUNCH_ID_HESS and EVENT_ID_HESS that only together uniquely identify an event within a given run (i.e. OBS_ID). Probably the scheme to uniquely identify events at the DL0 level for CTA will be even more complicated, because of the much larger number of telescopes and events.

So given that data taking and event identification is different for every IACT at low data levels and is already fixed for existing IACTs, we propose here to have an EVENT_ID that is simpler and works the same for all IACTs at the DL3 level.

As an example: for H.E.S.S. we achieve this by using an INT64 for EVENT_ID and to store EVENT_ID = (BUNCH_ID_HESS << 32) | (EVENT_ID_HESS), i.e. use the upper bits to contain the low-level bunch ID and the lower bits to contains the low-level event ID. This encoding is unique and reversible, i.e. it’s easy to go back to BUNCH_ID_HESS and EVENT_ID_HESS for a given EVENT_ID, and to low-level checks (e.g. look at the shower images for a given event that behaves strangely in reconstructed high-level parameters).


Currently in this format specification, event class EV_CLASS is a header key (i.e. the same for all events in a given event list) and EVENT_TYPE is a bitfield column (i.e. can have a different value for each event). Both are optional at this time, only used as provenance information, not by science tools to make any decisions how to analyse the data.

The reason for this is simply that we have not agreed yet on a scheme what event class and event type means, and how it should be used by science tools for analysis. Developing this will be one of the major topics for the next version of the spec. It is likely that a proper definition of event classes and types will not be compatible with what is currently defined here, so not filling EV_CLASS and EVENT_TYPE when creating DL3 data is not a bad idea.

To summarise a bit the discussions on this important point in the past years, they were mostly done by looking at what Fermi-LAT is doing and some prototyping in H.E.S.S. to export DL3 data to FITS.

The scheme in Fermi-LAT for event classes and event types is nicely summarised here or here. There event classes and types are key parts of the data model, used for EVENT to IRF association and even end users need to learn about them and pass event class and type information when using science tools.

One option could be to mostly adopt what Fermi-LAT does for IACTs. However, a major difference is that Fermi-LAT is a more stable detector, that has a CALDB of a very limited number or IRFs that can be used for all data, whereas for IACTs with changing telescope configurations, degrading mirrors, changing atmosphere, zenith angle, … most likely we will have to produce per-observation IRFs, and then it’s easier to bundle the IRFs with the EVENTS in one file, and the use of event class and type to link EVENTS and IRFs is no longer needed.

An alternative scheme is to use the term “event class” to describe a given analysis configuration. This is e.g. how we currently use EV_CLASS in H.E.S.S., we fill values like “standard” or “hard” or “loose” to describe a given full configuration that is the result of a calibration, event reconstruction and gamma-hadron separation pipeline. This is similar to what Fermi-LAT does, except less sophisticated, the event classes are completely independent, there is no nesting, and separate events and IRF files are produced for each class/configuration. To analyse data from a given class, the user chooses which set of files to download (e.g. “loose” for pulsars or “hard” for a bright source where a good PSF is needed), and then the science tools don’t need to do anything with EV_CLASS, it is just provenance information. Some people are experimenting with the use of EVENT_TYPE in a similar way as Fermi-LAT, e.g. to have event quality partitioning based on number of telescopes that saw a given event, or other criteria. Again, it is left to users to split events by EVENT_TYPE and produce IRFs for each event type and pass those for a joint fit to the science tools, as there is no agreement or implementation yet in the science tools to support EVENT_TYPE directly.

So to conclude and summarise again: EV_CLASS and EVENT_TYPE as mentioned here in this spec are optional and very preliminary. Defining event class and type for IACTs needs more prototyping by the science tools and current IACTs and CTA and discussion, and then a proposal for a specification.


The keyword OBS_MODE specifies the mode of the observation. This is used in cases where the instrument or detector can be configured to operate in different modes which significantly affect the resulting data, or to accomodate different types of instrument. More details can be found here

Most IACT data will be POINTING, indicating a constant pointing in equatorial coordinates.

In addition to the OGIP-defined values (POINTING, RASTER, SLEW and SCAN), we define the option DRIFT to accomodate ground-based wide-field instruments, in which local zenith/azimuth coordinates remain constant. In this case, the header keywords RA_PNT and DEC_PNT are no longer mandatory, and instead ALT_PNT and AZ_PNT are required.

It is likely that OBS_MODE in the future will be a key piece of information in the DL3 data model, defining the observation mode (e.g. pointed, divergent, slewing, …) and being required to analyse the data correctly.