Flexible Image Transport System


High Energy Astrophysics Science Archive Research Centre.

OGIP FITS Standards

The FITS Working Group in the Office of Guest Investigators Program has established conventions for FITS files for high-energy astrophysics projects.


The HEASARC’s calibration database (CALDB) system stores and indexes datasets associated with the calibration of high energy astronomical instrumentation.


IACT = imaging atmospheric Cherenkov telescope (see wikipedia article).

Observation = Run

For IACTs observations are usually conducted by pointing the array (or a sub-array) for a period of time (typically half an hour for current IACTs) at a fixed location in celestial coordinates (i.e. the telescopes slew in horizontal Alt/Az coordinates to keep the pointing position RA/DEC in the center of the field of view).

For current IACTs the term “run” is more common than “observation”, but for CTA probably the term “observation” will be used. So it’s recommended to use observation in these format specs.

Off Observation

The term “off observation” or “off run” refers to observations where most of the field of view contains no gamma-ray emission (apart from a possible diffuse extragalactic isotropic component, which is supposed to be very weak at TeV energies).

AGN observations are sometimes also considered “off observations”, because the fraction of the field of view containing their gamma-ray emission is often very small, and most of the field of view is empty.

For further info on background modeling see Berge (2007)