  The Cherenkov Telescope Array (CTA) is a future ground-based observatory
(with two locations, in the Northern and Southern Hemispheres) that will be
used in the study of the very-high-energy gamma-ray sky. CTA observations will
be proposed by external users or initiated by the observatory, with the
resulting measurements being processed by the CTA observatory and the reduced
data made accessible to the corresponding proposer. Instrument Response
Functions (IRFs) will also be provided to convert the quantities measured by
the array(s) into relevant science products (i.e. spectra, sky maps, light
curves).
  As the response of the telescopes depend on many correlated observational and
physical quantities (e.g. gamma-ray arrival direction, energy, telescope
orientation, background light, weather conditions etc.) the CTA IRFs could grow
into increasingly larger and larger file sizes, which can become unwieldy or
impractical for use in specific observation cases. To this end, a customized
IRF format (complying with the FITS standard) is under development to reduce
the IRF file sizes into more manageable levels.
  This proposed format is attractive due to its ability to store multiple
parameters (in chosen ranges) relating to instrument performance in both binned
and parameterized formats, for various array and observing conditions. Details
of the format, preliminary design and testing of the prototype will be provided
below.
