Representations of spectral coordinates in FITS

¹ National Radio Astronomy Observatory, PO Box O, Socorro, NM 87801-0387, USA e-mail: egreisen@nrao.edu
² Australia Telescope National Facility, PO Box 76, Epping, NSW 1710, Australia
³ National Optical Astronomy Observatories, PO Box 26732, Tucson, AZ 85719, USA
⁴ UCO/Lick Observatory, University of California, Santa Cruz, CA 95064, USA

Received: 12 July 2005
Accepted: 5 October 2005

Abstract

Greisen & Calabretta (2002, A&A, 395, 1061) describe a generalized method for specifying the coordinates of FITS data samples. Following that general method, Calabretta & Greisen (2002, A&A, 395, 1077) describe detailed conventions for defining celestial coordinates as they are projected onto a two-dimensional plane. The present paper extends the discussion to the spectral coordinates of wavelength, frequency, and velocity. World coordinate functions are defined for spectral axes sampled linearly in wavelength, frequency, or velocity, linearly in the logarithm of wavelength or frequency, as projected by ideal dispersing elements, and as specified by a lookup table.