FITS Keywords ============= The description of the keywords follows a convention found in other FITS keyword dictionaries, for example the list in http://fits.gsfc.nasa.gov/fits_dictionary.html. The keyword name is expressed, with the reference to the paper where it is included. Following the type of HDU where the keyword can appear. The value shows the kind of variable represented by the keyword. The comment is a example of the comment associated with the keyword and definition is a explanation in detail of the usage of the keyword. Primary header -------------- ==== ======== ======================== ============================================ Type Keyword Example Explanation ==== ======== ======================== ============================================ L SIMPLE T Standard FITS format I BITPIX 16 One of -64,-32,8,16,32 I NAXIS 2 # of axes in frame I NAXIS1 2048 # of pixels per row I NAXIS2 2048 # of rows S ORIGIN 'GTC' FITS originator S OBSERVAT 'ORM' Observatory S TELESCOP 'GTC' The telescope S INSTRUME 'EMIR' The instrument S OBJECT 'NGC 4594' Target designation S OBSERVER 'OBSERVER' Who adquired the data S DATE-OBS '2012-09-20T12:00:11.50' Date of the start of the observation S DATE '2012-09-20T12:14:12.78' Date the file was written ==== ======== ======================== ============================================ Required by the pipeline ------------------------ ==== ======== ======================== ============================================ Type Keyword Example Explanation ==== ======== ======================== ============================================ R AIRMASS 1.1908 Mean airmass of the observation R MJD-OBS 72343.34324 Modified JD of the start of the observation S IMAGETYP 'FLAT' Type of the image S OBSTYPE 'FLATON' Type of observation L READPROC T The frame has been preprocessed after readout S READMODE 'RAMP' The readmode used to adquire the image I READSAMP 12 Number of samples taken (equal to NAXIS1 if NAXIS is 3) R EXPOSED Photometric time? R DARKTIME TBD R EXPTIME TBD R ELAPSED Time between resets? I OBSID 567 Identifier of the observing block S OBSMODE 'DITHER_IMAGES' Identifier of the observing mode I OBSEXPN 30 # of exposures during the block I OBSSEQN 12 # of the image in the sequence ==== ======== ======================== ============================================ Coordinate system ----------------- The specifications of world coordinates are treated in a series of four papers. By World coordinates we mean coordinates that serve to locate a measurement in some multidimensional parameter space. They include, for example, a measurable quantity such as frequency or wavelength associated with each point of the spectrum or a longitude and latitude in a conventional spherical coordinate system. “Representation of world coordinates in FITSâ€, Greisen, E.W. & Clabretta, M.R. 2002 A&A 395, 1061 (hereafter Paper I) describes a very general method for specifying coordinates. A pixel-to-coordinate matrix PCj_i will replace CROTAj, units will be described with a new keyword CUNITj, and secondary sets of coordinate descriptions may be specified. A complete system of unit specification is described and is expected to supplement the IAU standard system of units. Methods for describing the coordinates of matrices in binary tables are also described. “Representation of celestial coordinates in FITSâ€, Clabretta, M.R. & Greisen, E.W. 2002 A&A 395, 1077 (hereafter Paper II) applies the general rules of Paper I to the specific problem of specifying celestial coordinates in a two-dimensional projection of the sky. The coordinate system is specified with the new keyword RADESYS and a large number of projections are defined. Oblique projections are described and illustrated. Several examples of header interpretation and construction are given including one that specifies coordinates on a planetary body rather than the celestial sphere. The application to binary tables is described. “Representation of spectral coordinates in FITSâ€, Greisen, E.W. et al. 2004 (hereafter Paper III) is still open to comments from the FITS community. It applies the general rules and practices developed in the first two papers to spectral coordinates, namely frequency, wavelength, velocity, and the radio and optical conventional velocities. These are defined and methods of computing one type of coordinate from a spectral axis gridded in another are given. A projection representative of optical spectrometers is also defined. Coordinate reference frames may be specified. “Representation of distortions in FITS world coordinate systemsâ€, Clabretta, M.R. et al. 2004 (hereafter Paper IV) is in preparation. It will define Distortion Correction Functions (DCFs) which may be used to correct for instrumental defects including celestial coordinate warps (plate defects), variation of actual frequency with celestial coordinate, refraction, and the like. The set of WCS keywords usable are those supported by wcslib library http://www.atnf.csiro.au/people/mcalabre/WCS/ Checksum convention ------------------- The CHECKSUM and DATASUM keywords that are embedded in the FITS header are used to verify the integrity of the HDU. See http://fits.gsfc.nasa.gov/registry/checksum.html ==== ======== ======================== ============================================ Type Keyword Example Explanation ==== ======== ======================== ============================================ S CHECKSUM 'ADFASASDLIEXV' HDU checksum S DATASUM '1929302939392 data unit checksum ==== ======== ======================== ============================================