The header has an unlimited number of parameters in the form key=value / comment that may be associated with and describe the data. The header consists of ASCII records of 80 characters (with optional line separators) each; both header and data have a blocksize of a 2880 bytes (or a small integer (1-10) factor thereof - the blocking factor). All major astronomical data reduction packages (AIPS, GIPSY, MIRIAD, IRAF, MIDAS etc.). can import/export their data from/to FITS format in some extent. However, there are some known FITS ‘‘dialects’’.
The FITS format was originally described in a classic paper by D.H. Wells, E.W. Greisen and R.H. Harten in Astronomy and Astrophysics Supplement Series, Vol. 44 (1981), pp. 363-370. A number of addenda have been published since then, which describe various extensions to the format. There is often information in the manuals of packages like AIPS and IRAF. An updated version is published by R. Hanish et al. in Astronomy and Astrophysics, Vol. 376 (2001), pp. 359. (bibcode: 2001A&A...376..359H)
NEMO has two library routines available to process fits files: fits(3NEMO) and fitsio(3NEMO) , the latter more specialized to read and write images and derived (and compatible with) from the MIRIAD fitsio interface.
SIMPLE T (standard FITS file) BITPIX Number of bits per datum [choice of 8, 16, 32, -32, -64, (64 experimental) (-32,-64 IEEE floating point, 8 unsigned, 16,32 twos-complement ints) NAXIS total number of axes (0, 1, 2, ... 999) in cube NAXISn size of a particular axis (n=1 through NAXIS) in cube PCOUNT number of extra parameters associated with cube [0 if not present] GCOUNT number of repetitions of the cube+parameter [1 if not present] CRVALn value of the coordinate at the reference pixel CRPIXn the reference pixel (often in range 1..NAXISn) CDELTn increment per pixel CTYPEn coordinate type (an ascii string) DATE-OBS Observing date: new Y2K format "yyyy-mm-ddThh:mm:ss" TIME-OBS Observing time, "hh:mm:ss", deprecated after new Y2K HISTORY history records (no ’=’ needed) COMMENT comment cards (no ’=’ needed) END last keyword in headerFor SIMPLE=T fits file the size of the data section (in bytes) can always be computed from the following formula:
|BITPIX/8| * GCOUNT * (PCOUNT + NAXIS1*NAXIS2* ... *NAXISm)where m is the value of NAXIS. This size must of course be rounded up to the nearest multiple of 2880 bytes. After this, either there is no more information on the medium, or another HDU can be read.
date +%Y-%m-%dT%H:%M:%S.%N 2020-12-18T13:15:49.648961250and this should be the new format (with as many sub-second digits as are needed).
Some common coordinate systems:
LINEAR Simple linear coordinate system RA, DEC Equatorial (followed by a projection type) ELON, ELAT Ecliptic (followed by a projection type) GLON, GLAT Galactic (followed by a projection type)
Some common coordinate system projection types:
RP Name ___ __ ___________________________________ AZP 90 Zenithal perspective TAN 90 Gnomic (AZP w/ mu = 0) SIN 90 Orthographic (AZP w/ mu = infty) NCP 90 North celestial pole (special case of SIN) STG 90 Stereographic (AZP w/ mu = 1) ARC 90 Zenithal equidistant ZPN 90 Zenithal polynomial ZEA 90 Zenithal equal-area AIR 90 Airy CYP 0 Cylindrical perspective CAR 0 Cartesian MER 0 Mercator CEA 0 Cylindrical equal area COP 90 Conical perspective COD 90 Conical equidistant COE 90 Conical equal-area COO 90 Conical orthomorphic BON 90 Bonne’s equal area PCO 0 Polyconic SFL 0 Sanson-Flamsteed GLS 0 GLobal Sinusoidal (Similar to SFL) PAR 0 Parabolic AIT 0 Hammer-Aitoff equal area all-sky MOL 0 Molweide CSC 0 Cobe Quadrilateralized Spherical Cube QSC 0 Quadrilateralized Spherical Cube TSC 0 Tangential Spherical Cube
30-may-88 written PJT 20-mar-90 Nemo’s FITS I/O library is being tested PJT 3-jul-94 doc updated with PCOUNT/GCOUNT PJT 28-sep-01 experimental bitpix 64 added PJT 18-dec-20 y2k date-obs reminder PJT
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